Android: Handler for AsyncTask
public class UpdateTask extends AsyncTask<Void, Void, Void> {
    private ProgressDialog progressDialog;
    private Handler handler;

public UpdateTask(Act activity) {

        progressDialog = ProgressDialog.show(Activity.this, "Wait",
                "Wait");
        progressDialog.dismiss();

        handler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                //run on UI Thread
                switch( msg.what ){
                    case MSG:
                        progressDialog.show();
                        break;
                    case DETACH:
                        progressDialog.dismiss();
                        break;
                }
            }
        };

    }


    void detach() {
          activity=null;
                  //problematic
          //progressDialog.dismiss();
          //handler.sendEmptyMessage(DETACH);
        }




    @Override
    protected Void doInBackground(Void... params) {
        handler.sendEmptyMessage(MSG);;
        return null;
    }

    protected void onPostExecute(Void result)    {
        if (activity==null) {
            Log.w("RotationAsync", "onPostExecute() skipped -- no activity");
          }
          else {
              //problematic
              //                  progressDialog.dismiss();
              handler.sendEmptyMessage(MSG);
                progressDialog = null;
          }
    }

};

private Handler handler = new Handler(new Handler.Callback() {
    @Override
public boolean handleMessage(Message msg) {

    switch( msg.what ){
                case MSG:
                    progressDialog.show();
                    break;
                case DETACH:
                    progressDialog.dismiss();
                    break;
            }

    return false;
}
});