Android 深刻理解 Notification 機制

時間 2019-11-17

標籤 android 深刻理解 notification 機制欄目 Android 简体版

原文原文鏈接

本文預計閱讀時間爲20分鐘

本文須要解決的問題

筆者最近正在作一個項目，裏面須要用到 Android Notification 機制來實現某些特定需求。我正好經過這個機會研究一下 Android Notification 相關的發送邏輯和接收邏輯，以及整理相關的筆記。我研究 Notification 機制的目的是解決如下我在使用過程當中所思考的問題：java

咱們建立的 Notification 實例最終以什麼樣的方式發送給系統？
系統是如何接收到 Notification 實例並顯示的？
咱們是否能攔截其餘 app 的 Notification 並獲取其中的信息？

什麼是 Android Notification 機制？

Notification，中文名翻譯爲通知，每一個 app 能夠自定義通知的樣式和內容等，它會顯示在系統的通知欄等區域。用戶能夠打開抽屜式通知欄查看通知的詳細信息。在實際生活中，Android Notification 機制有很普遍的應用，例如 IM app 的新消息通知，資訊 app 的新聞推送等等。android

源碼分析

本文的源碼基於 Android 7.0。windows

Notification 的發送邏輯

通常來講，若是咱們本身的 app 想發送一條新的 Notification，能夠參照如下代碼：設計模式

NotificationCompat.Builder mBuilder =
        new NotificationCompat.Builder(this)
        .setSmallIcon(R.drawable.notification_icon)
        .setWhen(System.currentTimeMillis())
        .setContentTitle("Test Notification Title")
        .setContentText("Test Notification Content!");
Intent resultIntent = new Intent(this, ResultActivity.class);

PendingIntent contentIntent =
        PendingIntent.getActivity(
            this, 
            0, 
            resultIntent, 
            PendingIntent.FLAG_UPDATE_CURRENT
        );
mBuilder.setContentIntent(resultPendingIntent);
NotificationManager mNotificationManager =
    (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
// mId allows you to update the notification later on.
mNotificationManager.notify(mId, mBuilder.build());
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能夠看到，咱們經過 NotificationCompat.Builder 新建了一個 Notification 對象，最後經過 NotificationManager#notify() 方法將 Notification 發送出去。app

NotificationManager#notify()

public void notify(int id, Notification notification) {
    notify(null, id, notification);
}

// 省略部分註釋
public void notify(String tag, int id, Notification notification) {
    notifyAsUser(tag, id, notification, new UserHandle(UserHandle.myUserId()));
}

/** * @hide */
public void notifyAsUser(String tag, int id, Notification notification, UserHandle user) {
    int[] idOut = new int[1];
    INotificationManager service = getService();
    String pkg = mContext.getPackageName();
    // Fix the notification as best we can.
    Notification.addFieldsFromContext(mContext, notification);
    if (notification.sound != null) {
        notification.sound = notification.sound.getCanonicalUri();
        if (StrictMode.vmFileUriExposureEnabled()) {
            notification.sound.checkFileUriExposed("Notification.sound");
        }
    }
    fixLegacySmallIcon(notification, pkg);
    if (mContext.getApplicationInfo().targetSdkVersion > Build.VERSION_CODES.LOLLIPOP_MR1) {
        if (notification.getSmallIcon() == null) {
            throw new IllegalArgumentException("Invalid notification (no valid small icon): "
                    + notification);
        }
    }
    if (localLOGV) Log.v(TAG, pkg + ": notify(" + id + ", " + notification + ")");
    final Notification copy = Builder.maybeCloneStrippedForDelivery(notification);
    try {
        // !!!
        service.enqueueNotificationWithTag(pkg, mContext.getOpPackageName(), tag, id,
                copy, idOut, user.getIdentifier());
        if (localLOGV && id != idOut[0]) {
            Log.v(TAG, "notify: id corrupted: sent " + id + ", got back " + idOut[0]);
        }
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}
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咱們能夠看到，到最後會調用 service.enqueueNotificationWithTag() 方法，這裏的是 service 是 INotificationManager 接口。若是熟悉 AIDL 等系統相關運行機制的話，就能夠看出這裏是代理類調用了代理接口的方法，實際方法實現是在 NotificationManagerService 當中。框架

NotificationManagerService#enqueueNotificationWithTag()

