概述

IntentService 是 Android 提供的一個異步自停止的服務(wù)子類，其目的用于解決開發(fā)者平時在使用 Service 時需要開啟子線程執(zhí)行耗時任務(wù)的繁瑣步驟及可能事情做完后忘記停止 Service 的情況。需要了解 HandlerThread 類和 Android 的消息機制就能很輕松的了解 IntentService 類了。

IntentService 基本用法

• 創(chuàng)建繼承 IntentService 的子類，并覆寫 onHandleIntent() 方法。
• 調(diào)用 context.startService() 就可以了;

IntentService 封裝

IntentService 是利用 HandleThread 再次封裝。源碼如下很少，但確非常精巧。

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);
}

• 從源碼中可以看出在 onCreate() 方法中，創(chuàng)建了子線程的 Looper 和 Handler，這樣就能從主線程中通知子線程做事了。(HandlerThread 類就是一個線程類，并在 run() 方法中創(chuàng)建屬于子線程的 Looper，所以呢只要創(chuàng)建的 Handler 對象關(guān)聯(lián) HandlerThread 對象的 Looper 自然就能進(jìn)行線程間的通訊了)。
• 在 IntentService 類的 onStartCommand() 可以看到調(diào)用了 onStart() 方法。onStart() 把服務(wù)啟動消息通過 Handler 發(fā)送了出去。
• 再接著，消息對象通過 Handler 發(fā)送了出去，下一個要看的方法就是 ServiceHandler 類關(guān)于 handleMessage() 方法的覆寫了。從源碼的 handleMessage() 方法可以看出，調(diào)用 onHandleIntent()，onHandleIntent 執(zhí)行完畢后就是停止此 Service 了。所以呢這就是需要覆寫 onHandleIntent 的原因了。
• onDestroy() 方法對 HandlerThread 創(chuàng)建的 looper 進(jìn)行了退出循環(huán)。