前言
在上一篇通知服务NotificationListenerService使用方法 中，我们已经介绍了如何使用NotificationListenerService来监听消息通知，在最后我们还模拟了如何实现微信自动抢红包功能。
那么NotificationListenerService是如何实现系统通知监听的呢？（本篇源码分析基于API 32）
NotificationListenerService方法集
NotificationLisenerService是Service的子类
public abstract class NotificationListenerService extends Service
除了Service的方法属性外，NotificationListenerService还为我们提供了收到通知、通知被移除、连接到通知管理器等方法，如下图所示。
一般业务中我们只关注有标签的那四个方法即可。
NotificationListenerService接收流程
既然NotificationListenerService是继承自Service的，我们先来看它的onBind方法，代码如下所示。
@Override
public IBinder onBind(Intent intent) {
if (mWrapper == null) {
mWrapper = new NotificationListenerWrapper();
}
return mWrapper;
}
在onBind方法中返回了一个NotificationListenerWrapper实例，NotificationListenerWrapper对象是定义在NotificationListenerService中的一个内部类。主要方法如下所示。
/** @hide */
protected class NotificationListenerWrapper extends INotificationListener.Stub {
@Override
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;
}
if (sbn == null) {
Log.w(TAG, "onNotificationPosted: Error receiving StatusBarNotification");
return;
}
try {
// convert icon metadata to legacy format for older clients
createLegacyIconExtras(sbn.getNotification());
maybePopulateRemoteViews(sbn.getNotification());
maybePopulatePeople(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();
}
}
...省略onNotificationRemoved等方法
}
NotificationListenerWrapper继承自INotificationListener.Stub，当我们看到Stub这一关键字的时候，就应该知道这里是使用AIDL实现了跨进程通信。
在NotificationListenerWrapper的onNotificationPosted中通过代码
mHandler.obtainMessage(MyHandler.MSG_ON_NOTIFICATION_POSTED,
args).sendToTarget();
将消息发送出去，handler接受后，又调用NotificationListernerService的onNotificationPosted方法，进而实现通知消息的监听。代码如下所示。
private final class MyHandler extends Handler {
public static final int MSG_ON_NOTIFICATION_POSTED = 1;
@Override
public void handleMessage(Message msg) {
if (!isConnected) {
return;
}
switch (msg.what) {
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;
...
}
}
}那么，消息通知发送时，又是如何与NotificationListenerWrapper通信的呢？
通知消息发送流程
当客户端发送一个通知的时候，会调用如下所示的代码
notificationManager.notify(1, notification)
notify又会调用notifyAsUser方法，代码如下所示
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();
}
}
紧接着又会走到INotificationManager的enqueueNotificationWithTag方法中，enqueueNotificationWithTag是声明在INotificationManager.aidl文件中的接口
/** {@hide} */
interface INotificationManager
{
@UnsupportedAppUsage
void cancelAllNotifications(String pkg, int userId);
...
void cancelToast(String pkg, IBinder token);
void finishToken(String pkg, IBinder token);
void enqueueNotificationWithTag(String pkg, String opPkg, String tag, int id,
in Notification notification, int userId);
...
}这个接口是在NotificationManagerService中实现的，接着我们转到NotificationManagerService中去查看，相关主要代码如下所示。
@VisibleForTesting
final IBinder mService = new INotificationManager.Stub() {
@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);
}
}
enqueueNotificationWithTag方法会走进enqueueNotificationInternal方法，在方法最后会通过Handler发送一个EnqueueNotificationRunnable，代码如下所示。
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) {
...
//构造StatusBarNotification，用于分发监听服务
final StatusBarNotification n = new StatusBarNotification(
pkg, opPkg, id, tag, notificationUid, callingPid, notification,
user, null, System.currentTimeMillis());
// setup local book-keeping
String channelId = notification.getChannelId();
if (mIsTelevision && (new Notification.TvExtender(notification)).getChannelId() != null) {
channelId = (new Notification.TvExtender(notification)).getChannelId();
}
...
