public class

BrowseSupportFragment

extends BaseSupportFragment

Gradle dependencies

compile group: 'androidx.leanback', name: 'leanback', version: '1.2.0-alpha02'

  • groupId: androidx.leanback
  • artifactId: leanback
  • version: 1.2.0-alpha02

Artifact androidx.leanback:leanback:1.2.0-alpha02 it located at Google repository (https://maven.google.com/)

Androidx artifact mapping:

androidx.leanback:leanback com.android.support:leanback-v17

Androidx class mapping:

androidx.leanback.app.BrowseSupportFragment android.support.v17.leanback.app.BrowseSupportFragment

Overview

A fragment for creating Leanback browse screens. It is composed of a RowsSupportFragment and a HeadersSupportFragment.

A BrowseSupportFragment renders the elements of its ObjectAdapter as a set of rows in a vertical list. The elements in this adapter must be subclasses of Row.

The HeadersSupportFragment can be set to be either shown or hidden by default, or may be disabled entirely. See BrowseSupportFragment.setHeadersState(int) for details.

By default the BrowseSupportFragment includes support for returning to the headers when the user presses Back. For Activities that customize ComponentActivity.onBackPressed(), you must disable this default Back key support by calling BrowseSupportFragment.setHeadersTransitionOnBackEnabled(boolean) with false and use BrowseSupportFragment.BrowseTransitionListener and BrowseSupportFragment.startHeadersTransition(boolean).

The recommended theme to use with a BrowseSupportFragment is .

Summary

Fields
public static final intHEADERS_DISABLED

The headers fragment is disabled and will never be shown.

public static final intHEADERS_ENABLED

The headers fragment is enabled and shown by default.

public static final intHEADERS_HIDDEN

The headers fragment is enabled and hidden by default.

from FragmentmPreviousWho
Constructors
publicBrowseSupportFragment()

Methods
public static BundlecreateArgs(Bundle args, java.lang.String title, int headersState)

Creates arguments for a browse fragment.

protected java.lang.ObjectcreateEntranceTransition()

Create entrance transition.

public voidenableMainFragmentScaling(boolean enable)

Enables scaling of main fragment when headers are present.

public voidenableRowScaling(boolean enable)

public ObjectAdaptergetAdapter()

Returns the adapter containing the rows for the fragment.

public intgetBrandColor()

Returns the brand color for the browse fragment.

public intgetHeadersState()

Returns the state of the headers column in the browse fragment.

public HeadersSupportFragmentgetHeadersSupportFragment()

Get currently bound HeadersSupportFragment or null if HeadersSupportFragment has not been created yet.

public FragmentgetMainFragment()

public final BrowseSupportFragment.MainFragmentAdapterRegistrygetMainFragmentRegistry()

public OnItemViewClickedListenergetOnItemViewClickedListener()

Returns the item Clicked listener.

public OnItemViewSelectedListenergetOnItemViewSelectedListener()

Returns an item selection listener.

public RowsSupportFragmentgetRowsSupportFragment()

Get RowsSupportFragment if it's bound to BrowseSupportFragment or null if either BrowseSupportFragment has not been created yet or a different fragment is bound to it.

public intgetSelectedPosition()

Gets position of currently selected row.

public RowPresenter.ViewHoldergetSelectedRowViewHolder()

public final booleanisHeadersTransitionOnBackEnabled()

Returns true if headers transition on back key support is enabled.

public booleanisInHeadersTransition()

Returns true if the headers transition is currently running.

public booleanisShowingHeaders()

Returns true if headers are shown.

public voidonCreate(Bundle savedInstanceState)

Called to do initial creation of a fragment.

public HeadersSupportFragmentonCreateHeadersSupportFragment()

Creates a new HeadersSupportFragment instance.

public ViewonCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)

Called to have the fragment instantiate its user interface view.

public voidonDestroy()

Called when the fragment is no longer in use.

public voidonDestroyView()

Called when the view previously created by Fragment.onCreateView(LayoutInflater, ViewGroup, Bundle) has been detached from the fragment.

protected voidonEntranceTransitionEnd()

Callback when entrance transition is ended.

protected voidonEntranceTransitionPrepare()

Callback when entrance transition is prepared.

protected voidonEntranceTransitionStart()

Callback when entrance transition is started.

public voidonSaveInstanceState(Bundle outState)

Called to ask the fragment to save its current dynamic state, so it can later be reconstructed in a new instance if its process is restarted.

public voidonStart()

Called when the Fragment is visible to the user.

public voidonStop()

Called when the Fragment is no longer started.

protected voidrunEntranceTransition(java.lang.Object entranceTransition)

Run entrance transition.

public voidsetAdapter(ObjectAdapter adapter)

Sets the adapter containing the rows for the fragment.

public voidsetBrandColor(int color)

Sets the brand color for the browse fragment.

public voidsetBrowseTransitionListener(BrowseSupportFragment.BrowseTransitionListener listener)

Sets a listener for browse fragment transitions.

public voidsetHeaderPresenterSelector(PresenterSelector headerPresenterSelector)

Sets the PresenterSelector used to render the row headers.

public voidsetHeadersState(int headersState)

Sets the state for the headers column in the browse fragment.

public final voidsetHeadersTransitionOnBackEnabled(boolean headersBackStackEnabled)

Enables/disables headers transition on back key support.

public voidsetOnItemViewClickedListener(OnItemViewClickedListener listener)

Sets an item clicked listener on the fragment.

public voidsetOnItemViewSelectedListener(OnItemViewSelectedListener listener)

Sets an item selection listener.

public voidsetSelectedPosition(int position)

Sets the selected row position with smooth animation.

public voidsetSelectedPosition(int position, boolean smooth)

Sets the selected row position.

public voidsetSelectedPosition(int rowPosition, boolean smooth, Presenter.ViewHolderTask rowHolderTask)

Selects a Row and perform an optional task on the Row.

public voidstartHeadersTransition(boolean withHeaders)

Starts a headers transition.

from BaseSupportFragmentgetProgressBarManager, onViewCreated, prepareEntranceTransition, startEntranceTransition
from BrandedSupportFragmentgetBadgeDrawable, getSearchAffordanceColor, getSearchAffordanceColors, getTitle, getTitleView, getTitleViewAdapter, installTitleView, isShowingTitle, onInflateTitleView, onPause, onResume, setBadgeDrawable, setOnSearchClickedListener, setSearchAffordanceColor, setSearchAffordanceColors, setTitle, setTitleView, showTitle, showTitle
from Fragmentdump, equals, getActivity, getAllowEnterTransitionOverlap, getAllowReturnTransitionOverlap, getArguments, getChildFragmentManager, getContext, getDefaultViewModelCreationExtras, getDefaultViewModelProviderFactory, getEnterTransition, getExitTransition, getFragmentManager, getHost, getId, getLayoutInflater, getLayoutInflater, getLifecycle, getLoaderManager, getParentFragment, getParentFragmentManager, getReenterTransition, getResources, getRetainInstance, getReturnTransition, getSavedStateRegistry, getSharedElementEnterTransition, getSharedElementReturnTransition, getString, getString, getTag, getTargetFragment, getTargetRequestCode, getText, getUserVisibleHint, getView, getViewLifecycleOwner, getViewLifecycleOwnerLiveData, getViewModelStore, hashCode, hasOptionsMenu, instantiate, instantiate, isAdded, isDetached, isHidden, isInLayout, isMenuVisible, isRemoving, isResumed, isStateSaved, isVisible, onActivityCreated, onActivityResult, onAttach, onAttach, onAttachFragment, onConfigurationChanged, onContextItemSelected, onCreateAnimation, onCreateAnimator, onCreateContextMenu, onCreateOptionsMenu, onDestroyOptionsMenu, onDetach, onGetLayoutInflater, onHiddenChanged, onInflate, onInflate, onLowMemory, onMultiWindowModeChanged, onOptionsItemSelected, onOptionsMenuClosed, onPictureInPictureModeChanged, onPrepareOptionsMenu, onPrimaryNavigationFragmentChanged, onRequestPermissionsResult, onViewStateRestored, postponeEnterTransition, postponeEnterTransition, registerForActivityResult, registerForActivityResult, registerForContextMenu, requestPermissions, requireActivity, requireArguments, requireContext, requireFragmentManager, requireHost, requireParentFragment, requireView, setAllowEnterTransitionOverlap, setAllowReturnTransitionOverlap, setArguments, setEnterSharedElementCallback, setEnterTransition, setExitSharedElementCallback, setExitTransition, setHasOptionsMenu, setInitialSavedState, setMenuVisibility, setReenterTransition, setRetainInstance, setReturnTransition, setSharedElementEnterTransition, setSharedElementReturnTransition, setTargetFragment, setUserVisibleHint, shouldShowRequestPermissionRationale, startActivity, startActivity, startActivityForResult, startActivityForResult, startIntentSenderForResult, startPostponedEnterTransition, toString, unregisterForContextMenu
from java.lang.Objectclone, finalize, getClass, notify, notifyAll, wait, wait, wait

Fields

public static final int HEADERS_ENABLED

The headers fragment is enabled and shown by default.

public static final int HEADERS_HIDDEN

The headers fragment is enabled and hidden by default.

public static final int HEADERS_DISABLED

The headers fragment is disabled and will never be shown.

Constructors

public BrowseSupportFragment()

Methods

public static Bundle createArgs(Bundle args, java.lang.String title, int headersState)

Creates arguments for a browse fragment.

Parameters:

args: The Bundle to place arguments into, or null if the method should return a new Bundle.
title: The title of the BrowseSupportFragment.
headersState: The initial state of the headers of the BrowseSupportFragment. Must be one of BrowseSupportFragment.HEADERS_ENABLED, BrowseSupportFragment.HEADERS_HIDDEN, or BrowseSupportFragment.HEADERS_DISABLED.

Returns:

A Bundle with the given arguments for creating a BrowseSupportFragment.

public void setBrandColor(int color)

Sets the brand color for the browse fragment. The brand color is used as the primary color for UI elements in the browse fragment. For example, the background color of the headers fragment uses the brand color.

Parameters:

color: The color to use as the brand color of the fragment.

public int getBrandColor()

Returns the brand color for the browse fragment. The default is transparent.

public void setAdapter(ObjectAdapter adapter)

Sets the adapter containing the rows for the fragment.

The items referenced by the adapter must be be derived from Row. These rows will be used by the rows fragment and the headers fragment (if not disabled) to render the browse rows.

Parameters:

adapter: An ObjectAdapter for the browse rows. All items must derive from Row.

public final BrowseSupportFragment.MainFragmentAdapterRegistry getMainFragmentRegistry()

public ObjectAdapter getAdapter()

Returns the adapter containing the rows for the fragment.

public void setOnItemViewSelectedListener(OnItemViewSelectedListener listener)

Sets an item selection listener.

public OnItemViewSelectedListener getOnItemViewSelectedListener()

Returns an item selection listener.

public RowsSupportFragment getRowsSupportFragment()

Get RowsSupportFragment if it's bound to BrowseSupportFragment or null if either BrowseSupportFragment has not been created yet or a different fragment is bound to it.

Returns:

RowsSupportFragment if it's bound to BrowseSupportFragment or null otherwise.

public Fragment getMainFragment()

Returns:

Current main fragment or null if not created.

public HeadersSupportFragment getHeadersSupportFragment()

Get currently bound HeadersSupportFragment or null if HeadersSupportFragment has not been created yet.

Returns:

Currently bound HeadersSupportFragment or null if HeadersSupportFragment has not been created yet.

public void setOnItemViewClickedListener(OnItemViewClickedListener listener)

Sets an item clicked listener on the fragment. OnItemViewClickedListener will override that item presenter sets during Presenter.onCreateViewHolder(ViewGroup). So in general, developer should choose one of the listeners but not both.

public OnItemViewClickedListener getOnItemViewClickedListener()

Returns the item Clicked listener.

public void startHeadersTransition(boolean withHeaders)

Starts a headers transition.

This method will begin a transition to either show or hide the headers, depending on the value of withHeaders. If headers are disabled for this browse fragment, this method will throw an exception.

Parameters:

withHeaders: True if the headers should transition to being shown, false if the transition should result in headers being hidden.

public boolean isInHeadersTransition()

Returns true if the headers transition is currently running.

public boolean isShowingHeaders()

Returns true if headers are shown.

public void setBrowseTransitionListener(BrowseSupportFragment.BrowseTransitionListener listener)

Sets a listener for browse fragment transitions.

Parameters:

listener: The listener to call when a browse headers transition begins or ends.

public void enableRowScaling(boolean enable)

Deprecated: use BrowseSupportFragment.enableMainFragmentScaling(boolean) instead.

Parameters:

enable: true to enable row scaling

public void enableMainFragmentScaling(boolean enable)

Enables scaling of main fragment when headers are present. For the page/row fragment, scaling is enabled only when both this method and BrowseSupportFragment.MainFragmentAdapter.isScalingEnabled() are enabled.

Parameters:

enable: true to enable row scaling

public void onSaveInstanceState(Bundle outState)

Called to ask the fragment to save its current dynamic state, so it can later be reconstructed in a new instance if its process is restarted. If a new instance of the fragment later needs to be created, the data you place in the Bundle here will be available in the Bundle given to Fragment.onCreate(Bundle), Fragment.onCreateView(LayoutInflater, ViewGroup, Bundle), and Fragment.onViewCreated(View, Bundle).

This corresponds to and most of the discussion there applies here as well. Note however: this method may be called at any time before Fragment.onDestroy(). There are many situations where a fragment may be mostly torn down (such as when placed on the back stack with no UI showing), but its state will not be saved until its owning activity actually needs to save its state.

Parameters:

outState: Bundle in which to place your saved state.

public void onCreate(Bundle savedInstanceState)

Called to do initial creation of a fragment. This is called after Fragment.onAttach(Activity) and before Fragment.onCreateView(LayoutInflater, ViewGroup, Bundle).

Note that this can be called while the fragment's activity is still in the process of being created. As such, you can not rely on things like the activity's content view hierarchy being initialized at this point. If you want to do work once the activity itself is created, add a LifecycleObserver on the activity's Lifecycle, removing it when it receives the callback.

Any restored child fragments will be created before the base Fragment.onCreate method returns.

Parameters:

savedInstanceState: If the fragment is being re-created from a previous saved state, this is the state.

public void onDestroyView()

Called when the view previously created by Fragment.onCreateView(LayoutInflater, ViewGroup, Bundle) has been detached from the fragment. The next time the fragment needs to be displayed, a new view will be created. This is called after Fragment.onStop() and before Fragment.onDestroy(). It is called regardless of whether Fragment.onCreateView(LayoutInflater, ViewGroup, Bundle) returned a non-null view. Internally it is called after the view's state has been saved but before it has been removed from its parent.

public void onDestroy()

Called when the fragment is no longer in use. This is called after Fragment.onStop() and before Fragment.onDetach().

public HeadersSupportFragment onCreateHeadersSupportFragment()

Creates a new HeadersSupportFragment instance. Subclass of BrowseSupportFragment may override and return an instance of subclass of HeadersSupportFragment, e.g. when app wants to replace presenter to render HeaderItem.

Returns:

A new instance of HeadersSupportFragment or its subclass.

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)

Called to have the fragment instantiate its user interface view. This is optional, and non-graphical fragments can return null. This will be called between Fragment.onCreate(Bundle) and Fragment.onViewCreated(View, Bundle).

A default View can be returned by calling Fragment.Fragment(int) in your constructor. Otherwise, this method returns null.

It is recommended to only inflate the layout in this method and move logic that operates on the returned View to Fragment.onViewCreated(View, Bundle).

If you return a View from here, you will later be called in Fragment.onDestroyView() when the view is being released.

Parameters:

inflater: The LayoutInflater object that can be used to inflate any views in the fragment,
container: If non-null, this is the parent view that the fragment's UI should be attached to. The fragment should not add the view itself, but this can be used to generate the LayoutParams of the view.
savedInstanceState: If non-null, this fragment is being re-constructed from a previous saved state as given here.

Returns:

Return the View for the fragment's UI, or null.

public void setHeaderPresenterSelector(PresenterSelector headerPresenterSelector)

Sets the PresenterSelector used to render the row headers.

Parameters:

headerPresenterSelector: The PresenterSelector that will determine the Presenter for each row header.

public void setSelectedPosition(int position)

Sets the selected row position with smooth animation.

public int getSelectedPosition()

Gets position of currently selected row.

Returns:

Position of currently selected row.

public RowPresenter.ViewHolder getSelectedRowViewHolder()

Returns:

selected row ViewHolder inside fragment created by BrowseSupportFragment.MainFragmentRowsAdapter.

public void setSelectedPosition(int position, boolean smooth)

Sets the selected row position.

public void setSelectedPosition(int rowPosition, boolean smooth, Presenter.ViewHolderTask rowHolderTask)

Selects a Row and perform an optional task on the Row. For example setSelectedPosition(10, true, new ListRowPresenterSelectItemViewHolderTask(5)) scrolls to 11th row and selects 6th item on that row. The method will be ignored if RowsSupportFragment has not been created (i.e. before BrowseSupportFragment.onCreateView(LayoutInflater, ViewGroup, Bundle)).

Parameters:

rowPosition: Which row to select.
smooth: True to scroll to the row, false for no animation.
rowHolderTask: Optional task to perform on the Row. When the task is not null, headers fragment will be collapsed.

public void onStart()

Called when the Fragment is visible to the user. This is generally tied to of the containing Activity's lifecycle.

public void onStop()

Called when the Fragment is no longer started. This is generally tied to of the containing Activity's lifecycle.

public final void setHeadersTransitionOnBackEnabled(boolean headersBackStackEnabled)

Enables/disables headers transition on back key support. This is enabled by default. The BrowseSupportFragment will add a back stack entry when headers are showing. Running a headers transition when the back key is pressed only works when the headers state is BrowseSupportFragment.HEADERS_ENABLED or BrowseSupportFragment.HEADERS_HIDDEN.

NOTE: If an Activity has its own onBackPressed() handling, you must disable this feature. You may use BrowseSupportFragment.startHeadersTransition(boolean) and BrowseSupportFragment.BrowseTransitionListener in your own back stack handling.

public final boolean isHeadersTransitionOnBackEnabled()

Returns true if headers transition on back key support is enabled.

public void setHeadersState(int headersState)

Sets the state for the headers column in the browse fragment. Must be one of BrowseSupportFragment.HEADERS_ENABLED, BrowseSupportFragment.HEADERS_HIDDEN, or BrowseSupportFragment.HEADERS_DISABLED.

Parameters:

headersState: The state of the headers for the browse fragment.

public int getHeadersState()

Returns the state of the headers column in the browse fragment.

protected java.lang.Object createEntranceTransition()

Create entrance transition. Subclass can override to load transition from resource or construct manually. Typically app does not need to override the default transition that browse and details provides.

protected void runEntranceTransition(java.lang.Object entranceTransition)

Run entrance transition. Subclass may use TransitionManager to perform go(Scene) or beginDelayedTransition(). App should not override the default implementation of browse and details fragment.

protected void onEntranceTransitionPrepare()

Callback when entrance transition is prepared. This is when fragment should stop user input and animations.

protected void onEntranceTransitionStart()

Callback when entrance transition is started. This is when fragment should stop processing layout.

protected void onEntranceTransitionEnd()

Callback when entrance transition is ended.

Source

/*
 * Copyright (C) 2014 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */
package androidx.leanback.app;

import static androidx.recyclerview.widget.RecyclerView.NO_POSITION;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Color;
import android.graphics.Rect;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewGroup.MarginLayoutParams;
import android.view.ViewTreeObserver;

import androidx.annotation.ColorInt;
import androidx.core.view.ViewCompat;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentActivity;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentManager.BackStackEntry;
import androidx.fragment.app.FragmentTransaction;
import androidx.leanback.R;
import androidx.leanback.transition.TransitionHelper;
import androidx.leanback.transition.TransitionListener;
import androidx.leanback.util.StateMachine.Event;
import androidx.leanback.util.StateMachine.State;
import androidx.leanback.widget.BrowseFrameLayout;
import androidx.leanback.widget.InvisibleRowPresenter;
import androidx.leanback.widget.ListRow;
import androidx.leanback.widget.ObjectAdapter;
import androidx.leanback.widget.OnItemViewClickedListener;
import androidx.leanback.widget.OnItemViewSelectedListener;
import androidx.leanback.widget.PageRow;
import androidx.leanback.widget.Presenter;
import androidx.leanback.widget.PresenterSelector;
import androidx.leanback.widget.Row;
import androidx.leanback.widget.RowHeaderPresenter;
import androidx.leanback.widget.RowPresenter;
import androidx.leanback.widget.ScaleFrameLayout;
import androidx.leanback.widget.TitleViewAdapter;
import androidx.leanback.widget.VerticalGridView;
import androidx.recyclerview.widget.RecyclerView;

import java.util.HashMap;
import java.util.Map;

/**
 * A fragment for creating Leanback browse screens. It is composed of a
 * RowsSupportFragment and a HeadersSupportFragment.
 * <p>
 * A BrowseSupportFragment renders the elements of its {@link ObjectAdapter} as a set
 * of rows in a vertical list. The elements in this adapter must be subclasses
 * of {@link Row}.
 * <p>
 * The HeadersSupportFragment can be set to be either shown or hidden by default, or
 * may be disabled entirely. See {@link #setHeadersState} for details.
 * <p>
 * By default the BrowseSupportFragment includes support for returning to the headers
 * when the user presses Back. For Activities that customize {@link
 * FragmentActivity#onBackPressed()}, you must disable this default Back key support by
 * calling {@link #setHeadersTransitionOnBackEnabled(boolean)} with false and
 * use {@link BrowseSupportFragment.BrowseTransitionListener} and
 * {@link #startHeadersTransition(boolean)}.
 * <p>
 * The recommended theme to use with a BrowseSupportFragment is
 * {@link androidx.leanback.R.style#Theme_Leanback_Browse}.
 * </p>
 */
public class BrowseSupportFragment extends BaseSupportFragment {

    // BUNDLE attribute for saving header show/hide status when backstack is used:
    static final String HEADER_STACK_INDEX = "headerStackIndex";
    // BUNDLE attribute for saving header show/hide status when backstack is not used:
    static final String HEADER_SHOW = "headerShow";
    private static final String IS_PAGE_ROW = "isPageRow";
    private static final String CURRENT_SELECTED_POSITION = "currentSelectedPosition";

    /**
     * State to hide headers fragment.
     */
    final State STATE_SET_ENTRANCE_START_STATE = new State("SET_ENTRANCE_START_STATE") {
        @Override
        public void run() {
            setEntranceTransitionStartState();
        }
    };

    /**
     * Event for Header fragment view is created, we could perform
     * {@link #setEntranceTransitionStartState()} to hide headers fragment initially.
     */
    final Event EVT_HEADER_VIEW_CREATED = new Event("headerFragmentViewCreated");

    /**
     * Event for {@link #getMainFragment()} view is created, it's additional requirement to execute
     * {@link #onEntranceTransitionPrepare()}.
     */
    final Event EVT_MAIN_FRAGMENT_VIEW_CREATED = new Event("mainFragmentViewCreated");

    /**
     * Event that data for the screen is ready, this is additional requirement to launch entrance
     * transition.
     */
    final Event EVT_SCREEN_DATA_READY = new Event("screenDataReady");

    @Override
    void createStateMachineStates() {
        super.createStateMachineStates();
        mStateMachine.addState(STATE_SET_ENTRANCE_START_STATE);
    }

    @Override
    void createStateMachineTransitions() {
        super.createStateMachineTransitions();
        // when headers fragment view is created we could setEntranceTransitionStartState()
        mStateMachine.addTransition(STATE_ENTRANCE_ON_PREPARED, STATE_SET_ENTRANCE_START_STATE,
                EVT_HEADER_VIEW_CREATED);

        // add additional requirement for onEntranceTransitionPrepare()
        mStateMachine.addTransition(STATE_ENTRANCE_ON_PREPARED,
                STATE_ENTRANCE_ON_PREPARED_ON_CREATEVIEW,
                EVT_MAIN_FRAGMENT_VIEW_CREATED);
        // add additional requirement to launch entrance transition.
        mStateMachine.addTransition(STATE_ENTRANCE_ON_PREPARED,  STATE_ENTRANCE_PERFORM,
                EVT_SCREEN_DATA_READY);
    }

    final class BackStackListener implements FragmentManager.OnBackStackChangedListener {
        int mLastEntryCount;
        int mIndexOfHeadersBackStack;

        BackStackListener() {
            mLastEntryCount = getFragmentManager().getBackStackEntryCount();
            mIndexOfHeadersBackStack = -1;
        }

        void load(Bundle savedInstanceState) {
            if (savedInstanceState != null) {
                mIndexOfHeadersBackStack = savedInstanceState.getInt(HEADER_STACK_INDEX, -1);
                mShowingHeaders = mIndexOfHeadersBackStack == -1;
            } else {
                if (!mShowingHeaders) {
                    getFragmentManager().beginTransaction()
                            .addToBackStack(mWithHeadersBackStackName).commit();
                }
            }
        }

        void save(Bundle outState) {
            outState.putInt(HEADER_STACK_INDEX, mIndexOfHeadersBackStack);
        }


        @Override
        public void onBackStackChanged() {
            if (getFragmentManager() == null) {
                Log.w(TAG, "getFragmentManager() is null, stack:", new Exception());
                return;
            }
            int count = getFragmentManager().getBackStackEntryCount();
            // if backstack is growing and last pushed entry is "headers" backstack,
            // remember the index of the entry.
            if (count > mLastEntryCount) {
                BackStackEntry entry = getFragmentManager().getBackStackEntryAt(count - 1);
                if (mWithHeadersBackStackName.equals(entry.getName())) {
                    mIndexOfHeadersBackStack = count - 1;
                }
            } else if (count < mLastEntryCount) {
                // if popped "headers" backstack, initiate the show header transition if needed
                if (mIndexOfHeadersBackStack >= count) {
                    if (!isHeadersDataReady()) {
                        // if main fragment was restored first before BrowseSupportFragment's adapter gets
                        // restored: don't start header transition, but add the entry back.
                        getFragmentManager().beginTransaction()
                                .addToBackStack(mWithHeadersBackStackName).commit();
                        return;
                    }
                    mIndexOfHeadersBackStack = -1;
                    if (!mShowingHeaders) {
                        startHeadersTransitionInternal(true);
                    }
                }
            }
            mLastEntryCount = count;
        }
    }

    /**
     * Listener for transitions between browse headers and rows.
     */
    public static class BrowseTransitionListener {
        /**
         * Callback when headers transition starts.
         *
         * @param withHeaders True if the transition will result in headers
         *        being shown, false otherwise.
         */
        public void onHeadersTransitionStart(boolean withHeaders) {
        }
        /**
         * Callback when headers transition stops.
         *
         * @param withHeaders True if the transition will result in headers
         *        being shown, false otherwise.
         */
        public void onHeadersTransitionStop(boolean withHeaders) {
        }
    }

    private final class SetSelectionRunnable implements Runnable {
        static final int TYPE_INVALID = -1;
        static final int TYPE_INTERNAL_SYNC = 0;
        static final int TYPE_USER_REQUEST = 1;

        private int mPosition;
        private int mType;
        private boolean mSmooth;

        SetSelectionRunnable() {
            reset();
        }

        void post(int position, int type, boolean smooth) {
            // Posting the set selection, rather than calling it immediately, prevents an issue
            // with adapter changes.  Example: a row is added before the current selected row;
            // first the fast lane view updates its selection, then the rows fragment has that
            // new selection propagated immediately; THEN the rows view processes the same adapter
            // change and moves the selection again.
            if (type >= mType) {
                mPosition = position;
                mType = type;
                mSmooth = smooth;
                mBrowseFrame.removeCallbacks(this);
                if (!mStopped) {
                    mBrowseFrame.post(this);
                }
            }
        }

        @Override
        public void run() {
            setSelection(mPosition, mSmooth);
            reset();
        }

        public void stop() {
            // remove possible callback when stop, it will be re-added in start().
            mBrowseFrame.removeCallbacks(this);
        }

        public void start() {
            if (mType != TYPE_INVALID) {
                mBrowseFrame.post(this);
            }
        }

        private void reset() {
            mPosition = -1;
            mType = TYPE_INVALID;
            mSmooth = false;
        }
    }

    /**
     * Possible set of actions that {@link BrowseSupportFragment} exposes to clients. Custom
     * fragments can interact with {@link BrowseSupportFragment} using this interface.
     */
    public interface FragmentHost {
        /**
         * Fragments are required to invoke this callback once their view is created
         * inside {@link Fragment#onViewCreated} method. {@link BrowseSupportFragment} starts the entrance
         * animation only after receiving this callback. Failure to invoke this method
         * will lead to fragment not showing up.
         *
         * @param fragmentAdapter {@link MainFragmentAdapter} used by the current fragment.
         */
        void notifyViewCreated(MainFragmentAdapter fragmentAdapter);

        /**
         * Fragments mapped to {@link PageRow} are required to invoke this callback once their data
         * is created for transition, the entrance animation only after receiving this callback.
         * Failure to invoke this method will lead to fragment not showing up.
         *
         * @param fragmentAdapter {@link MainFragmentAdapter} used by the current fragment.
         */
        void notifyDataReady(MainFragmentAdapter fragmentAdapter);

        /**
         * Show or hide title view in {@link BrowseSupportFragment} for fragments mapped to
         * {@link PageRow}.  Otherwise the request is ignored, in that case BrowseSupportFragment is fully
         * in control of showing/hiding title view.
         * <p>
         * When HeadersSupportFragment is visible, BrowseSupportFragment will hide search affordance view if
         * there are other focusable rows above currently focused row.
         *
         * @param show Boolean indicating whether or not to show the title view.
         */
        void showTitleView(boolean show);
    }

    /**
     * Default implementation of {@link FragmentHost} that is used only by
     * {@link BrowseSupportFragment}.
     */
    private final class FragmentHostImpl implements FragmentHost {
        boolean mShowTitleView = true;

        FragmentHostImpl() {
        }

        @Override
        public void notifyViewCreated(MainFragmentAdapter fragmentAdapter) {
            mStateMachine.fireEvent(EVT_MAIN_FRAGMENT_VIEW_CREATED);
            if (!mIsPageRow) {
                // If it's not a PageRow: it's a ListRow, so we already have data ready.
                mStateMachine.fireEvent(EVT_SCREEN_DATA_READY);
            }
        }

        @Override
        public void notifyDataReady(MainFragmentAdapter fragmentAdapter) {
            // If fragment host is not the currently active fragment (in BrowseSupportFragment), then
            // ignore the request.
            if (mMainFragmentAdapter == null || mMainFragmentAdapter.getFragmentHost() != this) {
                return;
            }

            // We only honor showTitle request for PageRows.
            if (!mIsPageRow) {
                return;
            }

            mStateMachine.fireEvent(EVT_SCREEN_DATA_READY);
        }

        @Override
        public void showTitleView(boolean show) {
            mShowTitleView = show;

            // If fragment host is not the currently active fragment (in BrowseSupportFragment), then
            // ignore the request.
            if (mMainFragmentAdapter == null || mMainFragmentAdapter.getFragmentHost() != this) {
                return;
            }

