IntentService理解总结

平常用的不是特别多，但是要知道内部原理，看看源码总结一下

首先为什么会有这个东西？

我们知道服务中 service 中的代码是运行在主线程中的，不能做耗时的操作，于是那如果想做耗时操作怎么做呢？就出来 IntentService 这么个东西，用来处理后台服务中的耗时操作并且可以在任务完成之后，自动停止服务。

基本使用

public class MyIntentService extends IntentService {

    public MyIntentService() {
    super("MyIntentService"); 
    }

    //处理异步任务
    @Override
    protected void onHandleIntent(Intent intent) {
    // 打印当前线程的id
    Log.d("MyIntentService", "Thread id is " + Thread.currentThread(). getId());
    }

    @Override
    public void onDestroy() {
    super.onDestroy();
    Log.d("MyIntentService", "onDestroy executed");
    }
}
//开启服务
Intent intentService = new Intent(this, MyIntentService.class);
startService(intentService);

通过打印当前线程的 id 我们可以知道当前线程不是在主线程中，执行完成后，会自动执行 onDestroy() 方法。以上属于 IntentService 的基本用法。

有什么好处呢？

• 省去在 Service 中手动开线程的麻烦
• 当操作任务完成后自动停止服务

源码

/**
 * IntentService is a base class for {@link Service}s that handle asynchronous
 * requests (expressed as {@link Intent}s) on demand.  Clients send requests
 * through {@link android.content.Context#startService(Intent)} calls; the
 * service is started as needed, handles each Intent in turn using a worker
 * thread, and stops itself when it runs out of work.
 *
 * <p>This "work queue processor" pattern is commonly used to offload tasks
 * from an application's main thread.  The IntentService class exists to
 * simplify this pattern and take care of the mechanics.  To use it, extend
 * IntentService and implement {@link #onHandleIntent(Intent)}.  IntentService
 * will receive the Intents, launch a worker thread, and stop the service as
 * appropriate.
 使用它，需要继承IntentService,并且实现 onHandleIntent方法，IntentService会收到这些Intent,开启一个工作线程，在合适的时间会自动停止服务
 *
 * <p>All requests are handled on a single worker thread -- they may take as
 * long as necessary (and will not block the application's main loop), but
 * only one request will be processed at a time.
 所有的请求被处理在一个单一的工作线程中，不会阻塞主线程，但是在同一时间指挥处理一个请求
 *
 * <div class="special reference">
 * <h3>Developer Guides</h3>
 * <p>For a detailed discussion about how to create services, read the
 * <a href="{@docRoot}guide/components/services.html">Services</a> developer
 * guide.</p>
 * </div>
 *官网文档链接
 * @see android.os.AsyncTask
 */
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) {
            //异步处理任务
            onHandleIntent((Intent)msg.obj);
            //停止自身服务
            //stopSelf()： 会立马结束服务

			//stopSelf(int startId)： 等待所有消息都处理完后才终止服务
            stopSelf(msg.arg1);
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public IntentService(String name) {
        super();
        mName = name;
    }

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     * with your preferred semantics.
     *
     * <p>If enabled is true,
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
     * {@link #onHandleIntent(Intent)} returns, the process will be restarted
     * and the intent redelivered.  If multiple Intents have been sent, only
     * the most recent one is guaranteed to be redelivered.
     *
     * <p>If enabled is false (the default),
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     */
    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();
        //创建了一个HandlerThread
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();
		//取出HandlerThread线程的 looper ,由于 `HandlerThread` 又是一个异步线程，当我们把 
        //Looper 传递给 ServiceHandler时，使得 ServiceHandler也变成了一个异步执行的线程
        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 {@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;
    }

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

    /**
     * Unless you provide binding for your service, you don't need to implement this
     * method, because the default implementation returns null.
     * @see android.app.Service#onBind
     */
    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     * This method is invoked on the worker thread with a request to process.
     * Only one Intent is processed at a time, but the processing happens on a
     * worker thread that runs independently from other application logic.
     * So, if this code takes a long time, it will hold up other requests to
     * the same IntentService, but it will not hold up anything else.
     * When all requests have been handled, the IntentService stops itself,
     * so you should not call {@link #stopSelf}.
     *
     * @param intent The value passed to {@link
     *               android.content.Context#startService(Intent)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

总结

IntentService.java 总结来说就是：一个 HandlerThread + ServiceHandler , 以队列的形式处理异步操作的类。

IntentService 是继承与 Service 的，在 onCreate() 时，创建了 HandlerThread 实例（ HandlerThread 又是啥呢，它是 Thread 子类，内部拿到一个当前线程的 Looper, Looper 一直轮询消息，获得消息，处理消息），而 IntentService 内部有一个ServiceHandler,ServiceHandler 的创建的 Looper 是拿的是 HandlerThread 的 Looper , IntentService 在 onStart() 通过发送 Message, 在处理 Message 调用的是 onHandleIntent() 方法。