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这些图像。
有一个后台任务(当前为AsyncTask),定期获取新的图像。
我没有收到任何关于线程错误等方面的错误消息,只是AsyncTask的第二个实例不会运行doInBackground方法。
以下是Activity中的一些代码:
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);
}
DownloadTask
的代码如下:
@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);
}
处理程序将被调用,AsyncTask中的onPreExecute也将被调用,并且DownloadTask(就在onCreate中)的初始运行也有效。
根据这个问题:Android SDK AsyncTask doInBackground not running (subclass),可能与SDK15有关。
感谢任何提示。
更新:由于我收到了评论,指出处理程序可能不在UI线程中(这很奇怪,因为Thread.currentThread在onCreate和处理程序的handleMessage方法中都是相同的),因此我修改了handleMessage方法:
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);
}
}
mDownloadTask = new DownloadTask();
两次中所做的事情吗? - Sebastian Roth.execute((Void[]) null)
,可以写成.execute()
。 - Hauleth