Android中判断app何时启动和关闭的技术研究

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。

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);
            }
        }
    }
}

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();
}

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);
        }
    }
}