IntentService

Android上的後臺任務主要有三種實現方式，
一是按照常規的Java方式，自己寫執行緒，
二是使用SDK封裝好的後臺任務類AsyncTask，
三是使用Service。

     執行緒和AsyncTask都是和Activity的生命週期繫結的，而Service有自己的獨立生命週期。

1. IntentService是一個用於按需處理非同步請求的Service基類，呼叫方通過 startService(Intent)啟動服務， IntentService為每一個Intent開啟一個單獨的工作執行緒，並且在任務完成時自動終止服務 ****這種“工作佇列處理器”模式通常用於某個程式主執行緒之外的後臺任務。IntentService類簡化了這種機制。要使用這種工作佇列模式，只使用繼承IntentService並實現protected void onHandleIntent(Intent){ }方法。IntentService會接受Intents，啟動工作執行緒，並在合適的時候終止服務。

2. IntentService是繼承自Service的，從原始碼上看，它是Service、HandlerThread和Handler的強強聯合。

3. setIntentRedelivery(true),從字面理解是設定intent重投遞。如果設定為true，onStartCommand(Intent, int, int)將會返回START_REDELIVER_INTENT，如果onHandleIntent(Intent)返回之前程式死掉了，那麼程式將會重新啟動，intent重新投遞，如果有大量的intent投遞了，那麼只保證最近的intent會被重投遞。
   原始碼目錄\frameworks\base\core\java\android\app\IntentService.java:

   /** Copyright (C) 2008 The Android Open Source Project
    * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.*/
   package android.app;
   import android.content.Intent;
   import android.os.Handler;
   import android.os.HandlerThread;
   import android.os.IBinder;
   import android.os.Looper;
   import android.os.Message;
   
   /**
    * 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
    */
   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(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)}.
        */
       protected abstract void onHandleIntent(Intent intent);
   }