// 设置intent的白名点，是否盛典、是否后台启动等
if (notification.allPendingIntents != null) {
final int intentCount = notification.allPendingIntents.size();
if (intentCount > 0) {
final long duration = LocalServices.getService(
DeviceIdleInternal.class).getNotificationAllowlistDuration();
for (int i = 0; i < intentCount; i++) {
PendingIntent pendingIntent = notification.allPendingIntents.valueAt(i);
if (pendingIntent != null) {
mAmi.setPendingIntentAllowlistDuration(pendingIntent.getTarget(),
ALLOWLIST_TOKEN, duration,
TEMPORARY_ALLOWLIST_TYPE_FOREGROUND_SERVICE_ALLOWED,
REASON_NOTIFICATION_SERVICE,
"NotificationManagerService");
mAmi.setPendingIntentAllowBgActivityStarts(pendingIntent.getTarget(),
ALLOWLIST_TOKEN, (FLAG_ACTIVITY_SENDER | FLAG_BROADCAST_SENDER
| FLAG_SERVICE_SENDER));
}
}
}
}
...
mHandler.post(new EnqueueNotificationRunnable(userId, r, isAppForeground));
}
EnqueueNotificationRunnable源码如下所示。
protected class EnqueueNotificationRunnable implements Runnable {
private final NotificationRecord r;
private final int userId;
private final boolean isAppForeground;
EnqueueNotificationRunnable(int userId, NotificationRecord r, boolean foreground) {
this.userId = userId;
this.r = r;
this.isAppForeground = foreground;
}
@Override
public void run() {
synchronized (mNotificationLock) {
...
//将通知加入队列
mEnqueuedNotifications.add(r);
scheduleTimeoutLocked(r);
...
if (mAssistants.isEnabled()) {
mAssistants.onNotificationEnqueuedLocked(r);
mHandler.postDelayed(new PostNotificationRunnable(r.getKey()),
DELAY_FOR_ASSISTANT_TIME);
} else {
mHandler.post(new PostNotificationRunnable(r.getKey()));
}
}
}
}在EnqueueNotificationRunnable最后又会发送一个PostNotificationRunable，
PostNotificationRunable源码如下所示。
protected class PostNotificationRunnable implements Runnable {
private final String key;
PostNotificationRunnable(String key) {
this.key = key;
}
@Override
public void run() {
synchronized (mNotificationLock) {
try {
...
//发送通知
if (notification.getSmallIcon() != null) {
StatusBarNotification oldSbn = (old != null) ? old.getSbn() : null;
mListeners.notifyPostedLocked(r, old);
if ((oldSbn == null || !Objects.equals(oldSbn.getGroup(), n.getGroup()))
&& !isCritical(r)) {
mHandler.post(new Runnable() {
@Override
public void run() {
mGroupHelper.onNotificationPosted(
n, hasAutoGroupSummaryLocked(n));
}
});
} else if (oldSbn != null) {
final NotificationRecord finalRecord = r;
mHandler.post(() -> mGroupHelper.onNotificationUpdated(
finalRecord.getSbn(), hasAutoGroupSummaryLocked(n)));
}
} else {
//...
}
} finally {
...
}
}
}
}从代码中可以看出，PostNotificationRunable类中会调用notifyPostedLocked方法，这里你可能会有疑问：这里分明判断notification.getSmallIcon()是否为null，不为null时才会进入notifyPostedLocked方法。为什么这里直接默认了呢？这是因为在Android5.0中规定smallIcon不可为null，且NotificationListenerService仅适用于5.0以上，所以这里是必然会执行到notifyPostedLocked方法的。
其方法源码如下所示。
private void notifyPostedLocked(NotificationRecord r, NotificationRecord old,
boolean notifyAllListeners) {
try {
// Lazily initialized snapshots of the notification.
StatusBarNotification sbn = r.getSbn();
StatusBarNotification oldSbn = (old != null) ? old.getSbn() : null;
TrimCache trimCache = new TrimCache(sbn);
//循环通知每个ManagedServiceInfo对象
for (final ManagedServiceInfo info : getServices()) {
...
mHandler.post(() -> notifyPosted(info, sbnToPost, update));
}
} catch (Exception e) {
Slog.e(TAG, "Could not notify listeners for " + r.getKey(), e);
}
}
notifyPostedLocked方法最终会调用notifyPosted方法，我们再来看notifyPosted方法。
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) {
Slog.e(TAG, "unable to notify listener (posted): " + info, ex);
}
}
notifyPosted方法，最终会调用INotificationListerner的onNotificationPosted方法，这样就通知到了NotificationListenerService的onNotificationPosted方法。
上述方法的流程图如下图所示。
NotificationListenerService注册
在NotificationListenerService中通过registerAsSystemService方法注册服务，代码如下所示。
@SystemApi
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);
}
registerAsSystemService方法将NotificationListenerWrapper对象注册到NotificationManagerService中。如此就实现了对系统通知的监听。
总结
NotificationListenerService实现对系统通知的监听可以概括为三步：
- NotificationListenerService将 NotificationListenerWrapper注册到NotificationManagerService中。
- 当有通知被发送时 ，NotificationManagerService跨进程通知到每个NotificationListenerWrapper。
- NotificationListenerWrapper中信息由NotificationListenerService类中的Handler中处理，从而调用NotificationListenerService中对应的回调方法。