Android中判斷app何時啟動和關閉的技術研究

存在的問題

Android開發中不可避免的會遇到需要檢查app何時進入前台，何時被用戶關閉。奇怪的是，要達到這個目的並不容易。檢查app第一次啟動並不難，但要判斷它何時重新打開和關閉就沒有那麼簡單了。

這篇文章將介紹一種判斷app打開，重新打開和關閉的技術。

讓我們開始吧

判斷一個app打開和關閉的關鍵在於判斷它的activities是否正在前台顯示。讓我們先從簡單的例子開始，一個只有一個activity的app，而且不支持水平模式。這樣想要判斷app是打開還是關閉只需要檢查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!
}

上面例子的問題在於當需要支持水平模式時該方法就失效了。當我們旋轉設備時activity將會重建，onStart方法將被再次調用，這時將會錯誤的判斷為app第二次被打開。

為了處理設備旋轉的情況，我們需要增加一個校驗步驟。當activity退出時啟動一個定時器，用於判斷短時間內app的這個activity是否又被啟動，如果沒有，說明用戶真的退出了這個app，如果重新啟動了這個activity，說明用戶還逗留在這個app中。

這種校驗方式也適用於擁有多個activities的app，因為從app的一個activity跳轉到另一個activity也可以用這種校驗方式來處理。

使用這個技術我創建了一個管理類，所有的activities在可見和不可見時都會通知這個管理類。這個管理類為每個activity處理上述的校驗步驟，從而避免錯誤的檢測。它也提供了發布訂閱（觀察者）模式，任何對app啟動和關閉感興趣的模塊都可以通過它來得到對應的通知。

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 mForegroundActivity;
    private Set 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.
     * 


     * 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在可見性改變的需要發送通知

app中所有activities都要增加下面的代碼，用於可見性改變時通知管理類。最好的實現方式是把這段代碼加到工程的BaseActivity中。

@Override
protected void onStart() {
    super.onStart();
    AppForegroundStateManager.getInstance().onActivityVisible(this);
}

@Override
protected void onStop() {
    AppForegroundStateManager.getInstance().onActivityNotVisible(this);
    super.onStop();
}

訂閱app的前台可見性改變事件

在感興趣的模塊中訂閱app前台可見性改變事件，application類的onCreate函數是一個不錯的地方，它可以保證每次app啟動和關閉，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!
        }
    }

}

進一步的思考

有一些細節需要進一步討論，下面討論的幾點針對具體的應用可以做微調。

校驗時間

校驗定時器檢查app是否真的進入後台的時間間隔是多少合適呢？上面的代碼設置為30秒，原因如下。

當你的app在運行時，可能存在第三方的activities會覆蓋全屏幕，一些常見的例子是Google應用內購買和Facebook登錄注冊頁面。這些情況下你的app都會被迫進入後台，前台用於顯示這些第三方頁面。如果把這種情況當做用戶離開了你的app，顯然是不對的。30秒超時設置就是用來避免這種情況的。例如當用戶在30秒內完成應用內購買，大部分用戶都可以做得到，那麼就不會當做用戶突然離開app了。

如果你的app不存在上述這種情況，我建議可以把你的校驗時間設置為4秒，這樣對於低配設備當屏幕旋轉重新創建activity的時間間隔是合適的。

CPU休眠

可能存在的問題是當用戶關閉app或者app仍處於前台時用戶鎖屏了，這時CPU可能不會等到定時器檢測就休眠了。為了保證這種情況下定時器能夠正常檢測用戶退出app，我們需要持有wakelock防止CPU休眠直到app關閉事件被確認。實踐中相比使用wakelock，這種情況並不算問題。

判斷app是如何啟動的

現在我們已經知道如何檢測app何時啟動和關閉，但我們不知道app是如何啟動的。是用戶點擊通知欄消息？還是點擊一個鏈接？亦或是他們直接通過桌面圖標或最近使用啟動？

跟蹤啟動機制

首先我們需要知道在哪裡檢測app是如何啟動的。基於前面一個例子我們可以打印出app何時啟動，以及如何啟動。

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;
        }
    }
}

設置啟動機制

現在我們可以打印app何時啟動的機制，但我們沒有設置它。因此下一步就是在用戶通過鏈接或者通知啟動app時我們記下它。如果沒有通過這兩種方式設置過，說明用戶是通過點擊app圖標啟動的。

跟蹤鏈接點擊事件

為了跟蹤用戶點擊鏈接打開app，你需要找到代碼中處理鏈接的地方，並加入下面的代碼來跟蹤啟動機制。要確保這些代碼在activity的onStart()函數之前調用。在哪些地方加入下面的代碼取決於你的app架構了。

getApplication().setLaunchMechanism(LaunchMechanism.URL); 跟蹤通知事件

不幸的是跟蹤通知點擊需要更多技巧，通知顯示後，點擊它將會打開之前綁定好的一個PendingIntent，這裡的技巧是為通知的所有PendingIntents添加一個標識表明是由通知發出的。

例如當為通知創建PendingIntent時為每個intent添加如下代碼：

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);

到這一步我們需要做的就是在每個activity(統一在BaseActivity中添加)中檢查這個標識。當識別到這個標識時說明是從通知啟動的，這時可以把啟動機制設置為通過通知。這一步應該在onCreate中處理，這樣在app啟動到前台之前就設置好了(會觸發啟動機制的打印)。

@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);
        }
    }
}