procedure TFooForm.GetData();
begin
  RetrieveData := True; // Kreisenden Kringel anzeigen ;o)
  FSomeService.GetFooDataByDate( FDate, FooDataCallback );
end;

procedure TFooForm.FooDataCallback( AResult: TFooData; AException: Exception; var ADispose : Boolean );
begin
  RetrieveData := False;
  if Assigned( AException ) then
    begin
      // Exception-Meldung in der Form anzeigen
      ErrorMessageLabel.Caption := AException.Message;
    end
  else
    begin
      ErrorMessageLabel.Caption := '';
      PresentFooData( AResult ); // Daten im Formular anzeigen lassen
    end;
end;