源码简读之AsyncTask

源码注释中声明：适用于a few minutes的任务。
封装了Handler+线程池。
核心线程数：至少2个，最多4个。
最大线程数：CPU核数*2+1
线程保活时间：30s

方法

• onPreExecute()
• doInBackground()
• onProgressUpdate()
• publishProgress()
• onPostExecute()

初始化

// 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 workprivate 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;// 内部使用的BlockingQueueprivate static final BlockingQueue<Runnable> sPoolWorkQueue =        new LinkedBlockingQueue<Runnable>(128);// 初始化线程池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;}// 在初始化时，先创建一个Callable对象的mWorker线程对象，然后再转换为FutureTaskpublic AsyncTask() {    mWorker = new WorkerRunnable<Params, Result>() {        public Result call() throws Exception {            mTaskInvoked.set(true);            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);            //noinspection unchecked            Result result = doInBackground(mParams);            Binder.flushPendingCommands();            return postResult(result);        }    };    mFuture = new FutureTask<Result>(mWorker) {        @Override        protected void done() {            try {                postResultIfNotInvoked(get());            } catch (InterruptedException e) {                android.util.Log.w(LOG_TAG, e);            } catch (ExecutionException e) {                throw new RuntimeException("An error occurred while executing doInBackground()",                        e.getCause());            } catch (CancellationException e) {                postResultIfNotInvoked(null);            }        }    };}// 处理返回的数据，发送给 mHandler；private Result postResult(Result result) {    @SuppressWarnings("unchecked")    Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,            new AsyncTaskResult<Result>(this, result));    message.sendToTarget();    return result;}// mHandler是一个获取了MainLooper的对象，根据super(Looper.getMainLooper())就可以把该Handler转变成一个可以操作主线程的handler。private static class InternalHandler extends Handler {    public InternalHandler() {        super(Looper.getMainLooper());    }    @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})    @Override    public void handleMessage(Message msg) {        AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;        switch (msg.what) {            case MESSAGE_POST_RESULT:                // There is only one result                result.mTask.finish(result.mData[0]);                break;            case MESSAGE_POST_PROGRESS:                result.mTask.onProgressUpdate(result.mData);                break;        }    }}