Android后台任务（HandlerThread、AsyncTask、IntentService）

线程只是最基本的应用，在复杂的场景下使用Thread代码可读性会变的很多差而且也容易出错。
android为了简化开发者的工作量，提供了一些更容易使用的封装。

HandlerThread

为了让Handler在非主线程工作，可以使用HandlerThread。
如题使用如下：

    Handler handler;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        HandlerThread handlerThread = new HandlerThread("backThread",Process.THREAD_PRIORITY_BACKGROUND);
        handlerThread.start();
        handler = new Handler(handlerThread.getLooper());
    }

使用HandlerThread的Looper之后，Handler就是在HandlerThread所在线程中处理消息了。
其实HandlerThread就是新建了线程，然后调用了Looper.prepare();和Looper.loop();

AsyncTask

AsyncTask 现在使用的人已经不多了，它也是一个使用很简单的后台任务类，开发者不需要关注Thread和Handler就能在后台线程完成轻量级和时间较短的任务，并且最后在UI线程更新。

/*第一个为入参类型
 *第二个是进度类型
 *第三个是结果类型
private class DownloadFilesTask extends AsyncTask {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
             // Escape early if cancel() is called
             if (isCancelled()) break;
         }
         return totalSize;
     }

     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

下面可以看方法在哪个线程调用 ：

    @WorkerThread
    protected abstract Result doInBackground(Params... params); 

    @MainThread
    protected void onPreExecute() {
    }
    @MainThread
    protected void onPostExecute(Result result) {
    }

    @MainThread
    protected void onProgressUpdate(Progress... values) {
    }

其实AsyncTask有一个包含一个静态的线程池，最小同时运行两个线程，最多4个线程。

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    // We want at least 2 threads and at most 4 threads in the core pool,
    // preferring to have 1 less than the CPU count to avoid saturating
    // the CPU with background work
    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE_SECONDS = 30;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };

    private static final BlockingQueue sPoolWorkQueue =
            new LinkedBlockingQueue(128);

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

其中还有一个handler用于更新UI线程。

    private static Handler getMainHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

    private Handler getHandler() {
        return mHandler;
    }

由于都是静态的，所以其实所有的AsyncTask都是公用这个线程池和Handler。
AsyncTask使用起来也很简单，但是有一些限制，只能在UI线程实例化和调用。

new DownloadFilesTask().execute(url1,url2,url3);

IntentService

IntentService是为了处理异步任务而对Service的封装。
其实原理也很简单。

    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);
        }
    }

    @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);
    }

就是在Service的OnCreate里创建了一个HandlerThread，然后把需要处理的Intent发送到这个Handler在后台线程中处理并且完毕后调用stopSelf关闭服务。
使用上和Service一样，就是需要重写onHandleIntent方法。