Windows Message Not Fired on HTHELP Button

Question

I have a VCL form that is set for bsDialog with biHelp enabled ("?" icon in application bar).

I am following this example: http://delphi.about.com/od/adptips2006/qt/custom_bihelp.htm

However I cannot get the WMNCLBUTTONDOWN Windows Message to appear when I click the "?" button. It only seems to fire when I click on the title bar (like I was going to drag the window around.

Code:

procedure TMainFrm.WMNCLBUTTONDOWN(var Msg: TWMNCLButtonDown);
begin
  ShowMessage('WMNCLBUTTONDOWN Pre-Help') ;

  if Msg.HitTest = HTHELP then
    Msg.Result := 0 // "eat" the message
  else
    inherited;
 end;

procedure TMainFrm.WMNCLBUTTONUP(var Msg: TWMNCLButtonUp);
 begin

  if Msg.HitTest = HTHELP then
  begin
    Msg.Result := 0;
    ShowMessage('Need help?') ;
  end
  else
    inherited;
 end;

Again, I see the "Pre-Help" message when I click on the title bar, but not when I click on the "?" button. Why is this? I'm trying to show a separate form when that button is clicked.

Answer 1

The modal message loop of ShowMessage interferes with the message processing. Use OutputDebugString, for example, to see that the messages fire as your expect:

type
  TMainFrm = class(TForm)
  protected
    procedure WMNCLButtonDown(var Msg: TWMNCLButtonDown); 
      message WM_NCLBUTTONDOWN;
    procedure WMNCLButtonUp(var Msg: TWMNCLButtonUp); 
      message WM_NCLBUTTONUP;
  end;
....
procedure TMainFrm.WMNCLButtonDown(var Msg: TWMNCLButtonDown);
begin
  if Msg.HitTest = HTHELP then
  begin
    OutputDebugString('Help button down');
    Msg.Result := 0;
  end
  else
    inherited;
end;

procedure TMainFrm.WMNCLButtonUp(var Msg: TWMNCLButtonUp);
begin
  if Msg.HitTest = HTHELP then
  begin
    OutputDebugString('Help button up');
    Msg.Result := 0;
  end
  else
    inherited;
end;

Remember that buttons are not pressed until they are released. So you should not be taking action like showing dialogs when the button goes down. Wait until WM_NCLBUTTONUP before showing another dialog.