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;
    }