Android `AsyncTask`簡要分析

AsyncTask簡要分析

經典異步任務：AsyncTask,使用場景有：批量下載，批量拷貝等。官方文檔就直接給出了一個批量下載的示例。

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);

這里簡單看一下，AsyncTask中的幾個經典問題：

AsyncTask開了幾個線程？

AsyncTask怎么實現的線程調度？

好吧，就想到了這兩個問題。第一個問題，明顯是坑。答案是未知的。看手機?cpu個數了。

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;

然后是第二個問題。簡單回答就是一句話，通過開啟線程執行具體邏輯，然后通過handler將結果發送到主線程。

進一步追蹤的話，就會發現，AsyncTask = ThreadPoolExecutor + Handler 。

通過源碼可以看到，AsyncTask在構造方法里面，已經完成了全部的準備工作。

• MainHandler已經創建。

• FutureTask已經準備了。（說實話，FutureTask難過的讓人吐血）

  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 核心邏輯Binder.flushPendingCommands();} catch (Throwable tr) {mCancelled.set(true);throw tr;} finally {postResult(result); // 核心邏輯}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; // 核心邏輯exec.execute(mFuture); // 核心邏輯return this;}// 核心邏輯：handler，將結果發送到主線程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

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