Android 判断app何时是打开或者关闭的技术研究
jopen
10年前
原文链接：http://engineering.meetme.com/2015/04/android-determine-when-app-is-opened-or-closed/
存在的问题
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。
让我们开始吧
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：
@Override protected void onStart() { super.onStart(); // The Application has been opened! } @Override protected void onStop() { super.onStop(); // The Application has been closed! }
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：
1）把它添加到你的工程中
import android.app.Activity; import android.os.Handler; import android.os.Looper; import android.os.Message; import android.support.annotation.NonNull; import android.text.format.DateUtils; import java.lang.ref.Reference; import java.lang.ref.WeakReference; import java.util.HashSet; import java.util.Set; /** * This class is responsible for tracking all currently open activities. * By doing so this class can detect when the application is in the foreground * and when it is running in the background. */ public class AppForegroundStateManager { private static final String TAG = AppForegroundStateManager.class.getSimpleName(); private static final int MESSAGE_NOTIFY_LISTENERS = 1; public static final long APP_CLOSED_VALIDATION_TIME_IN_MS = 30 * DateUtils.SECOND_IN_MILLIS; // 30 Seconds private Reference<Activity> mForegroundActivity; private Set<OnAppForegroundStateChangeListener> mListeners = new HashSet<>(); private AppForegroundState mAppForegroundState = AppForegroundState.NOT_IN_FOREGROUND; private NotifyListenersHandler mHandler; // Make this class a thread safe singleton private static class SingletonHolder { public static final AppForegroundStateManager INSTANCE = new AppForegroundStateManager(); } public static AppForegroundStateManager getInstance() { return SingletonHolder.INSTANCE; } private AppForegroundStateManager() { // Create the handler on the main thread mHandler = new NotifyListenersHandler(Looper.getMainLooper()); } public enum AppForegroundState { IN_FOREGROUND, NOT_IN_FOREGROUND } public interface OnAppForegroundStateChangeListener { /** Called when the foreground state of the app changes */ public void onAppForegroundStateChange(AppForegroundState newState); } /** An activity should call this when it becomes visible */ public void onActivityVisible(Activity activity) { if (mForegroundActivity != null) mForegroundActivity.clear(); mForegroundActivity = new WeakReference<>(activity); determineAppForegroundState(); } /** An activity should call this when it is no longer visible */ public void onActivityNotVisible(Activity activity) { /* * The foreground activity may have been replaced with a new foreground activity in our app. * So only clear the foregroundActivity if the new activity matches the foreground activity. */ if (mForegroundActivity != null) { Activity ref = mForegroundActivity.get(); if (activity == ref) { // This is the activity that is going away, clear the reference mForegroundActivity.clear(); mForegroundActivity = null; } } determineAppForegroundState(); } /** Use to determine if this app is in the foreground */ public Boolean isAppInForeground() { return mAppForegroundState == AppForegroundState.IN_FOREGROUND; } /** * Call to determine the current state, update the tracking global, and notify subscribers if the state has changed. */ private void determineAppForegroundState() { /* Get the current state */ AppForegroundState oldState = mAppForegroundState; /* Determine what the new state should be */ final boolean isInForeground = mForegroundActivity != null && mForegroundActivity.get() != null; mAppForegroundState = isInForeground ? AppForegroundState.IN_FOREGROUND : AppForegroundState.NOT_IN_FOREGROUND; /* If the new state is different then the old state the notify subscribers of the state change */ if (mAppForegroundState != oldState) { validateThenNotifyListeners(); } } /** * Add a listener to be notified of app foreground state change events. * * @param listener */ public void addListener(@NonNull OnAppForegroundStateChangeListener listener) { mListeners.add(listener); } /** * Remove a listener from being notified of app foreground state change events. * * @param listener */ public void removeListener(OnAppForegroundStateChangeListener listener) { mListeners.remove(listener); } /** Notify all listeners the app foreground state has changed */ private void notifyListeners(AppForegroundState newState) { android.util.Log.i(TAG, "Notifying subscribers that app just entered state: " + newState); for (OnAppForegroundStateChangeListener listener : mListeners) { listener.onAppForegroundStateChange(newState); } } /** * This method will notify subscribes that the foreground state has changed when and if appropriate. * <br><br> * We do not want to just notify listeners right away when the app enters of leaves the foreground. When