Question

I have a Delphi XE2 project having components like Label1, BitBtn1 and Image1. I have implemented form dragging without the caption bar writing the following code:

  private
    { Private declarations }
    procedure WMNCHitTest(var Msg: TWMNCHitTest) ; message WM_NCHitTest;

and

procedure TMainForm.WMNCHitTest(var Msg: TWMNCHitTest) ;
begin
  inherited;
  if Msg.Result = htClient then Msg.Result := htCaption;
end;

In my form, the Image1.OnMouseMove and Label1.OnClick events are compulsory for my project, but they are not working. How can I drag the form from the client area except the Image1 and Label1 areas? I can do one thing that I may use one TPanel, but it will destroy the GlassFrame and SheetOfGlass properties of my form.

Answer 1

You will need to use the position information that is included in the WM_NCHITTEST message. Use that to determine whether or not there is a control present at that point. For example you could use the ControlAtPos method.

procedure TMainForm.WMNCHitTest(var Msg: TWMNCHitTest);
begin
  inherited;
  if ControlAtPos(ScreenToClient(Msg.Pos), True, True, True)=nil then
    if Msg.Result=htClient then
      Msg.Result := htCaption;
end;

This will allow dragging only if you have clicked on a point on the form at which there is no control present. You may wish to use an alternative criteria, but use of Msg.Pos is the key idea.