Android: Progress Dialog Not Displaying

I have a main activity that launches a child activity using the following code:

Intent intent = new Intent();
intent.setClassName(MyChildActivity.class.getPackage().getName(), MyChildActivity.class.getName());
((Activity)context).startActivity(intent);

I am trying to perform a time-consuming task in the child activity and would like to display a ProgressDialog while I do so. My code looks like this:

private ProgressDialog _progressDialog;

private OnClickListener btn_onClick = new OnClickListener() {
    public void onClick(View v) {
        _progressDialog = ProgressDialog.show(
            v.getContext(),
            "Please wait",
            "Performing task..."
        );

        TaskThread t = new ExportThread(v.getContext());
        t.start();
    }
};

private class TaskThread extends Thread{
    private Context _context;

    public TaskThread(Context context) {
        _context = context;
    }

    private Handler _handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            _progressDialog.dismiss();
        }
    };


    @Override
    public void run() {
        performTask(_context);
        _handler.sendEmptyMessage(0);
    }
}

For some reason, the ProgressDialog is not displaying. If I use that same code in the main activity, it works - but not in the child activity. In addition, the following code also fails to display the ProgressDialog (but the Toast does display):

private ProgressDialog _progressDialog;

private OnClickListener _btn_onClick = new OnClickListener() {
    public void onClick(View v) {
        _progressDialog = ProgressDialog.show(
            v.getContext(),
            "Please wait",
            "Performing task..."
        );

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        _progressDialog.dismiss();

        Toast.makeText(v.getContext(), "Done with progress dialog.", Toast.LENGTH_SHORT).show();
    }
};

Any ideas out there? Are we not allowed to display a ProgressDialog from a child activity?

Thank you.

why use thread instead of async task?

Async task implements the method onProgressUpdate and publishProgress which makes it easy to display and update UI/progress dialogs.

Here is some example code: http://android-projects.de/2010/12/08/threading-in-android-apps-wir-entwickeln-einen-zahler/