本文需要解決的問題

筆者最近正在做一個專案，裡面需要用到 Android Notification 機制來實現某些特定需求。我正好通過這個機會研究一下 Android Notification 相關的傳送邏輯和接收邏輯，以及整理相關的筆記。我研究 Notification 機制的目的是解決以下我在使用過程中所思考的問題：

1. 我們建立的 Notification 例項最終以什麼樣的方式傳送給系統？
2. 系統是如何接收到 Notification 例項並顯示的？
3. 我們是否能攔截其他 app 的 Notification 並獲取其中的資訊？

什麼是 Android Notification 機制？

Notification，中文名翻譯為通知，每個 app 可以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。使用者可以開啟抽屜式通知欄檢視通知的詳細資訊。在實際生活中，Android Notification 機制有很廣泛的應用，例如 IM app 的新訊息通知，資訊 app 的新聞推送等等。

原始碼分析

本文的原始碼基於 Android 7.0。

Notification 的傳送邏輯

一般來說，如果我們自己的 app 想傳送一條新的 Notification，可以參照以下程式碼：

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());
 

可以看到，我們通過 NotificationCompat.Builder 新建了一個 Notification 物件，最後通過 NotificationManager#notify() 方法將 Notification 傳送出去。

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();
    }
}
 

我們可以看到，到最後會呼叫 service.enqueueNotificationWithTag() 方法，這裡的是 service 是 INotificationManager 介面。如果熟悉 AIDL 等系統相關執行機制的話，就可以看出這裡是代理類呼叫了代理介面的方法，實際方法實現是在 NotificationManagerService 當中。

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;
}

這裡程式碼比較多，但通過註釋可以清晰地理清整個邏輯：

1. 首先檢查通知發起者是系統程序或者是檢視發起者傳送的是否是同個 app 的通知資訊，否則丟擲異常；
2. 除了系統的通知和已註冊的監聽器允許入佇列外，其他 app 的通知都會限制通知數上限和通知頻率上限；
3. 將 notification 的 PendingIntent 加入到白名單；
4. 將之前的 notification 進一步封裝為 StatusBarNotification 和 NotificationRecord，最後封裝到一個非同步執行緒 EnqueueNotificationRunnable 中

這裡有一個點，就是 mHandler，涉及到切換執行緒，我們先跟蹤一下 mHandler 是在哪個執行緒被建立。

mHandler 是 WorkerHandler 類的一個例項，在 NotificationManagerService#onStart() 方法中被建立，而 NotificationManagerService 是系統 Service，所以 EnqueueNotificationRunnable 的 run 方法會執行在 system_server 的主執行緒。

NotificationManagerService.EnqueueNotificationRunnable#run()

@Override
public void run() {
    synchronized(mNotificationList) {
        // 省略程式碼
        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);
    }
}

1. 省略的程式碼主要的工作是提取 notification 相關的屬性，同時通知 notification ranking service，有新的 notification 進來，然後對所有 notification 進行重新排序；
2. 然後到最後會呼叫 mListeners.notifyPostedLocked() 方法。這裡 mListeners 是 NotificationListeners 類的一個例項。

NotificationManagerService.NotificationListeners#notifyPostedLocked()<br />  -> 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);
    }
}

呼叫到最後會執行 listener.onNotificationPosted() 方法。通過全域性搜尋得知，listener 型別是 NotificationListenerService.NotificationListenerWrapper 的代理物件。

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();
        }
    }
}

這裡在一開始會從 sbnHolder 中獲取到 sbn 物件，sbn 隸屬於 StatusBarNotificationHolder 類，繼承於 IStatusBarNotificationHolder.Stub 物件。注意到這裡捕獲了一個 RemoteException，猜測涉及到跨程序呼叫，但我們不知道這段程式碼是在哪個程序中執行的，所以在這裡暫停跟蹤程式碼。

