Android中判断app何时启动和关闭的技术研究

    Android开发中不可避免的会遇到需要检查app何时进入前台，何时被用户关闭。奇怪的是，要达到这个目的并不容易。检查app第一次启动并不难，但要判断它何时重新打开和关闭就没有那么简单了。

    这篇文章将介绍一种判断app打开，重新打开和关闭的技术。

    判断一个app打开和关闭的关键在于判断它的activities是否正在前台显示。让我们先从简单的例子开始，一个只有一个activity的app，而且不支持水平模式。这样想要判断app是打开还是关闭只需要检查activity的onStart和onStop方法即可：

    上面例子的问题在于当需要支持水平模式时该方法就失效了。当我们旋转设备时activity将会重建，onStart方法将被再次调用，这时将会错误的判断为app第二次被打开。

    为了处理设备旋转的情况，我们需要增加一个校验步骤。当activity退出时启动一个定时器，用于判断短时间内app的这个activity是否又被启动，如果没有，说明用户真的退出了这个app，如果重新启动了这个activity，说明用户还逗留在这个app中。

    这种校验方式也适用于拥有多个activities的app，因为从app的一个activity跳转到另一个activity也可以用这种校验方式来处理。

    使用这个技术我创建了一个管理类，所有的activities在可见和不可见时都会通知这个管理类。这个管理类为每个activity处理上述的校验步骤，从而避免错误的检测。它也提供了发布订阅（观察者）模式，任何对app启动和关闭感兴趣的模块都可以通过它来得到对应的通知。

