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：
@Overrideprotected void onStart() { super.onStart(); // The Application has been opened!}@Overrideprotected 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
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);
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)。
@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);
}
}
}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。
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