照例来看一下IntentService的类说明
/**
* 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.
*
* <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.
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For a detailed discussion about how to create services, read the
* <a href="{@docRoot}guide/topics/fundamentals/services.html">Services</a> developer guide.</p>
* </div>
*
* @see android.os.AsyncTask
*/
IntentService是一个Service的子类，用于在需要的时候处理异步的请求，这些请求参数是通过Intent来传递的。客户端通过startService(Intent)来传递请求，IntentService会用一个工作线程轮流处理每个Intent，当所有的Intent都处理完成之后，它会自动停止自己。
所有的请求都会在一个单独的线程中处理，同一时间只能处理一个请求，意思是说所有的请求是按照一个一个地处理的，只有处理完了一个请求才会处理下一个请求。
接着来看成员变量
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;
private String mName;
private boolean mRedelivery;
保持了一个Looper和ServiceHandler的引用，Looper是Handler消息机制必不可少的对象。我们来看一下ServiceHandler。
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);
}
}
ServiceHandler是一个Handler的子类，在handleMessage中调用了onHandleIntent方法，并且自动停止了Service。但是工作线程在哪里呢？Service是运行在主线程中的，耗时任务同样无法在Service中运行。我们来看剩下的代码。
/**
* 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(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(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
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)}.
*/
@WorkerThread
protected abstract void onHandleIntent(Intent intent);
可以看到我们需要关注的是onCreate()、onStart()、onStartCommand()以及onDestroy()方法。
先来看onCreate()
@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);
}
创建了一个HandlerThread，这应该就是处理任务的线程了。接着初始化了mServiceLooper，然后创建了mServiceHandler = new ServiceHandler(mServiceLooper)。这是不是很熟悉呢，我们在使用HandlerThread结合Handler的时候就是这么使用的。
onStart()
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
onStart方法仅仅是将请求传递给mServiceHandler去处理而已。
onStartCommand
/**
* 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(Intent intent, int flags, int startId) {
onStart(intent, startId);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
onStartCommand方法也仅仅是将任务交给onStart方法去调度。当任务通过Intent传递过来的时候，onStartCommand方法将任务直接传递给onStart方法，由onStart方法将消息传递给Handler去处理。因为只有一个Thread去处理任务，所以同时过来多个任务时，也只是一个个地去执行。
onDestroy
@Override
public void onDestroy() {
mServiceLooper.quit();
}
在这里执行了Looper的退出方法，Looper退出了，Handler就不再处理消息，HandlerThread也会停止。
** 现在来所以下IntentService的大体流程**
首先我们会通过Intent将任务参数(比如说URL)打包，然后调用startService(Intent)来将参数传递到onStartCommand，接着onStartCommand将任务传递给onStart方法，由onStart方法使用Handler的消息机制，将任务即Intent传递给Handler去处理，由于该Handler是通过HandlerThread创建的，所以该Handler操作的代码是运行在HandlerThread中的，这样就实现了后台处理任务，并且不会阻塞UI线程。还要说一点的是，在任务执行完毕之后，IntentService会自动停止。
