Android `AsyncTask`簡要分析

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 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);

這裏簡單看一下，AsyncTask中的幾個經典問題：

1. AsyncTask開了幾個線程？
2. AsyncTask怎麼實現的線程調度？

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;

然後是第二個問題。簡單回答就是一句話，通過開啓線程執行具體邏輯，然後通過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在構造方法裏面，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 unchecked
                   result = 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) {
           @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