使用startForeground让android服务前台运行

最近在使用android 4.1系统的时候，发现在手机休眠一段时间后（1-2小时），后台运行的服务被强行kill掉，有可能是系统回收内存的一种机制，要想避免这种情况可以通过startForeground让服务前台运行，当stopservice的时候通过stopForeground去掉。

以下是android官方描述：

Running a Service in the Foreground

 A foreground service（前台服务） is a service that’s considered to be（被用户所认可的） something the user is actively aware of and thus not a candidate for（而不是一个候选的，可以在内存不足时，被系统杀死的） the system to kill when low on memory. A foreground service must provide a notification for the status bar（前台服务必须提供一个显示通知）, which is placed under the “Ongoing” heading（它是不可以忽略的）, which means that the notification cannot be dismissed unless the service is either stopped or removed from the foreground.（意思是通知信息不能被忽略，除非服务停止或主动移除，否则将一直显示。）

 For example, a music player that plays music from a service should be set to run in the foreground, because the user is explicitly aware of its operation（用户明确了解其运作）. The notification in the status bar might indicate the current song and allow the user to launch an activity to interact with the music player.

To request that your service run in the foreground, call startForeground(). This method takes two parameters: an integer that uniquely identifies the notification and the Notification for the status bar. For example:
Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),
System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ExampleActivity.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);

To remove the service from the foreground, call stopForeground(). This method takes a boolean, indicating whether to remove the status bar notification as well. This method does not stop the service. However, if you stop the service while it’s still running in the foreground, then the notification is also removed.

方法：

startForeground(int id, Notification notification)

参数：id 唯一的通知标识

 notification 通知信息 

stopForeground(boolean removeNotification) //停止前台服务

参数： removeNotification 是否移除之前的通知

Note: The methods startForeground() and stopForeground() were introduced in Android 2.0 (API Level 5). In order to run your service in the foreground on older versions of the platform, you must use the previoussetForeground() method—see the startForeground() documentation for information about how to provide backward compatibility.

For more information about notifications, see Creating Status Bar Notifications.
要想实现需求，我们只需要在onStartCommand里面调用 startForeground，然后再onDestroy里面调用stopForeground即可！

实际情况就譬如手机里面的音乐播放器一样，不管手机如何休眠，只要开始播放音乐了，就不会kill掉这个服务，一旦停止播放音乐，服务就可能被清掉。

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