自定义进度对话框显示停止在android上执行AsyncTask

Question

我已经实现了一个简单的自定义进度对话框，我在onPreExecute方法中显示了该对话框。但是在显示对话框之后AsycnTask没有进展到doInBackground方法。因此，对话框永远显示出来。当我注释掉对话框显示它工作正常。请参阅下面的代码。我该如何解决这个问题。

public class CustomProgressDialog  extends ProgressDialog
{

    public CustomProgressDialog(Context context) {
        super(context);
    }

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

    @Override
    public void show() {

        super.show();
    }

}


private CustomProgressDialog getProgressDialog() {
        CustomProgressDialog p = new CustomProgressDialog(activity);
        p.setCancelable(true);
        p.setOnCancelListener(new DialogInterface.OnCancelListener() {

            @Override
            public void onCancel(DialogInterface dialog) {
                loadMainFragment();
                if(imageTask != null)
                    imageTask.cancel(true);
            }
        });

 private class ImageDownloader extends AsyncTask<String, Void, Bitmap> {

           CustomProgressDialog  ringProgressDialog = null;
            Bitmap bitmap = null;

            @Override
            protected Bitmap doInBackground(String... param) {

                Bitmap b = downloadBitmap(param[0]);
                image = b;
                return b;
            }

            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                ringProgressDialog = getProgressDialog()
                ringProgressDialog.show();

                  // after showing this no progression 

            }

            @Override
            protected void onPostExecute(Bitmap result) {
               if(isCancelled())
                   return;

                if (result == null)
                    return;
                super.onPostExecute(result);
                bitmap = result;
                // do work

                if(ringProgressDialog != null)
                   ringProgressDialog.dismiss();
            }

            private Bitmap downloadBitmap(String url) {

                Bitmap bm = null;
                InputStream is = null;

                    try {
                        URL aURL = new URL(url);
                        URLConnection conn = aURL.openConnection();
                        conn.setConnectTimeout(Utils.timeout);
                        conn.setReadTimeout(Utils.timeout);
                        conn.connect();
                        is = conn.getInputStream();
                        bm = Utils.decodeBitmapFromInputStream(is,width,                width, true);}
            catch(Exception e){}
                return bm;

        }