@Override
public void enqueueNotificationWithTag(String pkg, String opPkg, String tag, int id, Notification notification, int[] idOut, int userId) throws RemoteException {
    enqueueNotificationInternal(pkg, opPkg, Binder.getCallingUid(),
            Binder.getCallingPid(), tag, id, notification, idOut, userId);
}

void enqueueNotificationInternal(final String pkg, final String opPkg, final int callingUid, final int callingPid, final String tag, final int id, final Notification notification, int[] idOut, int incomingUserId) {
    if (DBG) {
        Slog.v(TAG, "enqueueNotificationInternal: pkg=" + pkg + " id=" + id
             + " notification=" + notification);
    }
    checkCallerIsSystemOrSameApp(pkg);
    final boolean isSystemNotification = isUidSystem(callingUid) || ("android".equals(pkg));
    final boolean isNotificationFromListener = mListeners.isListenerPackage(pkg);

    final int userId = ActivityManager.handleIncomingUser(callingPid,
            callingUid, incomingUserId, true, false, "enqueueNotification", pkg);
    final UserHandle user = new UserHandle(userId);

    // Fix the notification as best we can.
    try {
        final ApplicationInfo ai = getContext().getPackageManager().getApplicationInfoAsUser(
                pkg, PackageManager.MATCH_DEBUG_TRIAGED_MISSING,
                (userId == UserHandle.USER_ALL) ? UserHandle.USER_SYSTEM : userId);
        Notification.addFieldsFromContext(ai, userId, notification);
    } catch (NameNotFoundException e) {
        Slog.e(TAG, "Cannot create a context for sending app", e);
        return;
    }

    mUsageStats.registerEnqueuedByApp(pkg);

    if (pkg == null || notification == null) {
        throw new IllegalArgumentException("null not allowed: pkg=" + pkg
             + " id=" + id + " notification=" + notification);
    }
    final StatusBarNotification n = new StatusBarNotification(
            pkg, opPkg, id, tag, callingUid, callingPid, 0, notification,
            user);

    // Limit the number of notifications that any given package except the android
    // package or a registered listener can enqueue. Prevents DOS attacks and deals with leaks.
    if (!isSystemNotification && !isNotificationFromListener) {
        synchronized (mNotificationList) {
            if(mNotificationsByKey.get(n.getKey()) != null) {
                // this is an update, rate limit updates only
                final float appEnqueueRate = mUsageStats.getAppEnqueueRate(pkg);
                if (appEnqueueRate > mMaxPackageEnqueueRate) {
                    mUsageStats.registerOverRateQuota(pkg);
                    final long now = SystemClock.elapsedRealtime();
                    if ((now - mLastOverRateLogTime) > MIN_PACKAGE_OVERRATE_LOG_INTERVAL) {
                        Slog.e(TAG, "Package enqueue rate is " + appEnqueueRate
                                + ". Shedding events. package=" + pkg);
                            mLastOverRateLogTime = now;
                    }
                    return;
                }
            }

            int count = 0;
            final int N = mNotificationList.size();
            for (int i=0; i<N; i++) {
                final NotificationRecord r = mNotificationList.get(i);
                if (r.sbn.getPackageName().equals(pkg) && r.sbn.getUserId() == userId) {
                    if (r.sbn.getId() == id && TextUtils.equals(r.sbn.getTag(), tag)) {
                        break;  // Allow updating existing notification
                    }
                    count++;
                    if (count >= MAX_PACKAGE_NOTIFICATIONS) {
                        mUsageStats.registerOverCountQuota(pkg);
                        Slog.e(TAG, "Package has already posted " + count
                                + " notifications. Not showing more. package=" + pkg);
                        return;
                    }
                }
            }
        }
    }

    // Whitelist pending intents.
    if (notification.allPendingIntents != null) {
        final int intentCount = notification.allPendingIntents.size();
        if (intentCount > 0) {
            final ActivityManagerInternal am = LocalServices
                    .getService(ActivityManagerInternal.class);
            final long duration = LocalServices.getService(
                    DeviceIdleController.LocalService.class).getNotificationWhitelistDuration();
            for (int i = 0; i < intentCount; i++) {
                PendingIntent pendingIntent = notification.allPendingIntents.valueAt(i);
                if (pendingIntent != null) {
                    am.setPendingIntentWhitelistDuration(pendingIntent.getTarget(), duration);
                }
            }
        }
    }

    // Sanitize inputs
    notification.priority = clamp(notification.priority, Notification.PRIORITY_MIN,
            Notification.PRIORITY_MAX);

    // setup local book-keeping
    final NotificationRecord r = new NotificationRecord(getContext(), n);
    mHandler.post(new EnqueueNotificationRunnable(userId, r));

    idOut[0] = id;
}
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這裏代碼比較多，但經過註釋能夠清晰地理清整個邏輯：異步

首先檢查通知發起者是系統進程或者是查看發起者發送的是不是同個 app 的通知信息，不然拋出異常；
除了系統的通知和已註冊的監聽器容許入隊列外，其餘 app 的通知都會限制通知數上限和通知頻率上限；
將 notification 的 PendingIntent 加入到白名單；
將以前的 notification 進一步封裝爲 StatusBarNotification 和 NotificationRecord，最後封裝到一個異步線程 EnqueueNotificationRunnable 中

這裏有一個點，就是 mHandler，涉及到切換線程，咱們先跟蹤一下 mHandler 是在哪一個線程被建立。ide

mHandler 是 WorkerHandler 類的一個實例，在 NotificationManagerService#onStart() 方法中被建立，而 NotificationManagerService 是系統 Service，因此 EnqueueNotificationRunnable 的 run 方法會運行在 system_server 的主線程。oop

NotificationManagerService.EnqueueNotificationRunnable#run()

@Override
public void run() {
    synchronized(mNotificationList) {
        // 省略代碼
        if (notification.getSmallIcon() != null) {
            StatusBarNotification oldSbn = (old != null) ? old.sbn : null;
            mListeners.notifyPostedLocked(n, oldSbn);
        } else {
            Slog.e(TAG, "Not posting notification without small icon: " + notification);
            if (old != null && !old.isCanceled) {
                mListeners.notifyRemovedLocked(n);
            }
            // ATTENTION: in a future release we will bail out here
            // so that we do not play sounds, show lights, etc. for invalid
            // notifications
            Slog.e(TAG, "WARNING: In a future release this will crash the app: " + n.getPackageName());
        }
        buzzBeepBlinkLocked(r);
    }
}
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省略的代碼主要的工做是提取 notification 相關的屬性，同時通知 notification ranking service，有新的 notification 進來，而後對全部 notification 進行從新排序；
而後到最後會調用 mListeners.notifyPostedLocked() 方法。這裏 mListeners 是 NotificationListeners 類的一個實例。

NotificationManagerService.NotificationListeners#notifyPostedLocked()
-> NotificationManagerService.NotificationListeners#notifyPosted()

public void notifyPostedLocked(StatusBarNotification sbn, StatusBarNotification oldSbn) {
    // Lazily initialized snapshots of the notification.
    TrimCache trimCache = new TrimCache(sbn);
    for (final ManagedServiceInfo info: mServices) {
        boolean sbnVisible = isVisibleToListener(sbn, info);
        boolean oldSbnVisible = oldSbn != null ? isVisibleToListener(oldSbn, info) : false;
        // This notification hasn't been and still isn't visible -> ignore.
        if (!oldSbnVisible && !sbnVisible) {
            continue;
        }
        final NotificationRankingUpdate update = makeRankingUpdateLocked(info);
        // This notification became invisible -> remove the old one.
        if (oldSbnVisible && !sbnVisible) {
            final StatusBarNotification oldSbnLightClone = oldSbn.cloneLight();
            mHandler.post(new Runnable() {
                @Override
                public void run() {
                    notifyRemoved(info, oldSbnLightClone, update);
                }
            });
            continue;
        }
        final StatusBarNotification sbnToPost = trimCache.ForListener(info);
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                notifyPosted(info, sbnToPost, update);
            }
        });
    }
}

private void notifyPosted(final ManagedServiceInfo info, final StatusBarNotification sbn, NotificationRankingUpdate rankingUpdate) {
    final INotificationListener listener = (INotificationListener) info.service;
    StatusBarNotificationHolder sbnHolder = new StatusBarNotificationHolder(sbn);
    try {
        listener.onNotificationPosted(sbnHolder, rankingUpdate);
    } catch (RemoteException ex) {
        Log.e(TAG, "unable to notify listener (posted): " + listener, ex);
    }
}
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調用到最後會執行 listener.onNotificationPosted() 方法。經過全局搜索得知，listener 類型是 NotificationListenerService.NotificationListenerWrapper 的代理對象。源碼分析

NotificationListenerService.NotificationListenerWrapper#onNotificationPosted()

public void onNotificationPosted(IStatusBarNotificationHolder sbnHolder, NotificationRankingUpdate update) {
    StatusBarNotification sbn;
    try {
        sbn = sbnHolder.get();
    } catch (RemoteException e) {
        Log.w(TAG, "onNotificationPosted: Error receiving StatusBarNotification", e);
        return;
    }
    try {
        // convert icon metadata to legacy format for older clients
        createLegacyIconExtras(sbn.getNotification());
        maybePopulateRemoteViews(sbn.getNotification());
    } catch (IllegalArgumentException e) {
        // warn and drop corrupt notification
        Log.w(TAG, "onNotificationPosted: can't rebuild notification from " + sbn.getPackageName());
        sbn = null;
    }
    // protect subclass from concurrent modifications of (@link mNotificationKeys}.
    synchronized(mLock) {
        applyUpdateLocked(update);
        if (sbn != null) {
            SomeArgs args = SomeArgs.obtain();
            args.arg1 = sbn;
            args.arg2 = mRankingMap;
            mHandler.obtainMessage(MyHandler.MSG_ON_NOTIFICATION_POSTED, args).sendToTarget();
        } else {
            // still pass along the ranking map, it may contain other information
            mHandler.obtainMessage(MyHandler.MSG_ON_NOTIFICATION_RANKING_UPDATE, mRankingMap).sendToTarget();
        }
    }
}
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這裏在一開始會從 sbnHolder 中獲取到 sbn 對象，sbn 隸屬於 StatusBarNotificationHolder 類，繼承於 IStatusBarNotificationHolder.Stub 對象。注意到這裏捕獲了一個 RemoteException，猜想涉及到跨進程調用，但咱們不知道這段代碼是在哪一個進程中執行的，因此在這裏暫停跟蹤代碼。

筆者以前是經過向系統發送通知的方式跟蹤源碼，發現走不通。故我的嘗試從另外一個角度入手，即系統接收咱們發過來的通知並顯示到通知欄這個方式入手跟蹤代碼。

系統如何顯示 Notification，即對於系統端來講，Notification 的接收邏輯

系統顯示 Notification 的過程，猜想是在 PhoneStatusBar.java 中，由於系統啓動的過程當中，會啓動 SystemUI 進程，初始化整個 Android 顯示的界面，包括系統通知欄。

PhoneStatusBar#start()
-> BaseStatusBar#start()

public void start() {
    // 省略代碼
    // Set up the initial notification state.
    try {
        mNotificationListener.registerAsSystemService(mContext,
                new ComponentName(mContext.getPackageName(), getClass().getCanonicalName()),
                UserHandle.USER_ALL);
    } catch (RemoteException e) {
        Log.e(TAG, "Unable to register notification listener", e);
    }
    // 省略代碼
}
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這段代碼中，會調用 NotificationListenerService#registerAsSystemService() 方法，涉及到咱們以前跟蹤代碼的類。咱們繼續跟進去看一下。

NotificationListenerService#registerAsSystemService()

public void registerAsSystemService(Context context, ComponentName componentName, int currentUser) throws RemoteException {
    if (mWrapper == null) {
        mWrapper = new NotificationListenerWrapper();
    }
    mSystemContext = context;
    INotificationManager noMan = getNotificationInterface();
    mHandler = new MyHandler(context.getMainLooper());
    mCurrentUser = currentUser;
    noMan.registerListener(mWrapper, componentName, currentUser);
}
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這裏會初始化一個 NotificationListenerWrapper 和 mHandler。因爲這是在 SystemUI 進程中去調用此方法將 NotificationListenerService 註冊爲系統服務，因此在前面分析的那裏：NotificationListenerService.NotificationListenerWrapper#onNotificationPosted()，這段代碼是運行在 SystemUI 進程，而 mHandler 則是運行在 SystemUI 主線程上的 Handler。因此，onNotificationPosted() 是運行在 SystemUI 進程中，它經過 sbn 從 system_server 進程中獲取到 sbn 對象。下一步是經過 mHandler 處理消息，查看 NotificationListenerService.MyHandler#handleMessage() 方法，得知當 message.what 爲 MSG_ON_NOTIFICATION_POSTED 時，調用的是 onNotificationPosted() 方法。

可是，NotificationListenerService 是一個抽象類，onNotificationPosted() 爲空方法，真正的實現是它的實例類。

觀察到以前 BaseStatusBar#start() 中，是調用了 mNotificationListener.registerAsSystemService() 方法。那麼，mNotificationListener 是在哪裏進行初始化呢？

BaseStatusBar.mNotificationListener#onNotificationPosted

private final NotificationListenerService mNotificationListener = new NotificationListenerService() {
    // 省略代碼
    
