本文基于Android 14源码，来探究通知的发送过程中所发生的步骤，以此来揭开通知的面纱。

1、通知的发送

通常我们会像下面这样来发送一个通知

Intent clickIntent = new Intent(mContext, Settings.MobileNetworkListActivity.class);
TaskStackBuilder stackBuilder =TaskStackBuilder.create(mContext).addNextIntent(clickIntent);
PendingIntent contentIntent =stackBuilder.getPendingIntent(0 /* requestCode */,PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE);Notification.Builder builder =new Notification.Builder(mContext, SIM_SETUP_CHANNEL_ID).setContentTitle(title).setContentText(text).setContentIntent(contentIntent).setSmallIcon(R.drawable.ic_sim_alert).setAutoCancel(true);
mNotificationManager.notify(SIM_ACTIVATION_NOTIFICATION_ID, builder.build());

其实最终都是通过notificationManager.notify(id, notification)来把通知发送出去，

2 NotificationManager#notify

//framework/base/core/java/android/app/NotificationManager.java
*** the same id has already been posted by your application and has not yet been canceled, it* will be replaced by the updated information.** @param id An identifier for this notification unique within your*        application.* @param notification A {@link Notification} object describing what to show the user. Must not*        be null.*/
public void notify(int id, Notification notification)
{notify(null, id, notification);
}public void notifyAsUser(String tag, int id, Notification notification, UserHandle user)
{INotificationManager service = getService();String pkg = mContext.getPackageName();try {if (localLOGV) Log.v(TAG, pkg + ": notify(" + id + ", " + notification + ")");service.enqueueNotificationWithTag(pkg, mContext.getOpPackageName(), tag, id,fixNotification(notification), user.getIdentifier());} catch (RemoteException e) {throw e.rethrowFromSystemServer();}
}

NotificationManager中，主要是拿到NotificationManagerService，然后调用其enqueueNotificationWithTag。

注意这里的参数：

pkg: 是发送通知的所在的包名

tag：此通知的字符串标识符

id：就是发送通知传入的int类型的id

notification：是传入的通过Notification.Builder构造的，经过fixNotification之后的notification

来看看这个fixNotification是做啥的。

private Notification fixNotification(Notification notification) {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);}}notification.reduceImageSizes(mContext);return Builder.maybeCloneStrippedForDelivery(notification);
}

一遍看下来，fixNotification主要是对通知的soundsmallIcon进行了处理，然后对通知的大小进行裁剪。

我们重点看下reduceImageSizes，因为如果通知非常多的话，通知里面icon通常也会很多，可能会带来性能问题。

void reduceImageSizes(Context context) {if (extras.getBoolean(EXTRA_REDUCED_IMAGES)) {return;}boolean isLowRam = ActivityManager.isLowRamDeviceStatic();if (mSmallIcon != null// Only bitmap icons can be downscaled.&& (mSmallIcon.getType() == Icon.TYPE_BITMAP|| mSmallIcon.getType() == Icon.TYPE_ADAPTIVE_BITMAP)) {Resources resources = context.getResources();int maxSize = resources.getDimensionPixelSize(isLowRam ? R.dimen.notification_small_icon_size_low_ram: R.dimen.notification_small_icon_size);mSmallIcon.scaleDownIfNecessary(maxSize, maxSize);}if (mLargeIcon != null || largeIcon != null) {Resources resources = context.getResources();Class<? extends Style> style = getNotificationStyle();int maxSize = resources.getDimensionPixelSize(isLowRam? R.dimen.notification_right_icon_size_low_ram: R.dimen.notification_right_icon_size);if (mLargeIcon != null) {mLargeIcon.scaleDownIfNecessary(maxSize, maxSize);}if (largeIcon != null) {largeIcon = Icon.scaleDownIfNecessary(largeIcon, maxSize, maxSize);}}reduceImageSizesForRemoteView(contentView, context, isLowRam);reduceImageSizesForRemoteView(headsUpContentView, context, isLowRam);reduceImageSizesForRemoteView(bigContentView, context, isLowRam);extras.putBoolean(EXTRA_REDUCED_IMAGES, true);
}private void reduceImageSizesForRemoteView(RemoteViews remoteView, Context context,boolean isLowRam) {if (remoteView != null) {Resources resources = context.getResources();int maxWidth = resources.getDimensionPixelSize(isLowRam? R.dimen.notification_custom_view_max_image_width_low_ramR.dimen.notification_custom_view_max_image_width);int maxHeight = resources.getDimensionPixelSize(isLowRam? R.dimen.notification_custom_view_max_image_height_low_ramR.dimen.notification_custom_view_max_image_height);remoteView.reduceImageSizes(maxWidth, maxHeight);}
}

首先对于icon来说，如果mSmallIcon不为空的话，通常情况下icon最大限制是48dp，也就是Bitmap宽高最大是48dp。后面对于mLargeIcon同样如此。

然后是RemoteView，对于contentView、headsUpContentView、bigContentView处理基本是一样的，它们的宽度限制是450dp，高度是284dp。

接下来，继续看NotificationManagerService里面的流程。

3 Fwk通知处理

3.1 enqueueNotification

//framework/base/services/core/java/com/android/server/notification/NotificationManagerService.java
@Override
public void enqueueNotificationWithTag(String pkg, String opPkg, String tag, int id,Notification notification, int userId) throws RemoteException {enqueueNotificationInternal(pkg, opPkg, Binder.getCallingUid(),Binder.getCallingPid(), tag, id, notification, userId,false /* byForegroundService */);
}void enqueueNotificationInternal(final String pkg, final String opPkg, final int callingUid, final int callingPid, final String tag, final int id, final Notification notification,int incomingUserId, boolean postSilently, boolean byForegroundService) {PostNotificationTracker tracker = acquireWakeLockForPost(pkg, callingUid);boolean enqueued = false;try {enqueued = enqueueNotificationInternal(pkg, opPkg, callingUid, callingPid,tag, id, notification, incomingUserId,postSilently, tracker, byForegroundService);} finally {if (!enqueued) {tracker.cancel();}}
}/*** @return True if we successfully processed the notification and handed off the task of* enqueueing it to a background thread; false otherwise.*/
private boolean enqueueNotificationInternal(final String pkg, final String opPkg, final int callingUid, final int callingPid, final String tag, final int id, final Notification notification, int incomingUserId,boolean postSilently, PostNotificationTracker tracker,boolean byForegroundService) {fixNotification(notification, pkg, tag, id, userId, notificationUid,policy, stripUijFlag);final StatusBarNotification n = new StatusBarNotification(pkg, opPkg, id, tag, notificationUid, callingPid, notification,user, null, System.currentTimeMillis());final NotificationRecord r = new NotificationRecord(getContext(), n, channel);r.setIsAppImportanceLocked(mPermissionHelper.isPermissionUserSet(pkg, userId));r.setPostSilently(postSilently);r.setFlagBubbleRemoved(false);r.setPkgAllowedAsConvo(mMsgPkgsAllowedAsConvos.contains(pkg));boolean isImportanceFixed = mPermissionHelper.isPermissionFixed(pkg, userId);r.setImportanceFixed(isImportanceFixed);mHandler.post(new EnqueueNotificationRunnable(userId, r, isAppForeground, tracker));return true;
}

enqueueNotificationInternal代码比较多。

首先，fixNotification里面主要是对通知的flags等进行适配，比如处理彩色通知权限 、全屏通知需要权限、多媒体类型的通知，权限检测、对Remote views 尺寸检查，太大的话设置为空等等。

然后创建状态栏通知对象StatusBarNotification，然后创建NotificationRecord，并将StatusBarNotification对象，最后，通过post来调动EnqueueNotificationRunnable。

protected class EnqueueNotificationRunnable implements Runnable {@Overridepublic void run() {boolean enqueued = false;try {enqueued = enqueueNotification();} finally {if (!enqueued) {mTracker.cancel();}}}
}

EnqueueNotificationRunnable调用的是enqueueNotification。

final ArrayList<NotificationRecord> mEnqueuedNotifications = new ArrayList<>();/*** @return True if we successfully enqueued the notification and handed off the task of* posting it to a background thread; false otherwise.*/
private boolean enqueueNotification() {synchronized (mNotificationLock) {mEnqueuedNotifications.add(r);scheduleTimeoutLocked(r); //如果有设置自动取消通知时间的final StatusBarNotification n = r.getSbn();if (DBG) Slog.d(TAG, "EnqueueNotificationRunnable.run for: " + n.getKey());NotificationRecord old = mNotificationsByKey.get(n.getKey());final int callingUid = n.getUid();final int callingPid = n.getInitialPid();final Notification notification = n.getNotification();final String pkg = n.getPackageName();final int id = n.getId();final String tag = n.getTag();// We need to fix the notification up a little for bubblesupdateNotificationBubbleFlags(r, isAppForeground);// Handle grouped notifications and bail out early if we// can to avoid extracting signals.handleGroupedNotificationLocked(r, old, callingUid, callingPid);mHandler.post(new PostNotificationRunnable(r.getKey(), r.getSbn().getPackageName(),r.getUid(), mTracker));return true;}
}

首先把前面创建的NotificationRecord对象，存入mEnqueuedNotifications中，然后从mEnqueuedNotifications中拿到StatusBarNotification，然后从StatusBarNotification中取出一些参数，接着调用updateNotificationBubbleFlags对bubbles进行适配。

3.2 postNotification

接着通过handleGroupedNotificationLocked对成组通知进行处理，最后调用PostNotificationRunnable。

protected class PostNotificationRunnable implements Runnable {@Overridepublic void run() {boolean posted = false;try {posted = postNotification();} finally {if (!posted) {mTracker.cancel();}}}
}/*** @return True if we successfully processed the notification and handed off the task of* notifying all listeners to a background thread; false otherwise.*/
private boolean postNotification() {//没有发送通知的权限，则为trueboolean appBanned = !areNotificationsEnabledForPackageInt(pkg, uid);boolean isCallNotification = isCallNotification(pkg, uid);synchronized (mNotificationLock) {NotificationRecord r = null;int N = mEnqueuedNotifications.size();for (int i = 0; i < N; i++) {final NotificationRecord enqueued = mEnqueuedNotifications.get(i);if (Objects.equals(key, enqueued.getKey())) {r = enqueued;break;}}final StatusBarNotification n = r.getSbn();final Notification notification = n.getNotification();boolean isCallNotificationAndCorrectStyle = isCallNotification&& notification.isStyle(Notification.CallStyle.class);if (!(notification.isMediaNotification() ||isCallNotificationAndCorrectStyle)&& (appBanned || isRecordBlockedLocked(r))) {//通知被禁止的mUsageStats.registerBlocked(r);if (DBG) {Slog.e(TAG, "Suppressing notification from package " + pkg);}return false;}//包是挂起状态final boolean isPackageSuspended =isPackagePausedOrSuspended(r.getSbn().getPackageName(), r.getUid());r.setHidden(isPackageSuspended);if (isPackageSuspended) {mUsageStats.registerSuspendedByAdmin(r);}//获取旧的通知记录NotificationRecord old = mNotificationsByKey.get(key);// Make sure the SBN has an instance ID for statsd logging.if (old == null || old.getSbn().getInstanceId() == null) {n.setInstanceId(mNotificationInstanceIdSequence.newInstanceId());} else {n.setInstanceId(old.getSbn().getInstanceId());}//获取在通知列表里的索引int index = indexOfNotificationLocked(n.getKey());if (index < 0) {//没找到，新加通知mNotificationList.add(r);mUsageStats.registerPostedByApp(r);mUsageStatsManagerInternal.reportNotificationPosted(r.getSbn().getOpPkg(),r.getSbn().getUser(), mTracker.getStartTime());final boolean isInterruptive = isVisuallyInterruptive(null, r);r.setInterruptive(isInterruptive);r.setTextChanged(isInterruptive);} else {//有旧的，更新通知old = mNotificationList.get(index);  // Potentially *changes* oldmNotificationList.set(index, r);mUsageStats.registerUpdatedByApp(r, old);mUsageStatsManagerInternal.reportNotificationUpdated(r.getSbn().getOpPkg(),r.getSbn().getUser(), mTracker.getStartTime());// Make sure we don't lose the foreground service state.notification.flags |=old.getNotification().flags & FLAG_FOREGROUND_SERVICE;r.isUpdate = true;final boolean isInterruptive = isVisuallyInterruptive(old, r);r.setTextChanged(isInterruptive);}mNotificationsByKey.put(n.getKey(), r);// Ensure if this is a foreground service that the proper additional// flags are set.if ((notification.flags & FLAG_FOREGROUND_SERVICE) != 0) {notification.flags |= FLAG_NO_CLEAR; //前台服务通知，添加必要的flag}//排序mRankingHelper.extractSignals(r);mRankingHelper.sort(mNotificationList);final int position = mRankingHelper.indexOf(mNotificationList, r);int buzzBeepBlinkLoggingCode = 0;if (!r.isHidden()) {buzzBeepBlinkLoggingCode = buzzBeepBlinkLocked(r); //获取声音震动闪灯的状态，封装成一个值返回}if (notification.getSmallIcon() != null) {NotificationRecordLogger.NotificationReported maybeReport =mNotificationRecordLogger.prepareToLogNotificationPosted(r, old,position, buzzBeepBlinkLoggingCode,getGroupInstanceId(r.getSbn().getGroupKey()));//更新状态栏通知notifyListenersPostedAndLogLocked(r, old, mTracker, maybeReport);posted = true;StatusBarNotification oldSbn = (old != null) ? old.getSbn() : null;//旧的通知为空或者新旧通知的group不一样。并且新的非紧急通知if (oldSbn == null|| !Objects.equals(oldSbn.getGroup(), n.getGroup())|| oldSbn.getNotification().flags != n.getNotification().flags) {if (!isCritical(r)) {mHandler.post(() -> {synchronized (mNotificationLock) {//发布新的状态栏通知mGroupHelper.onNotificationPosted(n, hasAutoGroupSummaryLocked(n));}});}}} else {Slog.e(TAG, "Not posting notification without small icon: " + notification);//通知没有设置smallIcon，这是有问题的if (old != null && !old.isCanceled) {mListeners.notifyRemovedLocked(r,NotificationListenerService.REASON_ERROR, r.getStats());mHandler.post(new Runnable() {@Overridepublic void run() {mGroupHelper.onNotificationRemoved(n);}});}// ATTENTION: in a future release we will bail out here// so that we do not play sounds, show lights, etc. for invalid// notificationsSlog.e(TAG, "WARNING: In a future release this will crash the app: "+ n.getPackageName());}} finally {//末尾从队列集合里移除当前已处理的通知int N = mEnqueuedNotifications.size();for (int i = 0; i < N; i++) {final NotificationRecord enqueued = mEnqueuedNotifications.get(i);if (Objects.equals(key, enqueued.getKey())) {mEnqueuedNotifications.remove(i);break;}}}
}@GuardedBy("mNotificationLock")
final ArrayList<NotificationRecord> mNotificationList = new ArrayList<>();@GuardedBy("mNotificationLock")
final ArrayMap<String, NotificationRecord> mNotificationsByKey = new ArrayMap<>();

