Service中onStartCommand的返回值问题

Android开发中，每次调用startService(Intent)时都会调用service对象的onStartCommand(Intent intent,int flag,int startId)方法，其返回值是一个常量，有四种类型：

/**

* Constant to return from {@link #onStartCommand}: if this service's

* process is killed while it is started (after returning from

* {@link #onStartCommand}), then leave it in the started state but

* don't retain this delivered intent. Later the system will try to

* re-create the service. Because it is in the started state, it will

* guarantee to call {@link #onStartCommand} after creating the new

* service instance; if there are not any pending start commands to be

* delivered to the service, it will be called with a null intent

* object, so you must take care to check for this.

*

* <p>This mode makes sense for things that will be explicitly started

* and stopped to run for arbitrary periods of time, such as a service

* performing background music playback.

*/

public static final int START_STICKY = 1;即当服务被杀死后，保留service的状态为开始状态，不会保留传递的Intent。随后系统会尝试重新创建service，由于服务状态为开始状态，所有创建是会调用onStartCommand方法。如果在此期间没有任何启动命令被传递到service，那么Intent参数为null。

/**

* Constant to return from {@link #onStartCommand}: if this service's

* process is killed while it is started (after returning from

* {@link #onStartCommand}), and there are no new start intents to

* deliver to it, then take the service out of the started state and

* don't recreate until a future explicit call to

* {@link Context#startService Context.startService(Intent)}. The

* service will not receive a {@link #onStartCommand(Intent, int, int)}

* call with a null Intent because it will not be re-started if there

* are no pending Intents to deliver.

*

* <p>This mode makes sense for things that want to do some work as a

* result of being started, but can be stopped when under memory pressure

* and will explicit start themselves again later to do more work. An

* example of such a service would be one that polls for data from

* a server: it could schedule an alarm to poll every N minutes by having

* the alarm start its service. When its {@link #onStartCommand} is

* called from the alarm, it schedules a new alarm for N minutes later,

* and spawns a thread to do its networking. If its process is killed

* while doing that check, the service will not be restarted until the

* alarm goes off.

*/

public static final int START_NOT_STICKY = 2;如果在执行完onStartCommand后，服务被杀死，系统不会自动重启服务。

/**

* Constant to return from {@link #onStartCommand}: if this service's

* process is killed while it is started (after returning from

* {@link #onStartCommand}), then it will be scheduled for a restart

* and the last delivered Intent re-delivered to it again via

* {@link #onStartCommand}. This Intent will remain scheduled for

* redelivery until the service calls {@link #stopSelf(int)} with the

* start ID provided to {@link #onStartCommand}. The

* service will not receive a {@link #onStartCommand(Intent, int, int)}

* call with a null Intent because it will will only be re-started if

* it is not finished processing all Intents sent to it (and any such

* pending events will be delivered at the point of restart).

*/

public static final int START_REDELIVER_INTENT = 3;重传Intent。如果执行完onStartCommand后被杀死，系统被自动重启服务，并将Intent传入。

/**

* Constant to return from {@link #onStartCommand}: compatibility

* version of {@link #START_STICKY} that does not guarantee that

* {@link #onStartCommand} will be called again after being killed.

*/

public static final int START_STICKY_COMPATIBILITY = 0;START_STICKY的兼容版本。