IntentService簡介及其使用

          IntentService是Service類的子類，用來處理異步請求。客戶端可以通過startService(Intent)方法傳遞請求給IntentService。IntentService在onCreate()函數中通過HandlerThread單獨開啓一個線程來處理所有Intent請求對象（通過startService的方式發送過來的）所對應的任務，這樣以免事務處理阻塞主線程。執行完所一個Intent請求對象所對應的工作之後，如果沒有新的Intent請求達到，則自動停止Service；否則執行下一個Intent請求所對應的任務。

　　IntentService在處理事務時，還是採用的Handler方式，創建一個名叫ServiceHandler的內部Handler，並把它直接綁定到HandlerThread所對應的子線程。 ServiceHandler把處理一個intent所對應的事務都封裝到叫做onHandleIntent的虛函數；因此我們直接實現虛函數onHandleIntent，再在裏面根據Intent的不同進行不同的事務處理就可以了。

另外，IntentService默認實現了Onbind（）方法，返回值爲null。

　　使用IntentService需要兩個步驟：

　　1、寫構造函數

　　2、實現虛函數onHandleIntent，並在裏面根據Intent的不同進行不同的事務處理就可以了。

好處：處理異步請求的時候可以減少寫代碼的工作量，比較輕鬆地實現項目的需求

注意：IntentService的構造函數一定是參數爲空的構造函數，然後再在其中調用super("name")這種形式的構造函數。

因爲Service的實例化是系統來完成的，而且系統是用參數爲空的構造函數來實例化Service的

關於Handler和Service的更多知識請閱讀《Looper和Handler》，《關於Handler技術》，《Service簡介》，《AIDL和Service實現兩進程通信》

Public Constructors
	IntentService(String name) Creates an IntentService.

Public Methods
IBinder	onBind(Intent intent) Unless you provide binding for your service, you don't need to implement this method, because the default implementation returns null.
void	onCreate() Called by the system when the service is first created.
void	onDestroy() Called by the system to notify a Service that it is no longer used and is being removed.
void	onStart(Intent intent, int startId) This method is deprecated. Implement onStartCommand(Intent, int, int) instead.
int	onStartCommand(Intent intent, int flags, int startId) You should not override this method for your IntentService.
void	setIntentRedelivery(boolean enabled) Sets intent redelivery preferences. If enabled is true, onStartCommand(Intent, int, int) will return START_REDELIVER_INTENT, so if this process dies before 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. If enabled is false (the default), onStartCommand(Intent, int, int) will return START_NOT_STICKY, and if the process dies, the Intent dies along with it. 設置爲true時，onStartCommand返回START_REDELIVER_INTENT，否則返回START_NOT_STICKY 關於此的更多內容請參考《Service簡介》

Protected Methods

abstract void

onHandleIntent(Intent intent) This method is invoked on the worker thread with a request to process. 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 stopSelf(). 該函數用於針對Intent的不同進行不同的事務處理就可以了.執行完所一個Intent請求對象所對應的工作之後，如果沒有新的Intent請求達到， 則自動停止Service；否則ServiceHandler會取得下一個Intent請求傳人該函數來處理其所對應的任務。

實例1

MyIntentService.java文件

package com.lenovo.robin.test;
import android.app.IntentService;
import android.content.Intent;
import android.util.Log;

public class MyIntentService extends IntentService {
final static String TAG="robin";
 public MyIntentService() {
  super("com.lenovo.robin.test.MyIntentService");
  Log.i(TAG,this+" is constructed");
 }
 @Override
 protected void onHandleIntent(Intent arg0) {
  Log.i(TAG,"begin onHandleIntent() in "+this);
  try {
   Thread.sleep(10*1000);
  } catch (InterruptedException e) {
     e.printStackTrace();
  }
  Log.i(TAG,"end onHandleIntent() in "+this);
 }
 public void onDestroy()
 {
  super.onDestroy();
  Log.i(TAG,this+" is destroy");
 }
}

啓動MyIntentServic的代碼片段 

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

AndroidManifest.xml文件代碼片段

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

運行結果

09-14 22:23:34.730: I/robin(30943): com.lenovo.robin.test.MyIntentService@40541370 is constructed

09-14 22:23:34.730: I/robin(30943): begin onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:23:44.730: I/robin(30943): end onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:23:44.730: I/robin(30943): begin onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:23:54.740: I/robin(30943): end onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:23:54.740: I/robin(30943): begin onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:24:04.739: I/robin(30943): end onHandleIntent() in com.lenovo.robin.test.MyIntentService@40541370

09-14 22:24:04.739: I/robin(30943): com.lenovo.robin.test.MyIntentService@40541370 is destroy

IntentService源碼

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;

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);
        }
    }

    public IntentService(String name) {
        super();
        mName = name;
    }

    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();
    }
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

   protected abstract void onHandleIntent(Intent intent);
}