Android Service startForeground() 可能不显示Notification问题

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：

		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：

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

    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：

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：

 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.

See Also

• stopForeground(boolean)

但是在我的eclipse中的javadoc却是看不到“must not be 0”的说明的！