3.2.1 通知禁止

postNotification中，首先会对比StatusBarNotification中key是否一致，然后判断如果通知被禁止，就返回。

3.2.2 加入或更新通知列表

接着从通知列表mNotificationList获取在通知列表里的索引，如果没有找到，说明是新通知，把它加入mNotificationList中，否则存在旧的通知，那么就把新通知的NotificationRecord更新到通知列表mNotificationList中。

然后把以通知的key为键，NotificationRecord为值，存到mNotificationsByKey中。

3.2.3 排序

然后对通知进行排序，并取出排序后通知的位置。

3.2.4 获取声音震动闪灯的状态

确定通知是否应该发出声音，震动，闪烁

/*** Determine whether this notification should attempt to make noise, vibrate, or flash the LED* @return buzzBeepBlink - bitfield (buzz ? 1 : 0) | (beep ? 2 : 0) | (blink ? 4 : 0)*/
int buzzBeepBlinkLocked(NotificationRecord record) {//汽车并且通知效果不可用if (mIsAutomotive && !mNotificationEffectsEnabledForAutomotive) {return 0;}boolean buzz = false;boolean beep = false;boolean blink = false;final String key = record.getKey();//重要性大于等于默认值3的才有效果的// Should this notification make noise, vibe, or use the LED?final boolean aboveThreshold =mIsAutomotive? record.getImportance() > NotificationManager.IMPORTANCE_DEFAULT: record.getImportance() >= NotificationManager.IMPORTANCE_DEFAULT;// Remember if this notification already owns the notification channels.boolean wasBeep = key != null && key.equals(mSoundNotificationKey);boolean wasBuzz = key != null && key.equals(mVibrateNotificationKey);// These are set inside the conditional if the notification is allowed to make noise.boolean hasValidVibrate = false;boolean hasValidSound = false;boolean sentAccessibilityEvent = false;// 如果通知将出现在状态栏中，它应该发送一个可访问性事件// If the notification will appear in the status bar, it should send an accessibility eventfinal boolean suppressedByDnd = record.isIntercepted()&& (record.getSuppressedVisualEffects() & SUPPRESSED_EFFECT_STATUS_BAR) != 0;if (!record.isUpdate&& record.getImportance() > IMPORTANCE_MIN&& !suppressedByDnd&& isNotificationForCurrentUser(record)) {sendAccessibilityEvent(record);sentAccessibilityEvent = true;}if (aboveThreshold && isNotificationForCurrentUser(record)) {if (mSystemReady && mAudioManager != null) {Uri soundUri = record.getSound();hasValidSound = soundUri != null && !Uri.EMPTY.equals(soundUri);VibrationEffect vibration = record.getVibration();// Demote sound to vibration if vibration missing & phone in vibration mode.if (vibration == null&& hasValidSound&& (mAudioManager.getRingerModeInternal()== AudioManager.RINGER_MODE_VIBRATE)&& mAudioManager.getStreamVolume(AudioAttributes.toLegacyStreamType(record.getAudioAttributes())) == 0) {boolean insistent = (record.getFlags() & Notification.FLAG_INSISTENT) != 0;vibration = mVibratorHelper.createFallbackVibration(insistent);}hasValidVibrate = vibration != null;boolean hasAudibleAlert = hasValidSound || hasValidVibrate;if (hasAudibleAlert && !shouldMuteNotificationLocked(record)) {if (!sentAccessibilityEvent) {sendAccessibilityEvent(record);sentAccessibilityEvent = true;}if (DBG) Slog.v(TAG, "Interrupting!");//持续更新，就是连续的通知boolean isInsistentUpdate = isInsistentUpdate(record);if (hasValidSound) {if (isInsistentUpdate) { // 声音为true// don't reset insistent sound, it's jarringbeep = true;} else {if (isInCall()) {playInCallNotification();beep = true;} else {beep = playSound(record, soundUri); //播放声音返回结果}if (beep) {mSoundNotificationKey = key;}}}final boolean ringerModeSilent =mAudioManager.getRingerModeInternal()== AudioManager.RINGER_MODE_SILENT;//非通话中，有设置震动，非静音模式if (!isInCall() && hasValidVibrate && !ringerModeSilent) {if (isInsistentUpdate) {buzz = true;} else {//震动buzz = playVibration(record, vibration, hasValidSound);if (buzz) {mVibrateNotificationKey = key;}}}// Try to start flash notification event whenever an audible and non-suppressed// notification is receivedmAccessibilityManager.startFlashNotificationEvent(getContext(),AccessibilityManager.FLASH_REASON_NOTIFICATION,record.getSbn().getPackageName());} else if ((record.getFlags() & Notification.FLAG_INSISTENT) != 0) {hasValidSound = false;}}}// If a notification is updated to remove the actively playing sound or vibrate,// cancel that feedback nowif (wasBeep && !hasValidSound) {  //旧通知有声音，新的没有clearSoundLocked(); //停止声音播放}if (wasBuzz && !hasValidVibrate) {clearVibrateLocked();}// light// release the lightboolean wasShowLights = mLights.remove(key); //旧的light状态if (canShowLightsLocked(record, aboveThreshold)) { //是否可以闪灯mLights.add(key); //加入集合updateLightsLocked(); //闪灯或者灭灯if (mUseAttentionLight && mAttentionLight != null) {mAttentionLight.pulse(); ///如果支持提示灯并且不为空的话}blink = true;} else if (wasShowLights) {updateLightsLocked();}//3种状态封装成一个intfinal int buzzBeepBlink = (buzz ? 1 : 0) | (beep ? 2 : 0) | (blink ? 4 : 0);if (buzzBeepBlink > 0) {// Ignore summary updates because we don't display most of the information.if (record.getSbn().isGroup() && record.getSbn().getNotification().isGroupSummary()) {if (DEBUG_INTERRUPTIVENESS) {Slog.v(TAG, "INTERRUPTIVENESS: "+ record.getKey() + " is not interruptive: summary");}} else if (record.canBubble()) {if (DEBUG_INTERRUPTIVENESS) {Slog.v(TAG, "INTERRUPTIVENESS: "+ record.getKey() + " is not interruptive: bubble");}} else {record.setInterruptive(true);if (DEBUG_INTERRUPTIVENESS) {Slog.v(TAG, "INTERRUPTIVENESS: "+ record.getKey() + " is interruptive: alerted");}}MetricsLogger.action(record.getLogMaker().setCategory(MetricsEvent.NOTIFICATION_ALERT).setType(MetricsEvent.TYPE_OPEN).setSubtype(buzzBeepBlink));EventLogTags.writeNotificationAlert(key, buzz ? 1 : 0, beep ? 1 : 0, blink ? 1 : 0);}record.setAudiblyAlerted(buzz || beep); //为true记录下震动或声音的开始时间，为false，时间为-1return buzzBeepBlink;
}