    @Override
    public void onNotificationPosted(final StatusBarNotification sbn, final RankingMap rankingMap) {
        if (DEBUG) Log.d(TAG, "onNotificationPosted: " + sbn);
        if (sbn != null) {
            mHandler.post(new Runnable() {
                @Override
                public void run() {
                    processForRemoteInput(sbn.getNotification());
                    String key = sbn.getKey();
                    mKeysKeptForRemoteInput.remove(key);
                    boolean isUpdate = mNotificationData.get(key) != null;
                    // In case we don't allow child notifications, we ignore children of
                    // notifications that have a summary, since we're not going to show them
                    // anyway. This is true also when the summary is canceled,
                    // because children are automatically canceled by NoMan in that case.
                    if (!ENABLE_CHILD_NOTIFICATIONS && mGroupManager.isChildInGroupWithSummary(sbn)) {
                        if (DEBUG) {
                            Log.d(TAG, "Ignoring group child due to existing summary: " + sbn);
                        }
                        // Remove existing notification to avoid stale data.
                        if (isUpdate) {
                            removeNotification(key, rankingMap);
                        } else {
                            mNotificationData.updateRanking(rankingMap);
                        }
                        return;
                    }
                    if (isUpdate) {
                        updateNotification(sbn, rankingMap);
                    } else {
                        addNotification(sbn, rankingMap, null /* oldEntry */ );
                    }
                }
            });
        }
    }
    // 省略代碼
}
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經過上述代碼，咱們知道了在 BaseStatusBar.java 中，建立了 NotificationListenerService 的實例對象，實現了 onNotificationPost() 這個抽象方法；
在 onNotificationPost() 中，經過 handler 進行消息處理，最終調用 addNotification() 方法

PhoneStatusBar#addNotification()

@Override
public void addNotification(StatusBarNotification notification, RankingMap ranking, Entry oldEntry) {
    if (DEBUG) Log.d(TAG, "addNotification key=" + notification.getKey());
    mNotificationData.updateRanking(ranking);
    Entry shadeEntry = createNotificationViews(notification);
    if (shadeEntry == null) {
        return;
    }
    boolean isHeadsUped = shouldPeek(shadeEntry);
    if (isHeadsUped) {
        mHeadsUpManager.showNotification(shadeEntry);
        // Mark as seen immediately
        setNotificationShown(notification);
    }
    if (!isHeadsUped && notification.getNotification().fullScreenIntent != null) {
        if (shouldSuppressFullScreenIntent(notification.getKey())) {
            if (DEBUG) {
                Log.d(TAG, "No Fullscreen intent: suppressed by DND: " + notification.getKey());
            }
        } else if (mNotificationData.getImportance(notification.getKey()) < NotificationListenerService.Ranking.IMPORTANCE_MAX) {
            if (DEBUG) {
                Log.d(TAG, "No Fullscreen intent: not important enough: " + notification.getKey());
            }
        } else {
            // Stop screensaver if the notification has a full-screen intent.
            // (like an incoming phone call)
            awakenDreams();
            // not immersive & a full-screen alert should be shown
            if (DEBUG) Log.d(TAG, "Notification has fullScreenIntent; sending fullScreenIntent");
            try {
                EventLog.writeEvent(EventLogTags.SYSUI_FULLSCREEN_NOTIFICATION, notification.getKey());
                notification.getNotification().fullScreenIntent.send();
                shadeEntry.notifyFullScreenIntentLaunched();
                MetricsLogger.count(mContext, "note_fullscreen", 1);
            } catch (PendingIntent.CanceledException e) {}
        }
    }
    // !!!
    addNotificationViews(shadeEntry, ranking);
    // Recalculate the position of the sliding windows and the titles.
    setAreThereNotifications();
}
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在這個方法中，最關鍵的方法是最後的 addNotificationViews() 方法。調用這個方法以後，你建立的 Notification 纔會被添加到系統通知欄上。

總結

跟蹤完整個過程當中，以前提到的問題也能夠一一解決了：

Q：咱們建立的 Notification 實例最終以什麼樣的方式發送給系統？

A：首先，咱們在 app 進程建立 Notification 實例，經過跨進程調用，傳遞到 system_server 進程的 NotificationManagerService 中進行處理，通過兩次異步調用，最後傳遞給在 NotificationManagerService 中已經註冊的 NotificationListenerWrapper。而 android 系統在初始化 systemui 進程的時候，會往 NotificationManagerService 中註冊監聽器（這裏指的就是 NotificationListenerWrapper）。這種實現方法就是基於咱們熟悉的一種設計模式：監聽者模式。