    这个管理类的使用分为三个步骤：

    2. import android.os.Handler;
    3. import android.os.Looper;
    4. import android.os.Message;
    5. import android.support.annotation.NonNull;
    6. import android.text.format.DateUtils;
    8. import java.lang.ref.Reference;
    9. import java.lang.ref.WeakReference;
    10. import java.util.HashSet;
    11. import java.util.Set;
    13. /**
    14.  * This class is responsible for tracking all currently open activities.
    15.  * By doing so this class can detect when the application is in the foreground
    16.  * and when it is running in the background.
    17.  */
    18. public class AppForegroundStateManager {
    19.     private static final String TAG = AppForegroundStateManager.class.getSimpleName();
    20.     private static final int MESSAGE_NOTIFY_LISTENERS = 1;
    21.     public static final long APP_CLOSED_VALIDATION_TIME_IN_MS = 30 * DateUtils.SECOND_IN_MILLIS; // 30 Seconds
    22.     private Reference<Activity> mForegroundActivity;
    23.     private Set<OnAppForegroundStateChangeListener> mListeners = new HashSet<>();
    24.     private AppForegroundState mAppForegroundState = AppForegroundState.NOT_IN_FOREGROUND;
    25.     private NotifyListenersHandler mHandler;
    27.     // Make this class a thread safe singleton
    28.     private static class SingletonHolder {
    29.         public static final AppForegroundStateManager INSTANCE = new AppForegroundStateManager();
    30.     }
    32.     public static AppForegroundStateManager getInstance() {
    33.         return SingletonHolder.INSTANCE;
    34.     }
    36.     private AppForegroundStateManager() {
    37.         // Create the handler on the main thread
    38.         mHandler = new NotifyListenersHandler(Looper.getMainLooper());
    39.     }
    41.     public enum AppForegroundState {
    42.         IN_FOREGROUND,
    43.         NOT_IN_FOREGROUND
    44.     }
    46.     public interface OnAppForegroundStateChangeListener {
    47.         /** Called when the foreground state of the app changes */
    48.         public void onAppForegroundStateChange(AppForegroundState newState);
    49.     }
    51.     /** An activity should call this when it becomes visible */
    52.     public void onActivityVisible(Activity activity) {
    53.         if (mForegroundActivity != null) mForegroundActivity.clear();
    54.         mForegroundActivity = new WeakReference<>(activity);
    55.         determineAppForegroundState();
    56.     }
    58.     /** An activity should call this when it is no longer visible */
    59.     public void onActivityNotVisible(Activity activity) {
    60.          * The foreground activity may have been replaced with a new foreground activity in our app.
    61.          * So only clear the foregroundActivity if the new activity matches the foreground activity.
    62.          */
    63.         if (mForegroundActivity != null) {
    64.             Activity ref = mForegroundActivity.get();
    66.             if (activity == ref) {
    68.                 mForegroundActivity.clear();
    69.                 mForegroundActivity = null;
    70.             }
    71.         }
    73.         determineAppForegroundState();
    74.     }
    76.     /** Use to determine if this app is in the foreground */
    77.     public Boolean isAppInForeground() {
    78.         return mAppForegroundState == AppForegroundState.IN_FOREGROUND;
    79.     }
    81.     /**
    82.      * Call to determine the current state, update the tracking global, and notify subscribers if the state has changed.
    83.      */
    84.     private void determineAppForegroundState() {
    85.         /* Get the current state */
    86.         AppForegroundState oldState = mAppForegroundState;
    88.         /* Determine what the new state should be */
    89.         final boolean isInForeground = mForegroundActivity != null && mForegroundActivity.get() != null;
    90.         mAppForegroundState = isInForeground ? AppForegroundState.IN_FOREGROUND : AppForegroundState.NOT_IN_FOREGROUND;
    92.         /* If the new state is different then the old state the notify subscribers of the state change */
    93.         if (mAppForegroundState != oldState) {
    94.             validateThenNotifyListeners();
    95.         }
    96.     }
    98.     /**
    99.      * Add a listener to be notified of app foreground state change events.
    100.      *
    101.      * @param listener
    102.      */
    103.     public void addListener(@NonNull OnAppForegroundStateChangeListener listener) {
    104.         mListeners.add(listener);
    105.     }
    107.     /**
    108.      * Remove a listener from being notified of app foreground state change events.
    109.      *
    110.      * @param listener
    111.      */
    112.     public void removeListener(OnAppForegroundStateChangeListener listener) {
    113.         mListeners.remove(listener);
    114.     }
    116.     /** Notify all listeners the app foreground state has changed */
    117.     private void notifyListeners(AppForegroundState newState) {
    118.         android.util.Log.i(TAG, "Notifying subscribers that app just entered state: " + newState);
    120.         for (OnAppForegroundStateChangeListener listener : mListeners) {
    121.             listener.onAppForegroundStateChange(newState);
    122.         }
    123.     }
    125.     /**
    126.      * This method will notify subscribes that the foreground state has changed when and if appropriate.
    127.      * <br><br>
    128.      * We do not want to just notify listeners right away when the app enters of leaves the foreground. When changing orientations or opening and
    129.      * Sent when we detect a change. We will not notify that a foreground change happened until the delay time has been reached. If a second
    130.      * foreground change is detected during the delay period then the notification will be canceled.
    131.      */
    132.     private void validateThenNotifyListeners() {
    134.         if (mHandler.hasMessages(MESSAGE_NOTIFY_LISTENERS)) {
    135.             android.util.Log.v(TAG, "Validation Failed: Throwing out app foreground state change notification");
    136.             mHandler.removeMessages(MESSAGE_NOTIFY_LISTENERS);
    137.         } else {
    138.             if (mAppForegroundState == AppForegroundState.IN_FOREGROUND) {
    139.                 // If the app entered the foreground then notify listeners right away; there is no validation time for this
    140.                 mHandler.sendEmptyMessage(MESSAGE_NOTIFY_LISTENERS);
    141.             } else {
    142.                 // We need to validate that the app entered the background. A delay is used to allow for time when the application went into the
    143.                 // background but we do not want to consider the app being backgrounded such as for in app purchasing flow and full screen ads.
    144.                 mHandler.sendEmptyMessageDelayed(MESSAGE_NOTIFY_LISTENERS, APP_CLOSED_VALIDATION_TIME_IN_MS);
    145.             }
    146.         }
    147.     }
    149.     private class NotifyListenersHandler extends Handler {
    150.         private NotifyListenersHandler(Looper looper) {
    151.             super(looper);
    152.         }
    154.         @Override
    155.         public void handleMessage(Message inputMessage) {
    156.             switch (inputMessage.what) {
    157.                 // The decoding is done
    158.                 case MESSAGE_NOTIFY_LISTENERS:
    159.                     /* Notify subscribers of the state change */
    160.                     android.util.Log.v(TAG, "App just changed foreground state to: " + mAppForegroundState);
    161.                     notifyListeners(mAppForegroundState);
    162.                     break;
    163.                 default:
    164.                     super.handleMessage(inputMessage);
    165.             }
    166.         }
    167.     }
    168. }

    2）Activities在可见性改变的需要发送通知

    app中所有activities都要增加下面的代码，用于可见性改变时通知管理类。最好的实现方式是把这段代码加到工程的BaseActivity中。

    3）订阅app的前台可见性改变事件

    在感兴趣的模块中订阅app前台可见性改变事件，application类的onCreate函数是一个不错的地方，它可以保证每次app启动和关闭，你都能得到通知。

    1. public class MyApplication extends Application {
    2.     @Override
    3.     public void onCreate() {
    4.         super.onCreate();
    5.         AppForegroundStateManager.getInstance().addListener(this);
    6.     }
    8.     @Override
    9.     public void onAppForegroundStateChange(AppForegroundStateManager.AppForegroundState newState) {
    10.         if (AppForegroundStateManager.AppForegroundState.IN_FOREGROUND == newState) {
    11.             // App just entered the foreground. Do something here!
    12.         } else {
    13.             // App just entered the background. Do something here!
    14.         }
    15.     }
    17. }

    有一些细节需要进一步讨论，下面讨论的几点针对具体的应用可以做微调。

    校验定时器检查app是否真的进入后台的时间间隔是多少合适呢？上面的代码设置为30秒，原因如下。

    当你的app在运行时，可能存在第三方的activities会覆盖全屏幕，一些常见的例子是Google应用内购买和Facebook登录注册页面。这些情况下你的app都会被迫进入后台，前台用于显示这些第三方页面。如果把这种情况当做用户离开了你的app，显然是不对的。30秒超时设置就是用来避免这种情况的。例如当用户在30秒内完成应用内购买，大部分用户都可以做得到，那么就不会当做用户突然离开app了。

    如果你的app不存在上述这种情况，我建议可以把你的校验时间设置为4秒，这样对于低配设备当屏幕旋转重新创建activity的时间间隔是合适的。

    CPU休眠

    现在我们已经知道如何检测app何时启动和关闭，但我们不知道app是如何启动的。是用户点击通知栏消息？还是点击一个链接？亦或是他们直接通过桌面图标或最近使用启动？

    首先我们需要知道在哪里检测app是如何启动的。基于前面一个例子我们可以打印出app何时启动，以及如何启动。

    设置启动机制

    现在我们可以打印app何时启动的机制，但我们没有设置它。因此下一步就是在用户通过链接或者通知启动app时我们记下它。如果没有通过这两种方式设置过，说明用户是通过点击app图标启动的。

    为了跟踪用户点击链接打开app，你需要找到代码中处理链接的地方，并加入下面的代码来跟踪启动机制。要确保这些代码在activity的onStart()函数之前调用。在哪些地方加入下面的代码取决于你的app架构了。

    1. getApplication().setLaunchMechanism(LaunchMechanism.URL);

    跟踪通知事件

    不幸的是跟踪通知点击需要更多技巧，通知显示后，点击它将会打开之前绑定好的一个PendingIntent，这里的技巧是为通知的所有PendingIntents添加一个标识表明是由通知发出的。

    例如当为通知创建PendingIntent时为每个intent添加如下代码：

    到这一步我们需要做的就是在每个activity(统一在BaseActivity中添加)中检查这个标识。当识别到这个标识时说明是从通知启动的，这时可以把启动机制设置为通过通知。这一步应该在onCreate中处理，这样在app启动到前台之前就设置好了(会触发启动机制的打印)。

    1. @Override
    2. public void onCreate(Bundle savedInstanceState) {
    3.     super.onCreate(savedInstanceState);
    5.     Intent intent = getIntent();
    6.     if (intent != null && intent.getExtras() != null) {
    7.         // Detect if the activity was launched by the user clicking on a notification
    8.         if (intent.getExtras().getBoolean(EXTRA_HANDLING_NOTIFICATION, false)) {
    9.             // Notify that the activity was opened by the user clicking on a notification.
    10.             getApplication().setLaunchMechanism(LaunchMechanism.NOTIFICATION);
    11.         }
    12. }

    本文接近尾声了，到这里你应该已经掌握了如何检测app何时启动和关闭，以及它是如何启动的。