IntentService 源码解析

Android系统中的Service一般是在后台长时间运行的，有时候我们需要一种轻量级的Service在后台运行结束之后就主动停止的，这时候推荐使用IntentService，可以看做是Service和HandlerThread的结合体，在完成了使命之后会自动停止，适合需要在工作线程处理UI无关任务的场景。

其具有如下几个特点：

• IntentService 是继承自 Service 并处理异步请求的一个类，在 IntentService 内有一个工作线程来处理耗时操作。
• 当任务执行完后，IntentService 会自动停止，不需要我们去手动结束。
• 如果启动 IntentService 多次，那么每一个耗时操作会以工作队列的方式在 IntentService 的 onHandleIntent 回调方法中执行，依次去执行，使用串行的方式，执行完自动结束。

// IntentService继承自Service，内部有一个HandlerThread对象。
public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;

    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);
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     */
    public IntentService(String name) {
        super();
        mName = name;
    }

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     */
    public void setIntentRedelivery(boolean enabled) {
        mRedelivery = enabled;
    }

    @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);
    }

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

    /**
     * You should not override this method for your IntentService. Instead,
     */
    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        mServiceLooper.quit();
    }

    /**
     * Unless you provide binding for your service,you don't need to implement this
     */
    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}