            // We only honor showTitle request for PageRows.
            if (!mIsPageRow) {
                return;
            }

            updateTitleViewVisibility();
        }
    }

    /**
     * Interface that defines the interaction between {@link BrowseSupportFragment} and its main
     * content fragment. The key method is {@link MainFragmentAdapter#getFragment()},
     * it will be used to get the fragment to be shown in the content section. Clients can
     * provide any implementation of fragment and customize its interaction with
     * {@link BrowseSupportFragment} by overriding the necessary methods.
     *
     * <p>
     * Clients are expected to provide
     * an instance of {@link MainFragmentAdapterRegistry} which will be responsible for providing
     * implementations of {@link MainFragmentAdapter} for given content types. Currently
     * we support different types of content - {@link ListRow}, {@link PageRow} or any subtype
     * of {@link Row}. We provide an out of the box adapter implementation for any rows other than
     * {@link PageRow} - {@link androidx.leanback.app.RowsSupportFragment.MainFragmentAdapter}.
     *
     * <p>
     * {@link PageRow} is intended to give full flexibility to developers in terms of Fragment
     * design. Users will have to provide an implementation of {@link MainFragmentAdapter}
     * and provide that through {@link MainFragmentAdapterRegistry}.
     * {@link MainFragmentAdapter} implementation can supply any fragment and override
     * just those interactions that makes sense.
     */
    public static class MainFragmentAdapter<T extends Fragment> {
        private boolean mScalingEnabled;
        private final T mFragment;
        FragmentHostImpl mFragmentHost;

        public MainFragmentAdapter(T fragment) {
            this.mFragment = fragment;
        }

        public final T getFragment() {
            return mFragment;
        }

        /**
         * Returns whether its scrolling.
         */
        public boolean isScrolling() {
            return false;
        }

        /**
         * Set the visibility of titles/hover card of browse rows.
         */
        public void setExpand(boolean expand) {
        }

        /**
         * For rows that willing to participate entrance transition,  this function
         * hide views if afterTransition is true,  show views if afterTransition is false.
         */
        public void setEntranceTransitionState(boolean state) {
        }

        /**
         * Sets the window alignment and also the pivots for scale operation.
         */
        public void setAlignment(int windowAlignOffsetFromTop) {
        }

        /**
         * Callback indicating transition prepare start.
         */
        public boolean onTransitionPrepare() {
            return false;
        }

        /**
         * Callback indicating transition start.
         */
        public void onTransitionStart() {
        }

        /**
         * Callback indicating transition end.
         */
        public void onTransitionEnd() {
        }

        /**
         * Returns whether row scaling is enabled.
         */
        public boolean isScalingEnabled() {
            return mScalingEnabled;
        }

        /**
         * Sets the row scaling property.
         */
        public void setScalingEnabled(boolean scalingEnabled) {
            this.mScalingEnabled = scalingEnabled;
        }

        /**
         * Returns the current host interface so that main fragment can interact with
         * {@link BrowseSupportFragment}.
         */
        public final FragmentHost getFragmentHost() {
            return mFragmentHost;
        }

        void setFragmentHost(FragmentHostImpl fragmentHost) {
            this.mFragmentHost = fragmentHost;
        }
    }

    /**
     * Interface to be implemented by all fragments for providing an instance of
     * {@link MainFragmentAdapter}. Both {@link RowsSupportFragment} and custom fragment provided
     * against {@link PageRow} will need to implement this interface.
     */
    public interface MainFragmentAdapterProvider {
        /**
         * Returns an instance of {@link MainFragmentAdapter} that {@link BrowseSupportFragment}
         * would use to communicate with the target fragment.
         */
        MainFragmentAdapter getMainFragmentAdapter();
    }

    /**
     * Interface to be implemented by {@link RowsSupportFragment} and its subclasses for providing
     * an instance of {@link MainFragmentRowsAdapter}.
     */
    public interface MainFragmentRowsAdapterProvider {
        /**
         * Returns an instance of {@link MainFragmentRowsAdapter} that {@link BrowseSupportFragment}
         * would use to communicate with the target fragment.
         */
        MainFragmentRowsAdapter getMainFragmentRowsAdapter();
    }

    /**
     * This is used to pass information to {@link RowsSupportFragment} or its subclasses.
     * {@link BrowseSupportFragment} uses this interface to pass row based interaction events to
     * the target fragment.
     */
    public static class MainFragmentRowsAdapter<T extends Fragment> {
        private final T mFragment;

        public MainFragmentRowsAdapter(T fragment) {
            if (fragment == null) {
                throw new IllegalArgumentException("Fragment can't be null");
            }
            this.mFragment = fragment;
        }

        public final T getFragment() {
            return mFragment;
        }
        /**
         * Set the visibility titles/hover of browse rows.
         */
        public void setAdapter(ObjectAdapter adapter) {
        }

        /**
         * Sets an item clicked listener on the fragment.
         */
        public void setOnItemViewClickedListener(OnItemViewClickedListener listener) {
        }

        /**
         * Sets an item selection listener.
         */
        public void setOnItemViewSelectedListener(OnItemViewSelectedListener listener) {
        }

        /**
         * Selects a Row and perform an optional task on the Row.
         */
        public void setSelectedPosition(int rowPosition,
                                        boolean smooth,
                                        final Presenter.ViewHolderTask rowHolderTask) {
        }

        /**
         * Selects a Row.
         */
        public void setSelectedPosition(int rowPosition, boolean smooth) {
        }

        /**
         * @return The position of selected row.
         */
        public int getSelectedPosition() {
            return 0;
        }

        /**
         * @param position Position of Row.
         * @return Row ViewHolder.
         */
        public RowPresenter.ViewHolder findRowViewHolderByPosition(int position) {
            return null;
        }
    }

    private boolean createMainFragment(ObjectAdapter adapter, int position) {
        Object item = null;
        if (!mCanShowHeaders) {
            // when header is disabled, we can decide to use RowsSupportFragment even no data.
        } else if (adapter == null || adapter.size() == 0) {
            return false;
        } else {
            if (position < 0) {
                position = 0;
            } else if (position >= adapter.size()) {
                throw new IllegalArgumentException(
                        String.format("Invalid position %d requested", position));
            }
            item = adapter.get(position);
        }

        boolean oldIsPageRow = mIsPageRow;
        Object oldPageRow = mPageRow;
        mIsPageRow = mCanShowHeaders && item instanceof PageRow;
        mPageRow = mIsPageRow ? item : null;
        boolean swap;

        if (mMainFragment == null) {
            swap = true;
        } else {
            if (oldIsPageRow) {
                if (mIsPageRow) {
                    if (oldPageRow == null) {
                        // fragment is restored, page row object not yet set, so just set the
                        // mPageRow object and there is no need to replace the fragment
                        swap = false;
                    } else {
                        // swap if page row object changes
                        swap = oldPageRow != mPageRow;
                    }
                } else {
                    swap = true;
                }
            } else {
                swap = mIsPageRow;
            }
        }

        if (swap) {
            mMainFragment = mMainFragmentAdapterRegistry.createFragment(item);
            if (!(mMainFragment instanceof MainFragmentAdapterProvider)) {
                throw new IllegalArgumentException(
                        "Fragment must implement MainFragmentAdapterProvider");
            }

            setMainFragmentAdapter();
        }

        return swap;
    }

    void setMainFragmentAdapter() {
        mMainFragmentAdapter = ((MainFragmentAdapterProvider) mMainFragment)
                .getMainFragmentAdapter();
        mMainFragmentAdapter.setFragmentHost(new FragmentHostImpl());
        if (!mIsPageRow) {
            if (mMainFragment instanceof MainFragmentRowsAdapterProvider) {
                setMainFragmentRowsAdapter(((MainFragmentRowsAdapterProvider) mMainFragment)
                        .getMainFragmentRowsAdapter());
            } else {
                setMainFragmentRowsAdapter(null);
            }
            mIsPageRow = mMainFragmentRowsAdapter == null;
        } else {
            setMainFragmentRowsAdapter(null);
        }
    }

    /**
     * Factory class responsible for creating fragment given the current item. {@link ListRow}
     * should return {@link RowsSupportFragment} or its subclass whereas {@link PageRow}
     * can return any fragment class.
     */
    public abstract static class FragmentFactory<T extends Fragment> {
        public abstract T createFragment(Object row);
    }

    /**
     * FragmentFactory implementation for {@link ListRow}.
     */
    public static class ListRowFragmentFactory extends FragmentFactory<RowsSupportFragment> {
        @Override
        public RowsSupportFragment createFragment(Object row) {
            return new RowsSupportFragment();
        }
    }

    /**
     * Registry class maintaining the mapping of {@link Row} subclasses to {@link FragmentFactory}.
     * BrowseRowFragment automatically registers {@link ListRowFragmentFactory} for
     * handling {@link ListRow}. Developers can override that and also if they want to
     * use custom fragment, they can register a custom {@link FragmentFactory}
     * against {@link PageRow}.
     */
    public final static class MainFragmentAdapterRegistry {
        private final Map<Class<?>, FragmentFactory> mItemToFragmentFactoryMapping =
                new HashMap<>();
        private final static FragmentFactory sDefaultFragmentFactory = new ListRowFragmentFactory();

        public MainFragmentAdapterRegistry() {
            registerFragment(ListRow.class, sDefaultFragmentFactory);
        }

        public void registerFragment(Class<?> rowClass, FragmentFactory factory) {
            mItemToFragmentFactoryMapping.put(rowClass, factory);
        }

        public Fragment createFragment(Object item) {
            FragmentFactory fragmentFactory = item == null ? sDefaultFragmentFactory :
                    mItemToFragmentFactoryMapping.get(item.getClass());
            if (fragmentFactory == null && !(item instanceof PageRow)) {
                fragmentFactory = sDefaultFragmentFactory;
            }

            return fragmentFactory.createFragment(item);
        }
    }

    static final String TAG = "BrowseSupportFragment";

    private static final String LB_HEADERS_BACKSTACK = "lbHeadersBackStack_";

    static final boolean DEBUG = false;

    /** The headers fragment is enabled and shown by default. */
    public static final int HEADERS_ENABLED = 1;

    /** The headers fragment is enabled and hidden by default. */
    public static final int HEADERS_HIDDEN = 2;

    /** The headers fragment is disabled and will never be shown. */
    public static final int HEADERS_DISABLED = 3;

    private MainFragmentAdapterRegistry mMainFragmentAdapterRegistry =
            new MainFragmentAdapterRegistry();
    MainFragmentAdapter mMainFragmentAdapter;
    Fragment mMainFragment;
    HeadersSupportFragment mHeadersSupportFragment;
    MainFragmentRowsAdapter mMainFragmentRowsAdapter;
    ListRowDataAdapter mMainFragmentListRowDataAdapter;

    private ObjectAdapter mAdapter;
    private PresenterSelector mAdapterPresenter;

    private int mHeadersState = HEADERS_ENABLED;
    private int mBrandColor = Color.TRANSPARENT;
    private boolean mBrandColorSet;