筆者之前是通過向系統傳送通知的方式跟蹤原始碼，發現走不通。故個人嘗試從另一個角度入手，即系統接收我們發過來的通知並顯示到通知欄這個方式入手跟蹤程式碼。

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

系統顯示 Notification 的過程，猜測是在 PhoneStatusBar.java 中，因為系統啟動的過程中，會啟動 SystemUI 程序，初始化整個 Android 顯示的介面，包括系統通知欄。

PhoneStatusBar#start()<br />  -> BaseStatusBar#start()

public void start() {
    // 省略程式碼
    // 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);
    }
    // 省略程式碼
}

這段程式碼中，會呼叫 NotificationListenerService#registerAsSystemService() 方法，涉及到我們之前跟蹤程式碼的類。我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); 
}

這裡會初始化一個 NotificationListenerWrapper 和 mHandler。由於這是在 SystemUI 程序中去呼叫此方法將 NotificationListenerService 註冊為系統服務，所以在前面分析的那裡：NotificationListenerService.NotificationListenerWrapper#onNotificationPosted()，這段程式碼是執行在 SystemUI 程序，而 mHandler 則是執行在 SystemUI 主執行緒上的 Handler。所以，onNotificationPosted() 是執行在 SystemUI 程序中，它通過 sbn 從 system_server 程序中獲取到 sbn 物件。下一步是通過 mHandler 處理訊息，檢視 NotificationListenerService.MyHandler#handleMessage() 方法，得知當 message.what 為 MSG_ON_NOTIFICATION_POSTED 時，呼叫的是 onNotificationPosted() 方法。

但是，NotificationListenerService 是一個抽象類，onNotificationPosted() 為空方法，真正的實現是它的例項類。

觀察到之前 BaseStatusBar#start() 中，是呼叫了 mNotificationListener.registerAsSystemService() 方法。那麼，mNotificationListener 是在哪裡進行初始化呢？

BaseStatusBar.mNotificationListener#onNotificationPosted

private final NotificationListenerService mNotificationListener = new NotificationListenerService() {
    // 省略程式碼
    
    @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 */ );
                    }
                }
            });
        }
    }
    // 省略程式碼
}

1. 通過上述程式碼，我們知道了在 BaseStatusBar.java 中，建立了 NotificationListenerService 的例項物件，實現了 onNotificationPost() 這個抽象方法；
2. 在 onNotificationPost() 中，通過 handler 進行訊息處理，最終呼叫 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();
}

在這個方法中，最關鍵的方法是最後的 addNotificationViews() 方法。呼叫這個方法之後，你建立的 Notification 才會被新增到系統通知欄上。

總結

跟蹤完整個過程中，之前提到的問題也可以一一解決了：

• Q：我們建立的 Notification 例項最終以什麼樣的方式傳送給系統？

A：首先，我們在 app 程序建立 Notification 例項，通過跨程序呼叫，傳遞到 system_server 程序的 NotificationManagerService 中進行處理，經過兩次非同步呼叫，最後傳遞給在 NotificationManagerService 中已經註冊的 NotificationListenerWrapper。而 android 系統在初始化 systemui 程序的時候，會往 NotificationManagerService 中註冊監聽器（這裡指的就是 NotificationListenerWrapper）。這種實現方法就是基於我們熟悉的一種設計模式：監聽者模式。

• Q：系統是如何獲取到 Notification 例項並顯示的？

A：上面提到，由於初始化的時候已經往 NotificationManagerService 註冊監聽器，所以系統 SystemUI 程序會接收到 Notification 例項之後經過進一步解析，然後構造出 Notification Views 並最終顯示在系統通知欄上。

• Q：我們是否能攔截 Notification 並獲取其中的資訊？

A：通過上面的流程，我個人認為可以通過 Xposed 等框架去 hook 其中幾個重要的方法去捕獲 Notification 例項，例如 hook NotificationManager#notify() 方法去獲取 Notification 例項。

這篇文章會同步到我的個人日誌，如有問題，請大家踴躍提