IntentService 解析

概述

IntentService 是 Android 提供的一个异步自停止的服务子类，其目的用于解决开发者平时在使用 Service 时需要开启子线程执行耗时任务的繁琐步骤及可能事情做完后忘记停止 Service 的情况。需要了解 HandlerThread 类和 Android 的消息机制就能很轻松的了解 IntentService 类了。

IntentService 基本用法

• 创建继承 IntentService 的子类，并覆写 onHandleIntent() 方法。
• 调用 context.startService() 就可以了;

IntentService 封装

IntentService 是利用 HandleThread 再次封装。源码如下很少，但确非常精巧。

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);
}

• 从源码中可以看出在 onCreate() 方法中，创建了子线程的 Looper 和 Handler，这样就能从主线程中通知子线程做事了。(HandlerThread 类就是一个线程类，并在 run() 方法中创建属于子线程的 Looper，所以呢只要创建的 Handler 对象关联 HandlerThread 对象的 Looper 自然就能进行线程间的通讯了)。
• 在 IntentService 类的 onStartCommand() 可以看到调用了 onStart() 方法。onStart() 把服务启动消息通过 Handler 发送了出去。
• 再接着，消息对象通过 Handler 发送了出去，下一个要看的方法就是 ServiceHandler 类关于 handleMessage() 方法的覆写了。从源码的 handleMessage() 方法可以看出，调用 onHandleIntent()，onHandleIntent 执行完毕后就是停止此 Service 了。所以呢这就是需要覆写 onHandleIntent 的原因了。
• onDestroy() 方法对 HandlerThread 创建的 looper 进行了退出循环。