changing orientations or opening and * closing the app quickly we briefly pass through a NOT_IN_FOREGROUND state that must be ignored. To accomplish this a delayed message will be * 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 * foreground change is detected during the delay period then the notification will be canceled. */ private void validateThenNotifyListeners() { // If the app has any pending notifications then throw out the event as the state change has failed validation if (mHandler.hasMessages(MESSAGE_NOTIFY_LISTENERS)) { android.util.Log.v(TAG, "Validation Failed: Throwing out app foreground state change notification"); mHandler.removeMessages(MESSAGE_NOTIFY_LISTENERS); } else { if (mAppForegroundState == AppForegroundState.IN_FOREGROUND) { // If the app entered the foreground then notify listeners right away; there is no validation time for this mHandler.sendEmptyMessage(MESSAGE_NOTIFY_LISTENERS); } else { // We need to validate that the app entered the background. A delay is used to allow for time when the application went into the // background but we do not want to consider the app being backgrounded such as for in app purchasing flow and full screen ads. mHandler.sendEmptyMessageDelayed(MESSAGE_NOTIFY_LISTENERS, APP_CLOSED_VALIDATION_TIME_IN_MS); } } } private class NotifyListenersHandler extends Handler { private NotifyListenersHandler(Looper looper) { super(looper); } @Override public void handleMessage(Message inputMessage) { switch (inputMessage.what) { // The decoding is done case MESSAGE_NOTIFY_LISTENERS: /* Notify subscribers of the state change */ android.util.Log.v(TAG, "App just changed foreground state to: " + mAppForegroundState); notifyListeners(mAppForegroundState); break; default: super.handleMessage(inputMessage); } } } }
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。
@Override protected void onStart() { super.onStart(); AppForegroundStateManager.getInstance().onActivityVisible(this); } @Override protected void onStop() { AppForegroundStateManager.getInstance().onActivityNotVisible(this); super.onStop(); }
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。
public class MyApplication extends Application { @Override public void onCreate() { super.onCreate(); AppForegroundStateManager.getInstance().addListener(this); } @Override public void onAppForegroundStateChange(AppForegroundStateManager.AppForegroundState newState) { if (AppForegroundStateManager.AppForegroundState.IN_FOREGROUND == newState) { // App just entered the foreground. Do something here! } else { // App just entered the background. Do something here! } } }
进一步的思考
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。
校验时间
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应用内购买和非死book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
可能存在的问题是当用户关闭app或者app仍处于前台时用户锁屏了，这时CPU可能不会等到定时器检测就休眠了。为了保证这种情况下定时器能够正常检测用户退出app，我们需要持有wakelock防止CPU休眠直到app关闭事件被确认。实践中相比使用wakelock，这种情况并不算问题。
判断app是如何启动的
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？
跟踪启动机制
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。
public class MyApplication extends Application { public final String TAG = MyApplication.class.getSimpleName(); public enum LaunchMechanism { DIRECT, NOTIFICATION, URL; } private LaunchMechanism mLaunchMechanism = LaunchMechanism.DIRECT; public void setLaunchMechanism(LaunchMechanism launchMechanism) { mLaunchMechanism = launchMechanism; } @Override public void onCreate() { super.onCreate(); AppForegroundStateManager.getInstance().addListener(this); } @Override public void onAppForegroundStateChange(AppForegroundStateManager.AppForegroundState newState) { if (AppForegroundStateManager.AppForegroundState.IN_FOREGROUND.equals(newState)) { // App just entered the foreground. Log.i(TAG, "App Just Entered the Foreground with launch mechanism of: " + mLaunchMechanism); } else { // App just entered the background. Set our launch mode back to the default of direct. mLaunchMechanism = LaunchMechanism.DIRECT; } } }
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。
跟踪链接点击事件
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。
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：
public static final String EXTRA_HANDLING_NOTIFICATION = "Notification.EXTRA_HANDLING_NOTIFICATION"; // Put an extra so we know when an activity launches if it is a from a notification intent.putExtra(EXTRA_HANDLING_NOTIFICATION, true);
@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Intent intent = getIntent(); if (intent != null && intent.getExtras() != null) { // Detect if the activity was launched by the user clicking on a notification if (intent.getExtras().getBoolean(EXTRA_HANDLING_NOTIFICATION, false)) { // Notify that the activity was opened by the user clicking on a notification. getApplication().setLaunchMechanism(LaunchMechanism.NOTIFICATION); } } }