    BrowseFrameLayout mBrowseFrame;
    private ScaleFrameLayout mScaleFrameLayout;
    boolean mHeadersBackStackEnabled = true;
    String mWithHeadersBackStackName;
    boolean mShowingHeaders = true;
    boolean mCanShowHeaders = true;
    private int mContainerListMarginStart;
    private int mContainerListAlignTop;
    private boolean mMainFragmentScaleEnabled = true;
    OnItemViewSelectedListener mExternalOnItemViewSelectedListener;
    private OnItemViewClickedListener mOnItemViewClickedListener;
    private int mSelectedPosition = -1;
    private float mScaleFactor;
    boolean mIsPageRow;
    Object mPageRow;
    boolean mStopped = true;

    private PresenterSelector mHeaderPresenterSelector;
    private final SetSelectionRunnable mSetSelectionRunnable = new SetSelectionRunnable();

    // transition related:
    Object mSceneWithHeaders;
    Object mSceneWithoutHeaders;
    private Object mSceneAfterEntranceTransition;
    Object mHeadersTransition;
    BackStackListener mBackStackChangedListener;
    BrowseTransitionListener mBrowseTransitionListener;

    private static final String ARG_TITLE = BrowseSupportFragment.class.getCanonicalName() + ".title";
    private static final String ARG_HEADERS_STATE =
        BrowseSupportFragment.class.getCanonicalName() + ".headersState";

    /**
     * Creates arguments for a browse fragment.
     *
     * @param args The Bundle to place arguments into, or null if the method
     *        should return a new Bundle.
     * @param title The title of the BrowseSupportFragment.
     * @param headersState The initial state of the headers of the
     *        BrowseSupportFragment. Must be one of {@link #HEADERS_ENABLED}, {@link
     *        #HEADERS_HIDDEN}, or {@link #HEADERS_DISABLED}.
     * @return A Bundle with the given arguments for creating a BrowseSupportFragment.
     */
    public static Bundle createArgs(Bundle args, String title, int headersState) {
        if (args == null) {
            args = new Bundle();
        }
        args.putString(ARG_TITLE, title);
        args.putInt(ARG_HEADERS_STATE, headersState);
        return args;
    }

    /**
     * Sets the brand color for the browse fragment. The brand color is used as
     * the primary color for UI elements in the browse fragment. For example,
     * the background color of the headers fragment uses the brand color.
     *
     * @param color The color to use as the brand color of the fragment.
     */
    public void setBrandColor(@ColorInt int color) {
        mBrandColor = color;
        mBrandColorSet = true;

        if (mHeadersSupportFragment != null) {
            mHeadersSupportFragment.setBackgroundColor(mBrandColor);
        }
    }

    /**
     * Returns the brand color for the browse fragment.
     * The default is transparent.
     */
    @ColorInt
    public int getBrandColor() {
        return mBrandColor;
    }

    /**
     * Wrapping app provided PresenterSelector to support InvisibleRowPresenter for SectionRow
     * DividerRow and PageRow.
     */
    private void updateWrapperPresenter() {
        if (mAdapter == null) {
            mAdapterPresenter = null;
            return;
        }
        final PresenterSelector adapterPresenter = mAdapter.getPresenterSelector();
        if (adapterPresenter == null) {
            throw new IllegalArgumentException("Adapter.getPresenterSelector() is null");
        }
        if (adapterPresenter == mAdapterPresenter) {
            return;
        }
        mAdapterPresenter = adapterPresenter;

        Presenter[] presenters = adapterPresenter.getPresenters();
        final Presenter invisibleRowPresenter = new InvisibleRowPresenter();
        final Presenter[] allPresenters = new Presenter[presenters.length + 1];
        System.arraycopy(allPresenters, 0, presenters, 0, presenters.length);
        allPresenters[allPresenters.length - 1] = invisibleRowPresenter;
        mAdapter.setPresenterSelector(new PresenterSelector() {
            @Override
            public Presenter getPresenter(Object item) {
                Row row = (Row) item;
                if (row.isRenderedAsRowView()) {
                    return adapterPresenter.getPresenter(item);
                } else {
                    return invisibleRowPresenter;
                }
            }

            @Override
            public Presenter[] getPresenters() {
                return allPresenters;
            }
        });
    }

    /**
     * Sets the adapter containing the rows for the fragment.
     *
     * <p>The items referenced by the adapter must be be derived from
     * {@link Row}. These rows will be used by the rows fragment and the headers
     * fragment (if not disabled) to render the browse rows.
     *
     * @param adapter An ObjectAdapter for the browse rows. All items must
     *        derive from {@link Row}.
     */
    public void setAdapter(ObjectAdapter adapter) {
        mAdapter = adapter;
        updateWrapperPresenter();
        if (getView() == null) {
            return;
        }

        updateMainFragmentRowsAdapter();
        mHeadersSupportFragment.setAdapter(mAdapter);
    }

    void setMainFragmentRowsAdapter(MainFragmentRowsAdapter mainFragmentRowsAdapter) {
        if (mainFragmentRowsAdapter == mMainFragmentRowsAdapter) {
            return;
        }
        // first clear previous mMainFragmentRowsAdapter and set a new mMainFragmentRowsAdapter
        if (mMainFragmentRowsAdapter != null) {
            // RowsFragment cannot change click/select listeners after view created.
            // The main fragment and adapter should be GCed as long as there is no reference from
            // BrowseSupportFragment to it.
            mMainFragmentRowsAdapter.setAdapter(null);
        }
        mMainFragmentRowsAdapter = mainFragmentRowsAdapter;
        if (mMainFragmentRowsAdapter != null) {
            mMainFragmentRowsAdapter.setOnItemViewSelectedListener(
                    new MainFragmentItemViewSelectedListener(mMainFragmentRowsAdapter));
            mMainFragmentRowsAdapter.setOnItemViewClickedListener(mOnItemViewClickedListener);
        }
        // second update mMainFragmentListRowDataAdapter set on mMainFragmentRowsAdapter
        updateMainFragmentRowsAdapter();
    }

    /**
     * Update mMainFragmentListRowDataAdapter and set it on mMainFragmentRowsAdapter.
     * It also clears old mMainFragmentListRowDataAdapter.
     */
    void updateMainFragmentRowsAdapter() {
        if (mMainFragmentListRowDataAdapter != null) {
            mMainFragmentListRowDataAdapter.detach();
            mMainFragmentListRowDataAdapter = null;
        }
        if (mMainFragmentRowsAdapter != null) {
            mMainFragmentListRowDataAdapter = mAdapter == null
                    ? null : new ListRowDataAdapter(mAdapter);
            mMainFragmentRowsAdapter.setAdapter(mMainFragmentListRowDataAdapter);
        }
    }

    public final MainFragmentAdapterRegistry getMainFragmentRegistry() {
        return mMainFragmentAdapterRegistry;
    }

    /**
     * Returns the adapter containing the rows for the fragment.
     */
    public ObjectAdapter getAdapter() {
        return mAdapter;
    }

    /**
     * Sets an item selection listener.
     */
    public void setOnItemViewSelectedListener(OnItemViewSelectedListener listener) {
        mExternalOnItemViewSelectedListener = listener;
    }

    /**
     * Returns an item selection listener.
     */
    public OnItemViewSelectedListener getOnItemViewSelectedListener() {
        return mExternalOnItemViewSelectedListener;
    }

    /**
     * Get RowsSupportFragment if it's bound to BrowseSupportFragment or null if either BrowseSupportFragment has
     * not been created yet or a different fragment is bound to it.
     *
     * @return RowsSupportFragment if it's bound to BrowseSupportFragment or null otherwise.
     */
    public RowsSupportFragment getRowsSupportFragment() {
        if (mMainFragment instanceof RowsSupportFragment) {
            return (RowsSupportFragment) mMainFragment;
        }

        return null;
    }

    /**
     * @return Current main fragment or null if not created.
     */
    public Fragment getMainFragment() {
        return mMainFragment;
    }

    /**
     * Get currently bound HeadersSupportFragment or null if HeadersSupportFragment has not been created yet.
     * @return Currently bound HeadersSupportFragment or null if HeadersSupportFragment has not been created yet.
     */
    public HeadersSupportFragment getHeadersSupportFragment() {
        return mHeadersSupportFragment;
    }

    /**
     * Sets an item clicked listener on the fragment.
     * OnItemViewClickedListener will override {@link View.OnClickListener} that
     * item presenter sets during {@link Presenter#onCreateViewHolder(ViewGroup)}.
     * So in general, developer should choose one of the listeners but not both.
     */
    public void setOnItemViewClickedListener(OnItemViewClickedListener listener) {
        mOnItemViewClickedListener = listener;
        if (mMainFragmentRowsAdapter != null) {
            mMainFragmentRowsAdapter.setOnItemViewClickedListener(listener);
        }
    }

    /**
     * Returns the item Clicked listener.
     */
    public OnItemViewClickedListener getOnItemViewClickedListener() {
        return mOnItemViewClickedListener;
    }

    /**
     * Starts a headers transition.
     *
     * <p>This method will begin a transition to either show or hide the
     * headers, depending on the value of withHeaders. If headers are disabled
     * for this browse fragment, this method will throw an exception.
     *
     * @param withHeaders True if the headers should transition to being shown,
     *        false if the transition should result in headers being hidden.
     */
    public void startHeadersTransition(boolean withHeaders) {
        if (!mCanShowHeaders) {
            throw new IllegalStateException("Cannot start headers transition");
        }
        if (isInHeadersTransition() || mShowingHeaders == withHeaders) {
            return;
        }
        startHeadersTransitionInternal(withHeaders);
    }

    /**
     * Returns true if the headers transition is currently running.
     */
    public boolean isInHeadersTransition() {
        return mHeadersTransition != null;
    }

    /**
     * Returns true if headers are shown.
     */
    public boolean isShowingHeaders() {
        return mShowingHeaders;
    }

    /**
     * Sets a listener for browse fragment transitions.
     *
     * @param listener The listener to call when a browse headers transition
     *        begins or ends.
     */
    public void setBrowseTransitionListener(BrowseTransitionListener listener) {
        mBrowseTransitionListener = listener;
    }

