Android Service startForeground() 可能不显示Notification问题

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我是靠谱客的博主狂野咖啡，这篇文章主要介绍Android Service startForeground() 可能不显示Notification问题，现在分享给大家，希望可以做个参考。

Android中的Service分前台服务和后台服务两类：

前台服务会在通知栏有一条不能被手动清除的Notification，当此前台服务由于内存不足而被系统kill掉的时候，此Notification也会同时消失，用户由此得知此服务已经停止了，起到一个通知用户服务是否还在工作；

后台服务则没有类似的Notification，即使被系统kill掉，用户也不会得到什么通知。

Service通过调用方法 startForeground (int id, Notification notification)方法设置是否为前台任务，如：

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Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),
System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ViewServerActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, getText(R.string.notification_title,
getText(R.string.notification_message), pendingIntent);
startForeground(ONGOING_NOTIFICATION, notification);

由此代码引发了一个需要特别注意的事情，代码片段中的NOGOING_NOTIFICATION的值，不能是0，否则就不会显示出Notification，究其原因只能看源码了：

service startForeground()和stopForeground()源码：

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public final void startForeground(int id, Notification notification) {
try {
mActivityManager.setServiceForeground(
new ComponentName(this, mClassName), mToken, id,
notification, true);
} catch (RemoteException ex) {
}
}

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public final void stopForeground(boolean removeNotification) {
try {
mActivityManager.setServiceForeground(
new ComponentName(this, mClassName), mToken, 0, null,
removeNotification);
} catch (RemoteException ex) {
}
}

可见是有mActivityManager.setServiceForeground()里面做了封装，mActivityManager是一个IActivityManager接口的实现类，此接口实现类的关系具体参考此博文：

http://blog.csdn.net/stonecao/article/details/6579710

其实最终运行到的是ActiveServices类中的setServiceForegroundLocked方法：

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public void setServiceForegroundLocked(ComponentName className, IBinder token,
int id, Notification notification, boolean removeNotification) {
final int userId = UserHandle.getCallingUserId();
final long origId = Binder.clearCallingIdentity();
try {
ServiceRecord r = findServiceLocked(className, token, userId);
if (r != null) {
if (id != 0) {
if (notification == null) {
throw new IllegalArgumentException("null notification");
}
if (r.foregroundId != id) {
r.cancelNotification();
r.foregroundId = id;
}
notification.flags |= Notification.FLAG_FOREGROUND_SERVICE;
r.foregroundNoti = notification;
r.isForeground = true;
r.postNotification();
if (r.app != null) {
updateServiceForegroundLocked(r.app, true);
}
getServiceMap(r.userId).ensureNotStartingBackground(r);
} else {
if (r.isForeground) {
r.isForeground = false;
if (r.app != null) {
mAm.updateLruProcessLocked(r.app, false, false);
updateServiceForegroundLocked(r.app, true);
}
}
if (removeNotification) {
r.cancelNotification();
r.foregroundId = 0;
r.foregroundNoti = null;
}
}
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}

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public void setServiceForegroundLocked(ComponentName className, IBinder token,
int id, Notification notification, boolean removeNotification) {
final int userId = UserHandle.getCallingUserId();
final long origId = Binder.clearCallingIdentity();
try {
ServiceRecord r = findServiceLocked(className, token, userId);
if (r != null) {
if (id != 0) {
if (notification == null) {
throw new IllegalArgumentException("null notification");
}
if (r.foregroundId != id) {
r.cancelNotification();
r.foregroundId = id;
}
notification.flags |= Notification.FLAG_FOREGROUND_SERVICE;
r.foregroundNoti = notification;
r.isForeground = true;
r.postNotification();
if (r.app != null) {
updateServiceForegroundLocked(r.app, true);
}
getServiceMap(r.userId).ensureNotStartingBackground(r);
} else {
if (r.isForeground) {
r.isForeground = false;
if (r.app != null) {
mAm.updateLruProcessLocked(r.app, false, false);
updateServiceForegroundLocked(r.app, true);
}
}
if (removeNotification) {
r.cancelNotification();
r.foregroundId = 0;
r.foregroundNoti = null;
}
}
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}

至此，我们应该很清楚了，如果传进来的id=0,则直接执行以下代码了：

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 if (removeNotification) {
r.cancelNotification();
r.foregroundId = 0;
r.foregroundNoti = null;
}

最后直接cancleNotification()，所以传id=0，就相当于取消notification显示！

从google上的文档看到以下参数说明：

Parameters

id	The identifier for this notification as per `NotificationManager.notify(int, Notification)`; must not be 0.
notification	The Notification to be displayed.