Android `AsyncTask`简要分析

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 called if (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 unchecked result = 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) { @Override protected 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

   @MainThread public 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