    /**
     * @deprecated use {@link BrowseSupportFragment#enableMainFragmentScaling(boolean)} instead.
     *
     * @param enable true to enable row scaling
     */
    @Deprecated
    public void enableRowScaling(boolean enable) {
        enableMainFragmentScaling(enable);
    }

    /**
     * Enables scaling of main fragment when headers are present. For the page/row fragment,
     * scaling is enabled only when both this method and
     * {@link MainFragmentAdapter#isScalingEnabled()} are enabled.
     *
     * @param enable true to enable row scaling
     */
    public void enableMainFragmentScaling(boolean enable) {
        mMainFragmentScaleEnabled = enable;
    }

    void startHeadersTransitionInternal(final boolean withHeaders) {
        if (getFragmentManager().isDestroyed()) {
            return;
        }
        if (!isHeadersDataReady()) {
            return;
        }
        mShowingHeaders = withHeaders;
        mMainFragmentAdapter.onTransitionPrepare();
        mMainFragmentAdapter.onTransitionStart();
        onExpandTransitionStart(!withHeaders, new Runnable() {
            @Override
            public void run() {
                mHeadersSupportFragment.onTransitionPrepare();
                mHeadersSupportFragment.onTransitionStart();
                createHeadersTransition();
                if (mBrowseTransitionListener != null) {
                    mBrowseTransitionListener.onHeadersTransitionStart(withHeaders);
                }
                TransitionHelper.runTransition(
                        withHeaders ? mSceneWithHeaders : mSceneWithoutHeaders, mHeadersTransition);
                if (mHeadersBackStackEnabled) {
                    if (!withHeaders) {
                        getFragmentManager().beginTransaction()
                                .addToBackStack(mWithHeadersBackStackName).commit();
                    } else {
                        int index = mBackStackChangedListener.mIndexOfHeadersBackStack;
                        if (index >= 0) {
                            BackStackEntry entry = getFragmentManager().getBackStackEntryAt(index);
                            getFragmentManager().popBackStackImmediate(entry.getId(),
                                    FragmentManager.POP_BACK_STACK_INCLUSIVE);
                        }
                    }
                }
            }
        });
    }

    boolean isVerticalScrolling() {
        // don't run transition
        return mHeadersSupportFragment.isScrolling() || mMainFragmentAdapter.isScrolling();
    }


    private final BrowseFrameLayout.OnFocusSearchListener mOnFocusSearchListener =
            new BrowseFrameLayout.OnFocusSearchListener() {
        @Override
        public View onFocusSearch(View focused, int direction) {
            // if headers is running transition,  focus stays
            if (mCanShowHeaders && isInHeadersTransition()) {
                return focused;
            }
            if (DEBUG) Log.v(TAG, "onFocusSearch focused " + focused + " + direction " + direction);

            if (getTitleView() != null && focused != getTitleView()
                    && direction == View.FOCUS_UP) {
                return getTitleView();
            }
            if (getTitleView() != null && getTitleView().hasFocus()
                    && direction == View.FOCUS_DOWN) {
                return mCanShowHeaders && mShowingHeaders
                        ? mHeadersSupportFragment.getVerticalGridView() : mMainFragment.getView();
            }

            boolean isRtl = ViewCompat.getLayoutDirection(focused)
                    == ViewCompat.LAYOUT_DIRECTION_RTL;
            int towardStart = isRtl ? View.FOCUS_RIGHT : View.FOCUS_LEFT;
            int towardEnd = isRtl ? View.FOCUS_LEFT : View.FOCUS_RIGHT;
            if (mCanShowHeaders && direction == towardStart) {
                if (isVerticalScrolling() || mShowingHeaders || !isHeadersDataReady()) {
                    return focused;
                }
                return mHeadersSupportFragment.getVerticalGridView();
            } else if (direction == towardEnd) {
                if (isVerticalScrolling()) {
                    return focused;
                } else if (mMainFragment != null && mMainFragment.getView() != null) {
                    return mMainFragment.getView();
                }
                return focused;
            } else if (direction == View.FOCUS_DOWN && mShowingHeaders) {
                // disable focus_down moving into PageFragment.
                return focused;
            } else {
                return null;
            }
        }
    };

    final boolean isHeadersDataReady() {
        return mAdapter != null && mAdapter.size() != 0;
    }

    private final BrowseFrameLayout.OnChildFocusListener mOnChildFocusListener =
            new BrowseFrameLayout.OnChildFocusListener() {

        @Override
        public boolean onRequestFocusInDescendants(int direction, Rect previouslyFocusedRect) {
            if (getChildFragmentManager().isDestroyed()) {
                return true;
            }
            // Make sure not changing focus when requestFocus() is called.
            if (mCanShowHeaders && mShowingHeaders) {
                if (mHeadersSupportFragment != null && mHeadersSupportFragment.getView() != null
                        && mHeadersSupportFragment.getView().requestFocus(
                                direction, previouslyFocusedRect)) {
                    return true;
                }
            }
            if (mMainFragment != null && mMainFragment.getView() != null
                    && mMainFragment.getView().requestFocus(direction, previouslyFocusedRect)) {
                return true;
            }
            return getTitleView() != null
                    && getTitleView().requestFocus(direction, previouslyFocusedRect);
        }

        @Override
        public void onRequestChildFocus(View child, View focused) {
            if (getChildFragmentManager().isDestroyed()) {
                return;
            }
            if (!mCanShowHeaders || isInHeadersTransition()) return;
            int childId = child.getId();
            if (childId == R.id.browse_container_dock && mShowingHeaders) {
                startHeadersTransitionInternal(false);
            } else if (childId == R.id.browse_headers_dock && !mShowingHeaders) {
                startHeadersTransitionInternal(true);
            }
        }
    };

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt(CURRENT_SELECTED_POSITION, mSelectedPosition);
        outState.putBoolean(IS_PAGE_ROW, mIsPageRow);

        if (mBackStackChangedListener != null) {
            mBackStackChangedListener.save(outState);
        } else {
            outState.putBoolean(HEADER_SHOW, mShowingHeaders);
        }
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final Context context = getContext();
        TypedArray ta = context.obtainStyledAttributes(R.styleable.LeanbackTheme);
        mContainerListMarginStart = (int) ta.getDimension(
                R.styleable.LeanbackTheme_browseRowsMarginStart, context.getResources()
                .getDimensionPixelSize(R.dimen.lb_browse_rows_margin_start));
        mContainerListAlignTop = (int) ta.getDimension(
                R.styleable.LeanbackTheme_browseRowsMarginTop, context.getResources()
                .getDimensionPixelSize(R.dimen.lb_browse_rows_margin_top));
        ta.recycle();

        readArguments(getArguments());

        if (mCanShowHeaders) {
            if (mHeadersBackStackEnabled) {
                mWithHeadersBackStackName = LB_HEADERS_BACKSTACK + this;
                mBackStackChangedListener = new BackStackListener();
                getFragmentManager().addOnBackStackChangedListener(mBackStackChangedListener);
                mBackStackChangedListener.load(savedInstanceState);
            } else {
                if (savedInstanceState != null) {
                    mShowingHeaders = savedInstanceState.getBoolean(HEADER_SHOW);
                }
            }
        }

        mScaleFactor = getResources().getFraction(R.fraction.lb_browse_rows_scale, 1, 1);
    }

    @Override
    public void onDestroyView() {
        setMainFragmentRowsAdapter(null);
        mPageRow = null;
        mMainFragmentAdapter = null;
        mMainFragment = null;
        mHeadersSupportFragment = null;
        mBrowseFrame = null;
        mScaleFrameLayout = null;
        mSceneAfterEntranceTransition = null;
        mSceneWithHeaders = null;
        mSceneWithoutHeaders = null;
        super.onDestroyView();
    }

    @Override
    public void onDestroy() {
        if (mBackStackChangedListener != null) {
            getFragmentManager().removeOnBackStackChangedListener(mBackStackChangedListener);
        }
        super.onDestroy();
    }

    /**
     * Creates a new {@link HeadersSupportFragment} instance. Subclass of BrowseSupportFragment may override and
     * return an instance of subclass of HeadersSupportFragment, e.g. when app wants to replace presenter
     * to render HeaderItem.
     *
     * @return A new instance of {@link HeadersSupportFragment} or its subclass.
     */
    public HeadersSupportFragment onCreateHeadersSupportFragment() {
        return new HeadersSupportFragment();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        if (getChildFragmentManager().findFragmentById(R.id.scale_frame) == null) {
            mHeadersSupportFragment = onCreateHeadersSupportFragment();

            createMainFragment(mAdapter, mSelectedPosition);
            FragmentTransaction ft = getChildFragmentManager().beginTransaction()
                    .replace(R.id.browse_headers_dock, mHeadersSupportFragment);

            if (mMainFragment != null) {
                ft.replace(R.id.scale_frame, mMainFragment);
            } else {
                // Empty adapter used to guard against lazy adapter loading. When this
                // fragment is instantiated, mAdapter might not have the data or might not
                // have been set. In either of those cases mFragmentAdapter will be null.
                // This way we can maintain the invariant that mMainFragmentAdapter is never
                // null and it avoids doing null checks all over the code.
                mMainFragmentAdapter = new MainFragmentAdapter<>(null);
                mMainFragmentAdapter.setFragmentHost(new FragmentHostImpl());
            }

            ft.commit();
        } else {
            mHeadersSupportFragment = (HeadersSupportFragment) getChildFragmentManager()
                    .findFragmentById(R.id.browse_headers_dock);
            mMainFragment = getChildFragmentManager().findFragmentById(R.id.scale_frame);

            mIsPageRow = savedInstanceState != null
                    && savedInstanceState.getBoolean(IS_PAGE_ROW, false);
            // mPageRow object is unable to restore, if its null and mIsPageRow is true, this is
            // the case for restoring, later if setSelection() triggers a createMainFragment(),
            // should not create fragment.

            mSelectedPosition = savedInstanceState != null
                    ? savedInstanceState.getInt(CURRENT_SELECTED_POSITION, 0) : 0;

            setMainFragmentAdapter();
        }

        mHeadersSupportFragment.setHeadersGone(!mCanShowHeaders);
        if (mHeaderPresenterSelector != null) {
            mHeadersSupportFragment.setPresenterSelector(mHeaderPresenterSelector);
        }
        mHeadersSupportFragment.setAdapter(mAdapter);
        mHeadersSupportFragment.setOnHeaderViewSelectedListener(mHeaderViewSelectedListener);
        mHeadersSupportFragment.setOnHeaderClickedListener(mHeaderClickedListener);

