Android:使用 AsyncTask 进行重复 Ajax 调用的含义 [英] Android: Implication of using AsyncTask to make repeated Ajax Calls

问题描述

我需要我的 Android 应用程序使用 AJAX 调用定期从服务器获取数据，并相应地更新 UI(只是一堆需要使用 setText() 更新的 TextView).请注意，这涉及 2 个任务:

I need my Android app to periodically fetch data from a server using AJAX calls, and update the UI accordingly (just a bunch of TextViews that need to be updated with setText()). Note that this involves 2 tasks:

1. 进行 AJAX 调用，并在收到响应后更新 UI - 为此，我使用了一个简单的 AsyncTask.
2. 定期重复上述操作.

我还没有想出一种优雅的方法来实现上面的第 2 点.目前，我只是从 OnPostExecute() 执行任务本身.我在 SO 上的这个线程 上读到，我不需要担心垃圾收集与 AsyncTask 对象有关.

I haven't figured out an elegant way to achieve Point 2 above. Currently, I am simply executing the task itself from OnPostExecute(). I read on this thread at SO that I need not worry about garbage collection as far as the AsyncTask objects are concerned.

但我仍然不确定如何设置一个计时器，以便在它到期后触发我的 AsyncTask.任何指针将不胜感激.这是我的代码:

But I'm still unsure as to how I set up a timer that will fire my AsyncTask after it expires. Any pointers will be appreciated. Here is my code:

public class MyActivity extends Activity {


    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        new AjaxRequestTask().execute(MY_REST_API_URL);

    }

    private void updateReadings(String newReadings) {
           //Update the UI
        }

    class AjaxRequestTask extends AsyncTask<String, Integer, String> {

        @Override
        protected String doInBackground(String... restApiUrl) {
                    //Do AJAX Request
                }

        @Override
        protected void onPostExecute(String result) {
            updateReadings(result);
                     /*Is there a more elegant way to achieve this than create a new AsyncTask object every 10 seconds?  Also, How can I update the UI if I create a timer here? */
            new AjaxRequestTask().execute(MY_REST_API_URL);
        }

    }

}

提前致谢

我尝试发布答案但无法发布，因为我没有在 8 小时内回复的声誉.

I tried posting an answer but couldn't do it since I don't have the reputation to answer within 8 hours.

好吧，所以我找到了解决方案.不过我不相信.

Well, so I found a solution. I'm not convinced however.

protected void onPostExecute(String result) {

            updateReadings(result);
            // super.onPostExecute(result);
            new Timer().schedule(
                    new TimerTask() {
                        @Override
                        public void run() {
                            new AjaxRequestTask().execute(MY_REST_API_URL);
                        }
                    }, 
                    TIMER_ONE_TIME_EXECUTION_DELAY
            );
        }

1. 我在使用它时有什么需要注意的方面吗?特别是，我看到 LogCat 中发生了很多 GC.另外，我想知道 AsyncTask 如何成为 GC 的候选对象，除非 onPostExecute() 完成?

1. Are there any flip sides that I should be aware of when I use this? In particular, I am seeing lots of GCs happening in the LogCat. Also, I am wondering how an AsyncTask can be candidate for GC unless the onPostExecute() completes?

如何停止"更新?我想到的一种方法是将第一个 AsyncTask 实例作为 Activity 的成员变量.这样，我可以在它上面调用 cancel(true) 并希望这会停止"任务.

How can I "stop" the updates? One way I thought of was to make the very first AsyncTask instance as a member variable of the Activity. That way, I can invoke cancel(true) on it and hope that this will "stop" the tasks.

<小时>

解决方案:

如果有人正在寻找类似的东西 - 我在这里提到的所有解决方案都不能令人满意.它们都遇到 OutOfMemory 问题.我没有调试 OOM 的细节，但我怀疑这可能是因为递归，或者因为将 HTTP 相关的对象作为 AsyncTask 中的成员变量而不是作为成员变量Activity(主要是因为没有重用 HTTP 和其他对象).

---

SOLUTION:

In case anyone is looking for something similar - none of the solutions I mentioned here work satisfactorily. They all suffer from OutOfMemory issues. I did not debug into the details of the OOM, but I suspect it could either be because of the recursion, or because of having HTTP-related objects as member variables in the AsyncTask rather than as members of the Activity (basically because of NOT reusing HTTP and other objects).

我为另一种方法放弃了这种方法 - 在我的 AsyncTask 的 doInBackground() 中无休止地进行 Ajax 调用；并在 onProgressUpdate() 中更新 UI.这样我也避免了维护太多线程或 Handler 来更新 UI 的开销(记住 UI 可以在 onProgressUpdate() 中更新).

I discarded this approach for a different one - making my Ajax Calls endlessly in the doInBackground() of my AsyncTask; and updating the UI in onProgressUpdate(). That way I also avoid the overhead of maintaining too many threads or Handlers for updating the UI (remember UI can be updated in onProgressUpdate() ).

这种方法还消除了对 Timers 和 TimerTasks 的需要，有利于使用 Thread.sleep() 代替.SO 上的这个线程 有更多细节和代码片段也是.

This approach also eliminates the need for Timers and TimerTasks, favoring the use of Thread.sleep() instead. This thread on SO has more details and a code snippet too.

推荐答案

在任何 View 上调用 postDelayed() 来安排一大块代码在主视图上运行一定延迟后的应用程序线程.在 AsyncTask 的 onPostExecute() 中执行此操作以创建并执行另一个 AsyncTask.

Call postDelayed() on any View to schedule a hunk of code to be run on the main application thread after a certain delay. Do this in onPostExecute() of the AsyncTask to create and execute another AsyncTask.

您可以使用 AlarmManager，正如其他人所引用的那样，但我同意您的看法，对于纯粹在活动中发生的计时来说，这感觉有点矫枉过正.

You could use AlarmManager, as others have cited, but I would agree with you that it feels a bit like overkill for timing that occurs purely within an activity.

话虽如此，如果无论活动是否存在都应该发生 AJAX 调用，请务必考虑切换到 AlarmManager 和 IntentService.

That being said, if the AJAX calls should be occurring regardless of whether the activity exists, definitely consider switching to AlarmManager and an IntentService.

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