Android `AsyncTask`简要分析

AsyncTask简要分析

经典异步任务：AsyncTask,使用场景有：批量下载，批量拷贝等。官方文档就直接给出了一个批量下载的示例。

    private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {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 calledif (isCancelled()) break;}return totalSize;}protected void onProgressUpdate(Integer... progress) {setProgressPercent(progress[0]);}protected void onPostExecute(Long result) {showDialog("Downloaded " + result + " bytes");}}// Once created, a task is executed very simply:new DownloadFilesTask().execute(url1, url2, url3);

这里简单看一下，AsyncTask中的几个经典问题：

1. AsyncTask开了几个线程？
2. AsyncTask怎么实现的线程调度？

好吧，就想到了这两个问题。第一个问题，明显是坑。答案是未知的。看手机 cpu个数了。

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;

然后是第二个问题。简单回答就是一句话，通过开启线程执行具体逻辑，然后通过handler将结果发送到主线程。

进一步追踪的话，就会发现，AsyncTask = ThreadPoolExecutor + Handler 。

通过源码可以看到，AsyncTask在构造方法里面，已经完成了全部的准备工作。

• MainHandler已经创建。

• FutureTask已经准备了。（说实话，FutureTask难过的让人吐血）

       public AsyncTask(@Nullable Looper callbackLooper) {mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()? getMainHandler(): new Handler(callbackLooper);mWorker = new WorkerRunnable<Params, Result>() {public Result call() throws Exception {mTaskInvoked.set(true);Result result = null;try {Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);//noinspection uncheckedresult = doInBackground(mParams);  // mParams 核心逻辑Binder.flushPendingCommands();} catch (Throwable tr) {mCancelled.set(true);throw tr;} finally {postResult(result);  // 核心逻辑}return result;}};mFuture = new FutureTask<Result>(mWorker) {@Overrideprotected void done() {try {postResultIfNotInvoked(get());  // 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);}}};}

• 系消息

    @MainThreadpublic final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,Params... params) {if (mStatus != Status.PENDING) {switch (mStatus) {case RUNNING:throw new IllegalStateException("Cannot execute task:"+ " the task is already running.");case FINISHED:throw new IllegalStateException("Cannot execute task:"+ " the task has already been executed "+ "(a task can be executed only once)");}}mStatus = Status.RUNNING;onPreExecute();mWorker.mParams = params;  // 核心逻辑exec.execute(mFuture);   // 核心逻辑return this;}// 核心逻辑：handler，将结果发送到主线程private Result postResult(Result result) {@SuppressWarnings("unchecked")Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,new AsyncTaskResult<Result>(this, result));message.sendToTarget();return result;}
  原文地址https://www.cnblogs.com/pythoncat/p/10514933.html