从Handler调用的AsyncTask不会执行doInBackground

Question

我正在使用的应用程序是使用后台线程通过API下载图像列表,然后以幻灯片形式显示图像.

有一个后台任务(当前是AsyncTask)来定期获取新图像.

我没有得到关于错误的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.

以下是Activity的一些代码:

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);
}

该DownloadTask如下所示:

@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);
}

将调用该处理程序,将调用onPreExecute(在AsyncTask中)并且DownloadTask的初始运行(右上onCreate)也可以.

根据这个问题:Android SDK AsyncTask doInBackground没有运行(子类),它可能与SDK15有关.

谢谢你的任何提示.

---

更新当我收到有关Handler可能不在UI线程中的注释时(这很奇怪,就像Thread.currentThread在onCreate和Handlers handleMessage方法中一样,我将handleMessage方法修改为:

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

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);
    }
}

Answer 1

感谢与Waqas的有益讨论(谢谢!)我终于在我的代码中发现了错误.事实上,上述所有内容都是正确的并且按原样工作.我的问题是第二个任务阻止了第一个任务,反之亦然.

在Google网上论坛中找到这篇文章可能是巧合:http : //groups.google.com/group/android-developers/browse_thread/thread/f0cd114c57ceefe3 ? tvc=2& q=AsyncTask+in+Android+4.0.建议参与线程的每个人都仔细阅读本讨论.

AsyncTask将线程模型切换到串行执行器(再次),这与我看来有2个AsyncTasks的方法不兼容.

最后,我将"下载"的处理切换为经典,Thread并在Handler必要时使用发布消息取消幻灯片.使用处理程序sendEmptyMessageDelayed我将在一段时间后简单地重新创建下载线程以刷新数据.

感谢所有的评论和答案.