my single threaded delphi 2009 app (not quite yet complete) has started to have a problem with Application.ProcessMessages hanging. my app has a TTimer object that fires every 100 ms to poll an external device. i use Application.ProcessMessages to update the screen when something changes so the app is still responsive.

one of these was in a grid OnMouseDown event. in there, it had an Application.ProcessMessages that essentially hung. removing that was no problem except that i soon discovered another Application.ProcessMessages that was also blocking.

i think what may be happening to me is that the TTimer is--in the app mode i'm currently debugging--probably taking too long to complete. i have prevented the TTimer.OnTimer event hander from re-entering the same code (see below):

procedure TfrmMeas.tmrCheckTimer(Sender: TObject);
begin
  if m_CheckTimerBusy then
    exit;

  m_CheckTimerBusy:=true;
  try
    PollForAndShowMeasurements;
  finally
    m_CheckTimerBusy:=false;
  end;
end;

what places would it be a bad practice to call Application.ProcessMessages? OnPaint routines springs to mind as something that wouldn't make sense.

any general recommendations?

i am surprised to see this kind of problem arise at this point in the development!