从Handler调用的AsyncTask不会执行doInBackground [英] AsyncTask called from Handler will not execute doInBackground

查看：18 发布时间：2022/1/7 22:10:00 android android-asynctask

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问题描述

我正在开发的应用程序使用后台线程通过 API 下载图像列表，然后在幻灯片中显示图像.

The application I'm working on is using a background thread to download a list of images through a API, then display the images in a slideshow.

有一个后台任务(当前为 AsyncTask)来定期获取新图像.

There is a background task (currently AsyncTask) to periodically fetch the new images.

我没有收到关于错误线程等的任何错误消息，只是 AsyncTasks 的第二个实例不会运行 doInBackground 方法.

I'm not getting any error messages about the wrong Thread etc, it's just that the AsyncTasks's second instance will not run the doInBackground method.

这是活动中的一些代码:

Here is some code from the Activity:

private DownloadTask mDownloadTask = null;
private Handler mHandler;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            if(mDownloadTask != null) {
                mDownloadTask.cancel(true);
            }
            mDownloadTask = new DownloadTask();
            mDownloadTask.execute((Void[]) null);
        }
    };

    mDownloadTask = new DownloadTask();
    mDownloadTask.execute((Void[]) null);
}

DownloadTask 看起来像这样:

@Override
protected List<String> doInBackground(Void... voids) {
     // Download list of URLs from server, etc.
}

@Override
protected void onPostExecute(List<String> urls) {
    mHandler.sendEmptyMessageDelayed(111, 5000);
}

将调用处理程序，将调用 onPreExecute(在 AsyncTask 中)并且 DownloadTask 的初始运行(就在 onCreate 中)也有效.

The handler will be called, the onPreExecute (in the AsyncTask) will be called and the initial run of DownloadTask (right in onCreate) also works.

根据这个问题:Android SDK AsyncTask doInBackground not running (subclass) ，可能与 SDK15 相关.

According to this question: Android SDK AsyncTask doInBackground not running (subclass) , it might be SDK15 related.

感谢您的提示.

更新因为我收到评论说 Handler 可能不在 UI 线程中(这很奇怪，因为 Thread.currentThread 在 onCreate 和 Handlers 中是相同的handleMessage 方法，我修改了 handleMessage 方法为:

Update As I received comments that the Handler might not be in the UI thread (which is weird, as Thread.currentThread is the same both in onCreate and the Handlers handleMessage method, I revised the handleMessage method to:

mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                if(mDownloadTask != null) {
                    mDownloadTask.cancel(true);
                }
                mDownloadTask = new DownloadTask();
                mDownloadTask.execute((Void[]) null);
            }
        });
    }
};

仍然没有成功.

更新完整的 DownloadTask 类

Update the full DownloadTask class

class DownloadTask extends AsyncTask<Void, Void, List<String>> {

    @Override
    protected void onPreExecute() {
        // Cancel the animation.
        if (mSlideshowAnimation != null) {
            mSlideshowAnimation.cancel(true);
        }

        mImageView1.setVisibility(View.GONE);
        mImageView2.setVisibility(View.GONE);
        animate(mProgressBar).alpha(1.0f).setDuration(500).start();

        Log.d(TAG, "Download preparation done.");
    }

    @Override
    protected List<String> doInBackground(Void... voids) {
        Log.d(TAG, "Download");
        SharedPreferences s = getSharedPreferences("access", Context.MODE_PRIVATE);
        String token = s.getString("token", null);

        Log.d(TAG, "Downloading slideshows.");

        List<String> urls = new ArrayList<String>();
        Slideshow[] slideshows = new Api(SlideshowActivity.this).getSlideshows(token);
        for (Slideshow slideshow : slideshows) {
            urls.addAll(slideshow.getAllPhotoUrls());
        }

        Log.d(TAG, "Downloading slideshows: " + slideshows.length);

        for (String url : urls) {
            try {
                url = Api.HOST + url;

                if (!Cache.fileExists(Cache.getCacheFilenameForUrl(SlideshowActivity.this, url))) {
                    Cache.cacheStream(SlideshowActivity.this, HttpHelper.download(SlideshowActivity.this, url), url);
                } else {
                    Log.d(TAG, "Cached: " + url);
                }
            } catch (IOException e) {
                Log.e(TAG, "Error while downloading.", e);
            }
        }

        Log.d(TAG, "Downloading slideshows finished.");

        return urls;
    }

    @Override
    protected void onPostExecute(List<String> urls) {
        Log.d(TAG, "download successful");
        animate(mProgressBar).alpha(0.0f).setDuration(500).start();

        mCurrentImageIndex = -1;
        mImageUrls = urls;

        mSlideshowAnimation = new SlideshowAnimation();
        mSlideshowAnimation.execute((Void[]) null);

        mHandler.sendEmptyMessageDelayed(111, 5000);
    }
}

从Handler调用的AsyncTask不会执行doInBackground [英] AsyncTask called from Handler will not execute doInBackground

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从Handler调用的AsyncTask不会执行doInBackground [英] AsyncTask called from Handler will not execute doInBackground

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