Originally posted by Even Charlton.

A common problem for new Android developers is how to handle screen rotations on the Android platform. Screen rotations cause all sorts of problems if you don’t anticipate them because the Activity gets torn down completely and then rebuilt. If you don’t handle it properly, AsyncTasks (threads) break completely. I’ll show the code first and then talk about why it works. Here’s the general pattern.

The code

public class MyActivity extends Activity {
    private MyTask mTask;
    @Override
    public void onCreate(Bundle savedInstanceState)
        super.onCreate(savedInstanceState)
        mTask = (MyTask) getLastNonConfigurationInstance();
        if(mTask == null) {
            mTask = new MyTask();
        }
        mTask.activity = this;
        if(mTask.getStatus() == AsyncTask.Status.PENDING) {
            mTask.execute();
        }
    }
    @Override
    public Object onRetainNonConfigurationInstance() {
        return mTask;
    }
    private static class MyTask extends AsyncTask<Params, Progress, Result>() {
        public MyActivity activity;
        @Override
        protected void onPreExecute() {
            // ...
        }
        @Override
        protected Result doInBackground(Params... params) {
            // ...
        }
        @Override
        protected void onProgressUpdate(Progress... updates) {
            // ...
        }
        @Override
        protected void onPostExecute(Result result) {
            // ...
        }
    }
}

The explanation

This works because MyTask is a static class—it will survive the class being torn down. All you then do is reattach it to the Activity when you recreate the Activity after the rotation. Note that you don’t just always call mTask.execute() — only call it if it hasn’t been called before.

Of course this might not be perfect for your situation, but I assume that you can make the necessary modifications for your specific case. Feel free to drop me a line if you have any questions.

注

从Api level 11开始引入了Loader类，该类同AsyncTask一样可以实现异步加载，同时在设计上考虑了configuation改变的场景。