        View root = inflater.inflate(R.layout.lb_browse_fragment, container, false);

        getProgressBarManager().setRootView((ViewGroup)root);

        mBrowseFrame = (BrowseFrameLayout) root.findViewById(R.id.browse_frame);
        mBrowseFrame.setOnChildFocusListener(mOnChildFocusListener);
        mBrowseFrame.setOnFocusSearchListener(mOnFocusSearchListener);

        installTitleView(inflater, mBrowseFrame, savedInstanceState);

        mScaleFrameLayout = (ScaleFrameLayout) root.findViewById(R.id.scale_frame);
        mScaleFrameLayout.setPivotX(0);
        mScaleFrameLayout.setPivotY(mContainerListAlignTop);

        if (mBrandColorSet) {
            mHeadersSupportFragment.setBackgroundColor(mBrandColor);
        }

        mSceneWithHeaders = TransitionHelper.createScene(mBrowseFrame, new Runnable() {
            @Override
            public void run() {
                showHeaders(true);
            }
        });
        mSceneWithoutHeaders =  TransitionHelper.createScene(mBrowseFrame, new Runnable() {
            @Override
            public void run() {
                showHeaders(false);
            }
        });
        mSceneAfterEntranceTransition = TransitionHelper.createScene(mBrowseFrame, new Runnable() {
            @Override
            public void run() {
                setEntranceTransitionEndState();
            }
        });

        return root;
    }

    void createHeadersTransition() {
        mHeadersTransition = TransitionHelper.loadTransition(getContext(),
                mShowingHeaders
                        ? R.transition.lb_browse_headers_in : R.transition.lb_browse_headers_out);

        TransitionHelper.addTransitionListener(mHeadersTransition, new TransitionListener() {
            @Override
            public void onTransitionStart(Object transition) {
            }
            @Override
            public void onTransitionEnd(Object transition) {
                mHeadersTransition = null;
                if (mMainFragmentAdapter != null) {
                    mMainFragmentAdapter.onTransitionEnd();
                    if (!mShowingHeaders && mMainFragment != null) {
                        View mainFragmentView = mMainFragment.getView();
                        if (mainFragmentView != null && !mainFragmentView.hasFocus()) {
                            mainFragmentView.requestFocus();
                        }
                    }
                }
                if (mHeadersSupportFragment != null) {
                    mHeadersSupportFragment.onTransitionEnd();
                    if (mShowingHeaders) {
                        VerticalGridView headerGridView = mHeadersSupportFragment.getVerticalGridView();
                        if (headerGridView != null && !headerGridView.hasFocus()) {
                            headerGridView.requestFocus();
                        }
                    }
                }

                // Animate TitleView once header animation is complete.
                updateTitleViewVisibility();

                if (mBrowseTransitionListener != null) {
                    mBrowseTransitionListener.onHeadersTransitionStop(mShowingHeaders);
                }
            }
        });
    }

    void updateTitleViewVisibility() {
        if (!mShowingHeaders) {
            boolean showTitleView;
            if (mIsPageRow && mMainFragmentAdapter != null) {
                // page fragment case:
                showTitleView = mMainFragmentAdapter.mFragmentHost.mShowTitleView;
            } else {
                // regular row view case:
                showTitleView = isFirstRowWithContent(mSelectedPosition);
            }
            if (showTitleView) {
                showTitle(TitleViewAdapter.FULL_VIEW_VISIBLE);
            } else {
                showTitle(false);
            }
        } else {
            // when HeaderFragment is showing,  showBranding and showSearch are slightly different
            boolean showBranding;
            boolean showSearch;
            if (mIsPageRow && mMainFragmentAdapter != null) {
                showBranding = mMainFragmentAdapter.mFragmentHost.mShowTitleView;
            } else {
                showBranding = isFirstRowWithContent(mSelectedPosition);
            }
            showSearch = isFirstRowWithContentOrPageRow(mSelectedPosition);
            int flags = 0;
            if (showBranding) flags |= TitleViewAdapter.BRANDING_VIEW_VISIBLE;
            if (showSearch) flags |= TitleViewAdapter.SEARCH_VIEW_VISIBLE;
            if (flags != 0) {
                showTitle(flags);
            } else {
                showTitle(false);
            }
        }
    }

    boolean isFirstRowWithContentOrPageRow(int rowPosition) {
        if (mAdapter == null || mAdapter.size() == 0) {
            return true;
        }
        for (int i = 0; i < mAdapter.size(); i++) {
            final Row row = (Row) mAdapter.get(i);
            if (row.isRenderedAsRowView() || row instanceof PageRow) {
                return rowPosition == i;
            }
        }
        return true;
    }

    boolean isFirstRowWithContent(int rowPosition) {
        if (mAdapter == null || mAdapter.size() == 0) {
            return true;
        }
        for (int i = 0; i < mAdapter.size(); i++) {
            final Row row = (Row) mAdapter.get(i);
            if (row.isRenderedAsRowView()) {
                return rowPosition == i;
            }
        }
        return true;
    }

    /**
     * Sets the {@link PresenterSelector} used to render the row headers.
     *
     * @param headerPresenterSelector The PresenterSelector that will determine
     *        the Presenter for each row header.
     */
    public void setHeaderPresenterSelector(PresenterSelector headerPresenterSelector) {
        mHeaderPresenterSelector = headerPresenterSelector;
        if (mHeadersSupportFragment != null) {
            mHeadersSupportFragment.setPresenterSelector(mHeaderPresenterSelector);
        }
    }

    private void setHeadersOnScreen(boolean onScreen) {
        MarginLayoutParams lp;
        View containerList;
        containerList = mHeadersSupportFragment.getView();
        if (containerList == null) {
            // Headers fragment has destroyed view.
            return;
        }
        lp = (MarginLayoutParams) containerList.getLayoutParams();
        lp.setMarginStart(onScreen ? 0 : -mContainerListMarginStart);
        containerList.setLayoutParams(lp);
    }

    void showHeaders(boolean show) {
        if (DEBUG) Log.v(TAG, "showHeaders " + show);
        mHeadersSupportFragment.setHeadersEnabled(show);
        setHeadersOnScreen(show);
        expandMainFragment(!show);
    }

    private void expandMainFragment(boolean expand) {
        MarginLayoutParams params = (MarginLayoutParams) mScaleFrameLayout.getLayoutParams();
        params.setMarginStart(!expand ? mContainerListMarginStart : 0);
        mScaleFrameLayout.setLayoutParams(params);
        mMainFragmentAdapter.setExpand(expand);

        setMainFragmentAlignment();
        final float scaleFactor = !expand
                && mMainFragmentScaleEnabled
                && mMainFragmentAdapter.isScalingEnabled() ? mScaleFactor : 1;
        mScaleFrameLayout.setLayoutScaleY(scaleFactor);
        mScaleFrameLayout.setChildScale(scaleFactor);
    }

    private HeadersSupportFragment.OnHeaderClickedListener mHeaderClickedListener =
        new HeadersSupportFragment.OnHeaderClickedListener() {
            @Override
            public void onHeaderClicked(RowHeaderPresenter.ViewHolder viewHolder, Row row) {
                if (!mCanShowHeaders || !mShowingHeaders || isInHeadersTransition()) {
                    return;
                }
                if (mMainFragment == null || mMainFragment.getView() == null) {
                    return;
                }
                startHeadersTransitionInternal(false);
                mMainFragment.getView().requestFocus();
            }
        };

    class MainFragmentItemViewSelectedListener implements OnItemViewSelectedListener {
        MainFragmentRowsAdapter mMainFragmentRowsAdapter;

        public MainFragmentItemViewSelectedListener(MainFragmentRowsAdapter fragmentRowsAdapter) {
            mMainFragmentRowsAdapter = fragmentRowsAdapter;
        }

        @Override
        public void onItemSelected(Presenter.ViewHolder itemViewHolder, Object item,
                RowPresenter.ViewHolder rowViewHolder, Row row) {
            int position = mMainFragmentRowsAdapter.getSelectedPosition();
            if (DEBUG) Log.v(TAG, "row selected position " + position);
            onRowSelected(position);
            if (mExternalOnItemViewSelectedListener != null) {
                mExternalOnItemViewSelectedListener.onItemSelected(itemViewHolder, item,
                        rowViewHolder, row);
            }
        }
    };

    private HeadersSupportFragment.OnHeaderViewSelectedListener mHeaderViewSelectedListener =
            new HeadersSupportFragment.OnHeaderViewSelectedListener() {
        @Override
        public void onHeaderSelected(RowHeaderPresenter.ViewHolder viewHolder, Row row) {
            int position = mHeadersSupportFragment.getSelectedPosition();
            if (DEBUG) Log.v(TAG, "header selected position " + position);
            // Layout of Headers Fragment in hidden state may triggers the onRowSelected and
            // reset to 0. Skip in that case.
            if (mShowingHeaders) {
                onRowSelected(position);
            }
        }
    };

    void onRowSelected(int position) {
        // even position is same, it could be data changed, always post selection runnable
        // to possibly swap main fragment.
        mSetSelectionRunnable.post(
                position, SetSelectionRunnable.TYPE_INTERNAL_SYNC, true);
    }

    void setSelection(int position, boolean smooth) {
        if (position == NO_POSITION) {
            return;
        }

        mSelectedPosition = position;
        if (mHeadersSupportFragment == null || mMainFragmentAdapter == null) {
            // onDestroyView() called
            return;
        }
        mHeadersSupportFragment.setSelectedPosition(position, smooth);
        replaceMainFragment(position);

        if (mMainFragmentRowsAdapter != null) {
            mMainFragmentRowsAdapter.setSelectedPosition(position, smooth);
        }

        updateTitleViewVisibility();
    }

    private void replaceMainFragment(int position) {
        if (createMainFragment(mAdapter, position)) {
            swapToMainFragment();
            expandMainFragment(!(mCanShowHeaders && mShowingHeaders));
        }
    }

    @SuppressWarnings("ReferenceEquality")
    final void commitMainFragment() {
        FragmentManager fm = getChildFragmentManager();
        Fragment currentFragment = fm.findFragmentById(R.id.scale_frame);
        if (currentFragment != mMainFragment) {
            fm.beginTransaction()
                    .replace(R.id.scale_frame, mMainFragment).commit();
        }
    }

    private final RecyclerView.OnScrollListener mWaitScrollFinishAndCommitMainFragment =
            new RecyclerView.OnScrollListener() {
        @SuppressWarnings("ReferenceEquality")
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            if (newState == RecyclerView.SCROLL_STATE_IDLE) {
                recyclerView.removeOnScrollListener(this);
                if (!mStopped) {
                    commitMainFragment();
                }
            }
        }
    };

    private void swapToMainFragment() {
        if (mStopped) {
            return;
        }
        final VerticalGridView gridView = mHeadersSupportFragment.getVerticalGridView();
        if (isShowingHeaders() && gridView != null
                && gridView.getScrollState() != RecyclerView.SCROLL_STATE_IDLE) {
            // if user is scrolling HeadersSupportFragment,  swap to empty fragment and wait scrolling
            // finishes.
            getChildFragmentManager().beginTransaction()
                    .replace(R.id.scale_frame, new Fragment()).commit();
            gridView.removeOnScrollListener(mWaitScrollFinishAndCommitMainFragment);
            gridView.addOnScrollListener(mWaitScrollFinishAndCommitMainFragment);
        } else {
            // Otherwise swap immediately
            commitMainFragment();
        }
    }

