Android中的线程形态-IntentService

• IntentService 可用于执行后台耗时的任务，当任务执行完时它会自动停止。IntentService比较适合执行一些优先级高的后台任务。它封装ThreadHandler和Handler

第一次启动IntentService的时候会调用onCreate（），onCreate方法会创建一个HandlerThread然后通过一个Looper来构造一个Handler对象。这样通过mServiceHandler 发送的消息最终都会在HandlerThread中执行。每次启动IntentService，它的onStartCommand方法就会被调用一次，IntentService在onStartCommand中处理每个后台任务的Intent。每个onStarCommand会调用onStart

    @Override
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

/**
     * You should not override this method for your IntentService. Instead,
     * override {@link #onHandleIntent}, which the system calls when the IntentService
     * receives a start request.
     * @see android.app.Service#onStartCommand
     */
    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

IntentService仅仅是通过mServiceHandler发送了一个消息，这个消息会在HandlerThread中被处理。mServiceHandler收到消息后，会将Intent对象传递给onHandlerIntent方法处理。

 @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

stopSelf（int startID）：会等所有消息都处理完后在终止服务。在终止之前需要判断最近启动服务的次数是否和startId相等。onHandleIntent（）执行完最后一个任务的时候，stopSelf（int startId）才会停止服务。当有多个后台任务同时存在时，后台任务会按照外界发起的顺序排队执行。

 private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }