/*
 * 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.test.runner.lifecycle;

import android.app.Activity;
import java.util.Collection;

/**
 * Interface for tests to use when they need to query the activity lifecycle state.
 *
 * <p>Activity lifecycle changes occur only on the UI thread - therefore listeners registered with
 * an ActivityLifecycleMonitor should expect to be invoked on the UI thread. The direct query
 * methods can only be called on the UI thread because otherwise they would not be able to return
 * consistent responses.
 *
 * <p>Retrieve instances of the monitor through {@link ActivityLifecycleMonitorRegistry}.
 *
 * <p>Detecting these lifecycle states requires support from Instrumentation, therefore do not
 * expect an instance to be present under any arbitrary instrumentation.
 */
public interface ActivityLifecycleMonitor {

  /**
   * Adds a new callback that will be notified when lifecycle changes occur.
   *
   * <p>Implementors will not hold a strong ref to the callback, the code which registers callbacks
   * is responsible for this. Code which registers callbacks should responsibly remove their
   * callback when it is no longer needed.
   *
   * <p>Callbacks are executed on the main thread of the application, and should take care not to
   * block or otherwise perform expensive operations as it will directly impact the application.
   *
   * @param callback an ActivityLifecycleCallback
   */
  void addLifecycleCallback(ActivityLifecycleCallback callback);

  /** Removes a previously registered lifecycle callback. */
  void removeLifecycleCallback(ActivityLifecycleCallback callback);

  /**
   * Returns the current lifecycle stage of a given activity.
   *
   * <p>This method can only return a consistent and correct answer from the main thread, therefore
   * callers should always invoke it from the main thread and implementors are free to throw an
   * exception if the call is not made on the main thread.
   *
   * <p>Implementors should ensure this method returns a consistent response if called from a
   * lifecycle callback also registered with this monitor (eg: it would be horriblely wrong if a
   * callback sees PAUSED and calls this method with the same activity and gets RESUMED.
   *
   * @param activity an activity in this application.
   * @return the lifecycle stage this activity is in.
   * @throws IllegalArgumentException if activity is unknown to the monitor.
   * @throws NullPointerException if activity is null.
   * @throws IllegalStateException if called off the main thread.
   */
  Stage getLifecycleStageOf(Activity activity);

  /**
   * Returns all activities in a given stage of their lifecycle.
   *
   * <p>This method can only return a consistant and correct answer from the main thread, therefore
   * callers should always invoke it from the main thread and implementors are free to throw an
   * exception if the call is not made on the main thread.
   *
   * <p>Implementors should ensure this method returns a consistant response if called from a
   * lifecycle callback also registered with this monitor (eg: it would be horriblely wrong if a
   * callback sees PAUSED and calls this method with the PAUSED and does not see its activity in the
   * response.
   *
   * <p>Callers should be aware that the monitor implementation may not hold strong references to
   * the Activities in the application. Therefore stages which are considered end stages or eligible
   * for garbage collection on low memory situations may not return an instance of a particular
   * activity if it has been garbage collected.
   *
   * @param stage the stage to query for.
   * @return a snapshot Collection of activities in the given stage. This collection may be empty.
   * @throws IllegalStateException if called from outside the main thread.
   */
  Collection<Activity> getActivitiesInStage(Stage stage);
}