3.2.5 更新通知

通知的更新分有SmallIcon的通知和没有的情况。

1、有SmallIcon通知：

/*** Asynchronously notify all listeners about a posted (new or updated) notification. This* should be called from {@link PostNotificationRunnable} to "complete" the post (since SysUI is* one of the NLSes, and will display it to the user).** <p>This method will call {@link PostNotificationTracker#finish} on the supplied tracker* when every {@link NotificationListenerService} has received the news.** <p>Also takes care of removing a notification that has been visible to a listener before,* but isn't anymore.*/
@GuardedBy("mNotificationLock")
private void notifyListenersPostedAndLogLocked(NotificationRecord r, NotificationRecord old,@NonNull PostNotificationTracker tracker,@Nullable NotificationRecordLogger.NotificationReported report) {List<Runnable> listenerCalls = mListeners.prepareNotifyPostedLocked(r, old, true);mHandler.post(() -> {for (Runnable listenerCall : listenerCalls) {listenerCall.run();}long postDurationMillis = tracker.finish();if (report != null) {report.post_duration_millis = postDurationMillis;mNotificationRecordLogger.logNotificationPosted(report);}});
}

首先调用notifyListenersPostedAndLogLocked更新状态栏通知。

/*** "Prepares" to notify all listeners about the posted notification.** <p>This method <em>does not invoke</em> the listeners; the caller should post each* returned {@link Runnable} on a suitable thread to do so.** @param notifyAllListeners notifies all listeners if true, else only notifies listeners*                           targeting <= O_MR1* @return A list of {@link Runnable} operations to notify all listeners about the posted* notification.*/
@VisibleForTesting
@GuardedBy("mNotificationLock")
List<Runnable> prepareNotifyPostedLocked(NotificationRecord r,NotificationRecord old, boolean notifyAllListeners) {if (isInLockDownMode(r.getUser().getIdentifier())) {return new ArrayList<>();}ArrayList<Runnable> listenerCalls = new ArrayList<>();try {// Lazily initialized snapshots of the notification.StatusBarNotification sbn = r.getSbn();StatusBarNotification oldSbn = (old != null) ? old.getSbn() : null;TrimCache trimCache = new TrimCache(sbn);for (final ManagedServiceInfo info : getServices()) {boolean sbnVisible = isVisibleToListener(sbn, r.getNotificationType(), info);boolean oldSbnVisible = (oldSbn != null)&& isVisibleToListener(oldSbn, old.getNotificationType(), info);// This notification hasn't been and still isn't visible -> ignore.if (!oldSbnVisible && !sbnVisible) {continue;}// If the notification is hidden, don't notifyPosted listeners targeting < P.// Instead, those listeners will receive notifyPosted when the notification is// unhidden.if (r.isHidden() && info.targetSdkVersion < Build.VERSION_CODES.P) {continue;}// If we shouldn't notify all listeners, this means the hidden state of// a notification was changed.  Don't notifyPosted listeners targeting >= P.// Instead, those listeners will receive notifyRankingUpdate.if (!notifyAllListeners && info.targetSdkVersion >= Build.VERSION_CODES.P) {continue;}final NotificationRankingUpdate update = makeRankingUpdateLocked(info);// This notification became invisible -> remove the old one.if (oldSbnVisible && !sbnVisible) {final StatusBarNotification oldSbnLightClone = oldSbn.cloneLight();listenerCalls.add(() -> notifyRemoved(info, oldSbnLightClone, update, null, REASON_USER_STOPPED));continue;}// Grant access before listener is notifiedfinal int targetUserId = (info.userid == UserHandle.USER_ALL)? UserHandle.USER_SYSTEM : info.userid;updateUriPermissions(r, old, info.component.getPackageName(), targetUserId);mPackageManagerInternal.grantImplicitAccess(targetUserId, null /* intent */,UserHandle.getAppId(info.uid),sbn.getUid(),false /* direct */, false /* retainOnUpdate */);final StatusBarNotification sbnToPost = trimCache.ForListener(info);listenerCalls.add(() -> notifyPosted(info, sbnToPost, update));}} catch (Exception e) {Slog.e(TAG, "Could not notify listeners for " + r.getKey(), e);}return listenerCalls;
}

