HandlerThread和IntentService

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前言

在这一篇文章中，会看HandlerThread和IntentService的源码。为什么一起讲它们呢？在Android中有一条思路，就是从java的线程，到Android中的消息机制，到将java线程和消息机制结合就是HandlerThread，而IntentService就是在HandlerThread基础上再与service结合在一起。

关于Android中的异步的东西，还有AsyncTask，AsyncTask是对java中的线程池的再次封装。进一步，可以联想到Loader.

回到本篇文章主题。

HandlerThread是一个直接继承于Thread的，并在run方法中将线程的Looper进行常规的初始化。而我们要做的就是提供一个Handler，并将Handler与HanderThread的Looper进行关联，通过Handler发送消息和处理消息。

IntentService直接继承于Service，在它的内部封装了HandlerThread的使用过程：提供一个Handler（即：ServiceHandler），并将Handler与HanderThread的Looper进行关联，然后它进一步将启动Service的Intent以消息的形式，通过Handler传给onHandleIntent方法，然后IntentService优雅的结束自己。我们要做的就是在onHandleIntent中做线程要做的事情。

源码

HandlerThread.jva

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;
    private @Nullable Handler mHandler;

    ...
    
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
    ...
}

HandlerThread的run方法做了主要的工作。创建Looper，在onLooperPrepare做开始循环前的初始化工作，开始Looper循环。

在具体使用HandlerThread时，就是创建Handler，将Handler与HandlerThread的Looper进行关联，然后通过Handler发送消息，处理消息。说明，消息是在HandlerThread线程中处理的。

IntentService.java

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;
    }

    ...

    @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);
}

IntentService是一个service子类,在onCreate中初始化了HandlerThread和ServiceHandler，并将ServiceHandler与HandlerThread的Looper进行关联。在onStart中，将启动服务的Intent封装进Message中，然后发给ServiceHandler。ServiceHandler再将Intent传递给onHandleIntent，最后优雅的结束自己。

在具体使用IntentService时，就是在onHandleIntent中正确的处理启动service的Intent即可。说明，onHandleIntent做的事情是在HandlerThread中进行的，因为HandlerThread的Looper与ServiceHandler已经关联，onHandleIntent是在ServiceHandler中被调用的（好啰嗦）。