unable to show a progress bar properly

unable to show a progress bar properly

i want to show in my activity a progress bar as a response to a button
click. i read in another question that i should use async task in order to
show/not show the progress bar but when i click on the button the progress
bar is not shown properly (it appears for much less time then it should)
any suggestions?
the activity code:
public void chooseContactFromList(View view){
ProgressBar pBar = (ProgressBar) findViewById(R.id.progressBar1);
circleActivity progressTask = (circleActivity) new
circleActivity(pBar).execute();
AlertDialog.Builder builder = new AlertDialog.Builder(this);
CharSequence[] cs=nameList.toArray(new CharSequence[nameList.size()]);
builder.setTitle("Make your selection");
builder.setItems(cs, new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int item) {
reciverNumber = phoneList.get(item);
}
});
AlertDialog alert = builder.create();
alert.show();
progressTask.cancel(true);
}
the AsyncTask code:
public class circleActivity extends AsyncTask<Void, Void, Void> {
private ProgressBar progressBar;
public circleActivity(ProgressBar pBar) {
progressBar=pBar;
}
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
progressBar.setVisibility(View.VISIBLE);
}
@Override
protected void onPostExecute(Void result) {
progressBar.setVisibility(View.INVISIBLE);
}
@Override
protected void onProgressUpdate(Void ... progress) {
}
@Override
protected Void doInBackground(Void... arg0) {
// TODO Auto-generated method stub
return null;
}
}
thanks