在notifyListenersPostedAndLogLocked中，主要是对是添加、移除通知等进行判断：

如果旧通知可见、新通知不可见，就将notifyRemoved加入listenerCalls队列

如果不需要移除，就是添加通知，将notifyPosted加入listenerCalls队列

然后notifyListenersPostedAndLogLocked中，调用listenerCall.run()，执行notifyRemoved或notifyPosted。

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 (android.os.DeadObjectException ex) {Slog.wtf(TAG, "unable to notify listener (posted): " + info, ex);} catch (RemoteException ex) {Slog.e(TAG, "unable to notify listener (posted): " + info, ex);}
}

notifyPosted中是先拿到INotificationListener，并调用它的onNotificationPosted。

在更新通知之后，如果旧的通知为空或者新旧通知的group不一样。并且新的非紧急通知，发布新的状态栏通知。

2、没有SmallIcon：

如果通知没有设置smallIcon，这是有问题的, 移除旧的通知。

最后，末尾从队列集合里移除当前已处理的通知。

4 INotificationListener的目的地

mService是从mService = asInterface(binder)取到的，然后保存在ManagedServiceInfo中。

ServiceConnection serviceConnection = new ServiceConnection() {IInterface mService;@Overridepublic void onServiceConnected(ComponentName name, IBinder binder) {Slog.v(TAG,  userid + " " + getCaption() + " service connected: " + name);boolean added = false;ManagedServiceInfo info = null;synchronized (mMutex) {mServicesRebinding.remove(servicesBindingTag);try {mService = asInterface(binder);info = newServiceInfo(mService, name,userid, isSystem, this, targetSdkVersion, uid);binder.linkToDeath(info, 0);added = mServices.add(info);} catch (RemoteException e) {Slog.e(TAG, "Failed to linkToDeath, already dead", e);}}if (added) {onServiceAdded(info);}}
}private ManagedServiceInfo newServiceInfo(IInterface service,ComponentName component, int userId, boolean isSystem, ServiceConnection connection,int targetSdkVersion, int uid) {return new ManagedServiceInfo(service, component, userId, isSystem, connection,targetSdkVersion, uid);
}

这里INotificationListener是从ManagedServiceInfo的service中取到的，

public ManagedServices(Context context, Object mutex, UserProfiles userProfiles,IPackageManager pm) {mContext = context;mMutex = mutex;mUserProfiles = userProfiles;mPm = pm;mConfig = getConfig();mApprovalLevel = APPROVAL_BY_COMPONENT;mUm = (UserManager) mContext.getSystemService(Context.USER_SERVICE);
}abstract protected Config getConfig();

从NotificationManagerService中拿到getConfig()，因此asInterface调用的是NotificationManagerService的asInterface。

//framework/base/services/core/java/com/android/server/notification/NotificationManagerService.java
public class NotificationAssistants extends ManagedServices {@Overrideprotected Config getConfig() {Config c = new Config();c.caption = "notification assistant";c.serviceInterface = NotificationAssistantService.SERVICE_INTERFACE;c.xmlTag = TAG_ENABLED_NOTIFICATION_ASSISTANTS;c.secureSettingName = Settings.Secure.ENABLED_NOTIFICATION_ASSISTANT;c.bindPermission = Manifest.permission.BIND_NOTIFICATION_ASSISTANT_SERVICE;c.settingsAction = Settings.ACTION_MANAGE_DEFAULT_APPS_SETTINGS;c.clientLabel = R.string.notification_ranker_binding_label;return c;}@Overrideprotected IInterface asInterface(IBinder binder) {return INotificationListener.Stub.asInterface(binder);}
}

