RetainFragEgActivity.java

public class RetainFragEgActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my);

        Button b = (Button) findViewById(R.id.aaa);
        b.setOnClickListener(new View.OnClickListener() {
            @Override public void onClick(View v) {
                RetainFragment retainFragment =
                        RetainFragment.getInstance(getFragmentManager());
                retainFragment.loadAsync();
            }
        });
    }

    private void handleResponse(String response) {
        // do something with the response...
    }

    public static class RetainFragment extends Fragment {
        private static final String TAG = "RetainFragment";

        private RetainFragment() {
        }

        public static RetainFragment getInstance(FragmentManager fm) {
            RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG);
            if (fragment == null) {
                fragment = new RetainFragment();
                fm.beginTransaction().add(fragment, TAG).commit();
            }
            return fragment;
        }

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

        public void loadAsync() {
            new AsyncTask<Void, Void, String>() {
                @Override protected String doInBackground(Void... params) {
                    // Do some work...
                    return null;
                }

                @Override protected void onPostExecute(String response) {
                    MyActivity myActivity = (MyActivity)getActivity();
                    if (myActivity != null) {
                        myActivity.handleResponse(response);
                    }
                }
            }.execute();
        }
    }
}