    /**
     * Sets the selected row position with smooth animation.
     */
    public void setSelectedPosition(int position) {
        setSelectedPosition(position, true);
    }

    /**
     * Gets position of currently selected row.
     * @return Position of currently selected row.
     */
    public int getSelectedPosition() {
        return mSelectedPosition;
    }

    /**
     * @return selected row ViewHolder inside fragment created by {@link MainFragmentRowsAdapter}.
     */
    public RowPresenter.ViewHolder getSelectedRowViewHolder() {
        if (mMainFragmentRowsAdapter != null) {
            int rowPos = mMainFragmentRowsAdapter.getSelectedPosition();
            return mMainFragmentRowsAdapter.findRowViewHolderByPosition(rowPos);
        }
        return null;
    }

    /**
     * Sets the selected row position.
     */
    public void setSelectedPosition(int position, boolean smooth) {
        mSetSelectionRunnable.post(
                position, SetSelectionRunnable.TYPE_USER_REQUEST, smooth);
    }

    /**
     * Selects a Row and perform an optional task on the Row. For example
     * <code>setSelectedPosition(10, true, new ListRowPresenterSelectItemViewHolderTask(5))</code>
     * scrolls to 11th row and selects 6th item on that row.  The method will be ignored if
     * RowsSupportFragment has not been created (i.e. before {@link #onCreateView(LayoutInflater,
     * ViewGroup, Bundle)}).
     *
     * @param rowPosition Which row to select.
     * @param smooth True to scroll to the row, false for no animation.
     * @param rowHolderTask Optional task to perform on the Row.  When the task is not null, headers
     * fragment will be collapsed.
     */
    public void setSelectedPosition(int rowPosition, boolean smooth,
            final Presenter.ViewHolderTask rowHolderTask) {
        if (mMainFragmentAdapterRegistry == null) {
            return;
        }
        if (rowHolderTask != null) {
            startHeadersTransition(false);
        }
        if (mMainFragmentRowsAdapter != null) {
            mMainFragmentRowsAdapter.setSelectedPosition(rowPosition, smooth, rowHolderTask);
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        mHeadersSupportFragment.setAlignment(mContainerListAlignTop);
        setMainFragmentAlignment();

        if (mCanShowHeaders && mShowingHeaders && mHeadersSupportFragment != null
                && mHeadersSupportFragment.getView() != null) {
            mHeadersSupportFragment.getView().requestFocus();
        } else if ((!mCanShowHeaders || !mShowingHeaders) && mMainFragment != null
                && mMainFragment.getView() != null) {
            mMainFragment.getView().requestFocus();
        }

        if (mCanShowHeaders) {
            showHeaders(mShowingHeaders);
        }

        mStateMachine.fireEvent(EVT_HEADER_VIEW_CREATED);

        mStopped = false;
        // if main fragment wasn't commited in stopped state, do it again in onStart()
        commitMainFragment();
        mSetSelectionRunnable.start();
    }

    @Override
    public void onStop() {
        mStopped = true;
        mSetSelectionRunnable.stop();
        super.onStop();
    }

    private void onExpandTransitionStart(boolean expand, final Runnable callback) {
        if (expand) {
            callback.run();
            return;
        }
        // Run a "pre" layout when we go non-expand, in order to get the initial
        // positions of added rows.
        new ExpandPreLayout(callback, mMainFragmentAdapter, getView()).execute();
    }

    private void setMainFragmentAlignment() {
        int alignOffset = mContainerListAlignTop;
        if (mMainFragmentScaleEnabled
                && mMainFragmentAdapter.isScalingEnabled()
                && mShowingHeaders) {
            alignOffset = (int) (alignOffset / mScaleFactor + 0.5f);
        }
        mMainFragmentAdapter.setAlignment(alignOffset);
    }

    /**
     * Enables/disables headers transition on back key support. This is enabled by
     * default. The BrowseSupportFragment will add a back stack entry when headers are
     * showing. Running a headers transition when the back key is pressed only
     * works when the headers state is {@link #HEADERS_ENABLED} or
     * {@link #HEADERS_HIDDEN}.
     * <p>
     * NOTE: If an Activity has its own onBackPressed() handling, you must
     * disable this feature. You may use {@link #startHeadersTransition(boolean)}
     * and {@link BrowseTransitionListener} in your own back stack handling.
     */
    public final void setHeadersTransitionOnBackEnabled(boolean headersBackStackEnabled) {
        mHeadersBackStackEnabled = headersBackStackEnabled;
    }

    /**
     * Returns true if headers transition on back key support is enabled.
     */
    public final boolean isHeadersTransitionOnBackEnabled() {
        return mHeadersBackStackEnabled;
    }

    private void readArguments(Bundle args) {
        if (args == null) {
            return;
        }
        if (args.containsKey(ARG_TITLE)) {
            setTitle(args.getString(ARG_TITLE));
        }
        if (args.containsKey(ARG_HEADERS_STATE)) {
            setHeadersState(args.getInt(ARG_HEADERS_STATE));
        }
    }

    /**
     * Sets the state for the headers column in the browse fragment. Must be one
     * of {@link #HEADERS_ENABLED}, {@link #HEADERS_HIDDEN}, or
     * {@link #HEADERS_DISABLED}.
     *
     * @param headersState The state of the headers for the browse fragment.
     */
    public void setHeadersState(int headersState) {
        if (headersState < HEADERS_ENABLED || headersState > HEADERS_DISABLED) {
            throw new IllegalArgumentException("Invalid headers state: " + headersState);
        }
        if (DEBUG) Log.v(TAG, "setHeadersState " + headersState);

        if (headersState != mHeadersState) {
            mHeadersState = headersState;
            switch (headersState) {
                case HEADERS_ENABLED:
                    mCanShowHeaders = true;
                    mShowingHeaders = true;
                    break;
                case HEADERS_HIDDEN:
                    mCanShowHeaders = true;
                    mShowingHeaders = false;
                    break;
                case HEADERS_DISABLED:
                    mCanShowHeaders = false;
                    mShowingHeaders = false;
                    break;
                default:
                    Log.w(TAG, "Unknown headers state: " + headersState);
                    break;
            }
            if (mHeadersSupportFragment != null) {
                mHeadersSupportFragment.setHeadersGone(!mCanShowHeaders);
            }
        }
    }

    /**
     * Returns the state of the headers column in the browse fragment.
     */
    public int getHeadersState() {
        return mHeadersState;
    }

    @Override
    protected Object createEntranceTransition() {
        return TransitionHelper.loadTransition(getContext(),
                R.transition.lb_browse_entrance_transition);
    }

    @Override
    protected void runEntranceTransition(Object entranceTransition) {
        TransitionHelper.runTransition(mSceneAfterEntranceTransition, entranceTransition);
    }

    @Override
    protected void onEntranceTransitionPrepare() {
        mHeadersSupportFragment.onTransitionPrepare();
        mMainFragmentAdapter.setEntranceTransitionState(false);
        mMainFragmentAdapter.onTransitionPrepare();
    }

    @Override
    protected void onEntranceTransitionStart() {
        mHeadersSupportFragment.onTransitionStart();
        mMainFragmentAdapter.onTransitionStart();
    }

    @Override
    protected void onEntranceTransitionEnd() {
        if (mMainFragmentAdapter != null) {
            mMainFragmentAdapter.onTransitionEnd();
        }

        if (mHeadersSupportFragment != null) {
            mHeadersSupportFragment.onTransitionEnd();
        }
    }

    void setSearchOrbViewOnScreen(boolean onScreen) {
        View searchOrbView = getTitleViewAdapter().getSearchAffordanceView();
        if (searchOrbView != null) {
            MarginLayoutParams lp = (MarginLayoutParams) searchOrbView.getLayoutParams();
            lp.setMarginStart(onScreen ? 0 : -mContainerListMarginStart);
            searchOrbView.setLayoutParams(lp);
        }
    }

    void setEntranceTransitionStartState() {
        setHeadersOnScreen(false);
        setSearchOrbViewOnScreen(false);
        // NOTE that mMainFragmentAdapter.setEntranceTransitionState(false) will be called
        // in onEntranceTransitionPrepare() because mMainFragmentAdapter is still the dummy
        // one when setEntranceTransitionStartState() is called.
    }

    void setEntranceTransitionEndState() {
        setHeadersOnScreen(mShowingHeaders);
        setSearchOrbViewOnScreen(true);
        mMainFragmentAdapter.setEntranceTransitionState(true);
    }

    private class ExpandPreLayout implements ViewTreeObserver.OnPreDrawListener {

        private final View mView;
        private final Runnable mCallback;
        private int mState;
        private MainFragmentAdapter mainFragmentAdapter;

        final static int STATE_INIT = 0;
        final static int STATE_FIRST_DRAW = 1;
        final static int STATE_SECOND_DRAW = 2;

        ExpandPreLayout(Runnable callback, MainFragmentAdapter adapter, View view) {
            mView = view;
            mCallback = callback;
            mainFragmentAdapter = adapter;
        }

        void execute() {
            mView.getViewTreeObserver().addOnPreDrawListener(this);
            mainFragmentAdapter.setExpand(false);
            // always trigger onPreDraw even adapter setExpand() does nothing.
            mView.invalidate();
            mState = STATE_INIT;
        }

        @Override
        public boolean onPreDraw() {
            if (getView() == null || getContext() == null) {
                mView.getViewTreeObserver().removeOnPreDrawListener(this);
                return true;
            }
            if (mState == STATE_INIT) {
                mainFragmentAdapter.setExpand(true);
                // always trigger onPreDraw even adapter setExpand() does nothing.
                mView.invalidate();
                mState = STATE_FIRST_DRAW;
            } else if (mState == STATE_FIRST_DRAW) {
                mCallback.run();
                mView.getViewTreeObserver().removeOnPreDrawListener(this);
                mState = STATE_SECOND_DRAW;
            }
            return false;
        }
    }
}