可看到实现类是NotificationAssistantService

//framework/base/core/java/android/service/notification/NotificationAssistantService.java
/*** The {@link Intent} that must be declared as handled by the service.*/
@SdkConstant(SdkConstant.SdkConstantType.SERVICE_ACTION)
public static final String SERVICE_INTERFACE= "android.service.notification.NotificationAssistantService";public abstract class NotificationAssistantService extendsNotificationListenerService {}

而 NotificationAssistantService又是继承的NotificationListenerService，所以3.2.4中的onNotificationPosted走到了这里。

//framework/base/core/java/android/service/notification/NotificationListenerService.java
public abstract class NotificationListenerService extends Service {protected class NotificationListenerWrapper extends INotificationListener.Stub {@Overridepublic void onNotificationPosted(IStatusBarNotificationHolder sbnHolder,NotificationRankingUpdate update) {// 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 informationmHandler.obtainMessage(MyHandler.MSG_ON_NOTIFICATION_RANKING_UPDATE,mRankingMap).sendToTarget();}}}}case MSG_ON_NOTIFICATION_POSTED: {SomeArgs args = (SomeArgs) msg.obj;StatusBarNotification sbn = (StatusBarNotification) args.arg1;RankingMap rankingMap = (RankingMap) args.arg2;args.recycle();onNotificationPosted(sbn, rankingMap);} break;/*** Implement this method to learn about new notifications as they are posted by apps.** @param sbn A data structure encapsulating the original {@link android.app.Notification}*            object as well as its identifying information (tag and id) and source*            (package name).* @param rankingMap The current ranking map that can be used to retrieve ranking information*                   for active notifications, including the newly posted one.*/public void onNotificationPosted(StatusBarNotification sbn, RankingMap rankingMap) {onNotificationPosted(sbn);}public void onNotificationPosted(StatusBarNotification sbn) {// optional}@Overridepublic IBinder onBind(Intent intent) {if (mWrapper == null) {mWrapper = new NotificationListenerWrapper();}return mWrapper;}@SystemApipublic 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);}
}

最终，NotificationListenerService的onNotificationPosted只是一个空实现，那么它的子类是谁呢？继续看。

//framework/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationListenerWithPlugins.java
public class NotificationListenerWithPlugins extends NotificationListenerService implements PluginListener<NotificationListenerController> {@Overridepublic void registerAsSystemService(Context context, ComponentName componentName,int currentUser) throws RemoteException {super.registerAsSystemService(context, componentName, currentUser);mPluginManager.addPluginListener(this, NotificationListenerController.class);}
}

SystemUI中的NotificationListenerWithPlugins实现了NotificationListenerService。

//framework/base/packages/SystemUI/src/com/android/systemui/statusbar/NotificationListener.java
public class NotificationListener extends NotificationListenerWithPlugins implementsPipelineDumpable {public void registerAsSystemService() {try {registerAsSystemService(mContext, new ComponentName(mContext.getPackageName(), getClass().getCanonicalName()),UserHandle.USER_ALL);} catch (RemoteException e) {Log.e(TAG, "Unable to register notification listener", e);}}@Overridepublic void onNotificationPosted(final StatusBarNotification sbn,final RankingMap rankingMap) {}
}

然后NotificationListener又继承了NotificationListenerWithPlugins。

//framework/base/packages/SystemUI/src/com/android/systemui/statusbar/notification/init/NotificationsControllerImpl.kt
override fun initialize(centralSurfaces: CentralSurfaces,presenter: NotificationPresenter,listContainer: NotificationListContainer,stackController: NotifStackController,notificationActivityStarter: NotificationActivityStarter,bindRowCallback: NotificationRowBinderImpl.BindRowCallback) {notificationListener.registerAsSystemService()
}

继续看SystemUI里面的listener是如何注册的。

//framework/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/CentralSurfacesImpl.java
@Override
public void start() {setUpPresenter()
}private void setUpPresenter() {mNotificationsController.initialize(this,mPresenter,mNotifListContainer,mStackScrollerController.getNotifStackController(),mNotificationActivityStarter,mCentralSurfacesComponent.getBindRowCallback());
}

最终是在CentralSurfacesImpl在start的时候注册的。