IntentService源码分析

1.前言

想必大家应该都使用过Service组件，为什么有service，还需要IntentService呢。它们有什么不同，带着这些问题，下面我们分析下源码。

2.IntentService源码分析

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.
    *
    * @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();
       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 {@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);
}

2.1 onCreate方法

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);
    }

public class HandlerThread extends Thread

可以看出在onCreate方法中HandlerThread变量。可以看出HandlerThread其实就是一个线程。在此启动一个线程。同时定义一个ServiceHandler的Handler，由于接收处理onStart发出的Message。接下来看看onStartCommand&onStart方法。

2.2 onStartCommand和onStart方法

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

在启动这个service的时候会调用onStartCommand方法，onStartCommand方法又会调用onStart方法，onStart干了什么事情呢？其实就是发送Message给到ServiceHandler中。

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);
        }
    }

然后处理onHandlerIntent()这个抽象方法。onHandlerIntent方法跑在onCreate方法开启的线程中。然后stopSelf()。

2.3 onDestroy方法

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

服务停止后会执行onDestroy方法，在onDestroy方法中会清除消息队列Message。所以IntentService多次启动，如果服务停止了，会清除所有消息队列的任务,后面的任务不会执行。因为都跑在一个消息队列中，多次启动服务不会执行onCreate方法，所以不会创建多个消息队列。

2.4 总结

由此可以看出Service和IntentService的区别
1.Service与IntentService本质上是一个Service组件。
2.Service不能处理耗时过长的任务，会引起ANR, 因为service运行在UI线程上。
3.Service需要处理耗时任务必须自己开启一个线程。
4.IntentService可以处理耗时任务，并且不需要关心是否开启工作线程，继承它就可以了，实现onHandleIntent抽象方法处理耗时操作就行了。
5.使用IntentService不需要关心关闭Service。在任务完成后会自动stopSelf掉。
6.由源码看出，IntentService不适合用bindService启动，因为onBind返回null，同时用bindService IntentService也不会回调onHandleIntent方法，也不会在service任务完成后停止。如果使用bindService启动IntentService，那仅仅是一个普通的Service。

3.如何使用

3.1继承IntentService实现onHandleIntent方法

/**
 * Created by zw on 2019/7/7 11:31
 */
public class MyIntentService extends IntentService {
    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public MyIntentService(String name) {
       // 可以定义工作线程名称
        super(name);
    }

    @Override
    protected void onHandleIntent(@Nullable Intent intent) {
      // 处理耗时任务
    }
}

3.2 AndriodManifest.xml中注册这个Service

<service android:name=".MyIntentService"/>

3.3 启动服务

startService(new Intent(this, MyIntentService.class));

That's All

IntentService源码onCreate方法里出现的Looper，Message, Handler另外MessageQueue将在接下来分析它们。