misuse of AsyncTask

[pangeneral]

According to our research, there are misuses about the following AsyncTask class:
de.geeksfactory.opacclient.frontend.LibraryListActivity.LoadLibrariesTask

The problems are:

1. LoadLibrariesTask is an inner class of LibraryListActivity, which means it holds strong reference the Activity, which can lead to memory leak when the Activity is destroyed and the AsyncTask did not finish;
2. The instance of LoadLibrariesTask are not cancelled before the Activity is destroyed, which can lead to the wrong invocation of onPostExecute() if the Activity is destroyed
3. The doInBackground() did not check the status of AsyncTask.

I think we can make following changes to fix the misuse problems:

1. LoadLibrariesTask should be a static inner class.
2. loadLibrariesTask, the instance of LibraryListActivity should invoke cancel() in the onDestroy() method of LibraryListActivity if it is not null
3. In the loop of doInBackground() method, check whether AsyncTask is cancelled via isCancelled(). If current AsyncTask is cancelled, jump out of the loop.

These are my suggestions above, thank you.

[raphaelm]

Feel free to submit a pull request for this, thanks!