异步消息处理机制相关面试问题-intentservice面试问题详解

IntentService是什么？

IntentService是继承并处理异步请求的一个类，在IntentService内有一个工作线程来处理耗时操作，启动IntentService方法和启动传统的Service一样，同时，当任务执行完后，IntentService会自动停止，而不需要我们手动的stopSelf()。另外，可以启动IntentService多次，而每个耗时操作会以工作队列的方式在IntentService的onHandleIntent回调方法中执行，并且，每次只会执行一个线程，执行完第一个再执行第二个。

• 它本质上是一种特殊的Service，继承自Service并且本身是一个抽象类。
• 它内部是通过HandlerThread和Handler实现异步操作的。

IntentService使用方法：

关于它的使用可以参考之前写的博客：http://www.cnblogs.com/webor2006/p/4305715.html

下面总结一下：创建IntentService时，只需要实现onHandleIntent()和构造方法，onHandleIntent()为异步方法，可以执行耗时操作。

IntentService源码解析：

跟HandlerThread的源代码类似，代码也不多，如下：

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(@Nullable 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(@Nullable 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
    @Nullable
    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)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

而首先先看一下它的onCreate()方法：

首先会创建一个HandlerThread,然后再根据它的Looper来创建一个Handler，如下：

此时就可以在Handler中执行异步任务了。那IntentService是如何启动异步任务的呢？这里可以看一下onStrartCommand()方法：

又调用了onStart()方法：

接着就会到handler的handleMessage()方法中去处理了：

而onHandleIntent()方法是个抽象方法，由咱们自己来实现：

而其中调用停止任务时传了一个参数：

这时就会等待所有任务处理完之后才会停止，而不是立马停止。也就是如果有很多个任务都要由IntentService处理完之后，才会去让IntentService去停止，而非执行完一个就立马停止了。

它的本质就是一个封装了HandlerThread和handler的异步框架。