AsyncTask:doInBackground()的返回值在哪里？

Question

调用时AsyncTask<Integer,Integer,Boolean>,其返回值为:

protected Boolean doInBackground(Integer... params)？

通常我们启动AsyncTask new AsyncTaskClassName().execute(param1,param2......);但它似乎没有返回值.

哪里可以doInBackground()找到返回值？

Answer 1

然后可以在onPostExecute中使用该值,您可能希望覆盖该值以便使用结果.

以下是Google文档的示例代码段:

 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));
          }
          return totalSize;
      }

      protected void onProgressUpdate(Integer... progress) {
          setProgressPercent(progress[0]);
      }

      protected void onPostExecute(Long result) {
          showDialog("Downloaded " + result + " bytes");
      }
 }

Answer 2

您可以doInBackground()通过调用类的get()方法来检索受保护的布尔值的返回值AsyncTask:

AsyncTaskClassName task = new AsyncTaskClassName();
bool result = task.execute(param1,param2......).get(); 

但要小心UI的响应性,因为get()等待计算完成并将阻止UI线程.
如果您正在使用内部类,则最好将该作业放入onPostExecute(布尔结果)方法中.

如果您只想更新用户界面,请AsyncTask为您提供以下两种选择:

要与执行的任务并行更新UI doInBackground()(例如更新a ProgressBar),您必须publishProgress()在doInBackground()方法内部调用.然后,您必须更新onProgressUpdate()方法中的UI .

要在任务完成时更新UI,您必须在onPostExecute()方法中执行此操作.

/** This method runs on a background thread (not on the UI thread) */
@Override
protected String doInBackground(String... params) {
    for (int progressValue = 0; progressValue  < 100; progressValue++) {
        publishProgress(progressValue);
    }
}

/** This method runs on the UI thread */
@Override
protected void onProgressUpdate(Integer... progressValue) {
    // TODO Update your ProgressBar here
}

/**
 * Called after doInBackground() method
 * This method runs on the UI thread
 */
@Override
protected void onPostExecute(Boolean result) {
   // TODO Update the UI thread with the final result
}

这样您就不必关心响应问题了.