IntentService详解和使用

1. IntentService概述

• 它本质是一种特殊的Service,继承自Service并且本身就是一个抽象类
• 它可以用于在后台执行耗时的异步任务，当任务完成后会自动停止
• 它拥有较高的优先级，不易被系统杀死（继承自Service的缘故），因此比较适合执行一些高优先级的异步任务
• 它内部通过HandlerThread和Handler实现异步操作
• 创建IntentService时，只需实现onHandleIntent和构造方法，onHandleIntent为异步方法，可以执行耗时操作
  2. 使用方法
  新建一个类，继承自IntentService

public class MyIntentServic  extends IntentService{
    /**
     需要实现一个无参的构造函数
     */
    private static final String TAG = "MyIntentServic";
    public MyIntentServic() {
        super(TAG);
    }
    /**
   实现onHandleIntent()方法。
     */
    @Override
    protected void onHandleIntent(Intent intent) {
       String tag=  intent.getStringExtra("tag");
        Log.e(TAG, "ag-------->"+tag);
        Log.e(TAG, "Thread.id----->"+Thread.currentThread());
        try {
            Thread.sleep(5*1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
 
    }
     /**
     并且还在其他生命周期方法中打印了log日志。
     */
    @Override
    public void onCreate() {
        super.onCreate();
        Log.e(TAG, "-------->onCreate:");
    }
 
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.e(TAG, "-------->onStartCommand:");
        return super.onStartCommand(intent, flags, startId);
    }
 
    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.e(TAG, "-------->onDestroy:");
    }
}

如何调用启动服务

 void startService() {
        Intent intent = new Intent(this,MyIntentServic.class);
        tag++;
        intent.putExtra("tag",tag+"");
        startService(intent);
    }

点击五次，查看打印log

打印log截图

注：和service一样，开启五次onCreate只执行一次，可见service实例只有一个，onStartCommand执行了5次,onHandleIntent也执行了5次

2. 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(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);
}

在onCreate()方法中，创建了一个线程HandlerThread对象，接着启动了该线程，然后将该线程的Looper对象赋值给内部类ServiceHandler。
ServiceHandler顾名思义，就是一个Handler，在它的handleMessage()方法中，调用了onHandleIntent((Intent)msg.obj)和stopSelf(msg.arg1)方法，而onHandleIntent((Intent)msg.obj)方法是一个抽象方法，需要用户自己实现；stopSelf(msg.arg1)方法，这不是停止Service的方法嘛！

参考这篇文章