The tabs of an owner drawn native "tab control" (TPageControl
in VCL, although its ascendant is appropriately named TCustomTabControl
- it is anyone's guess why the creative naming..), is expected to be painted by its parent control while processing WM_DRAWITEM
messages, as documented here.
The VCL takes the burden from the parent by mutating the message to a CN_DRAWITEM
message and sending it to the control itself. In this process the VCL has no further intervention. It just calls the OnDrawTab
message handler if it is assigned by user code, passing appropriate parameters.
So, it's not the VCL that draws the borders around tabs, but the OS itself. Also, evidently, it doesn't do this during processing of WM_DRAWITEM
messages but later in the painting process. You can verify this by putting an empty WM_DRAWITEM
handler on the parent of a page control. Result is, whatever we paint in the event handler, it will later get borders by the OS.
What we might try is to try to prevent what the OS draws take effect, we have the device context (as Canvas.Handle) after all. Unfortunately this route also is a dead end because the VCL, after the event handler returns, restores the device context's state.
The only way, then, we have is to completely abandon handling an OnDrawTab
event, and acting upon CN_DRAWITEM
message. Below sample code use an interposer class, but you can subclass the control any way you like. Make sure that OwnerDrawn
is set.
type
TPageControl = class(comctrls.TPageControl)
protected
procedure CNDrawitem(var Message: TWMDrawItem); message CN_DRAWITEM;
end;
TForm1 = class(TForm)
..
..
procedure TPageControl.CNDrawitem(var Message: TWMDrawItem);
var
Color: TColor;
Rect: TRect;
Rgn: HRGN;
begin
Color := 0;
// draw in different colors so we see where we've drawn
case Message.DrawItemStruct.itemID of
0: Color := $D0C0BF;
1: Color := $D0C0DF;
2: Color := $D0C0FF;
end;
SetDCBrushColor(Message.DrawItemStruct.hDC, Color);
// we don't want to get clipped in the passed rectangle
SelectClipRgn(Message.DrawItemStruct.hDC, 0);
// magic numbers corresponding to where the OS draw the borders
Rect := Message.DrawItemStruct.rcItem;
if Bool(Message.DrawItemStruct.itemState and ODS_SELECTED) then begin
Inc(Rect.Left, 2);
// Inc(Rect.Top, 1);
Dec(Rect.Right, 2);
Dec(Rect.Bottom, 3);
end else begin
Dec(Rect.Left, 2);
Dec(Rect.Top, 2);
Inc(Rect.Right, 2);
Inc(Rect.Bottom);
end;
FillRect(Message.DrawItemStruct.hDC, Rect,
GetStockObject(DC_BRUSH));
// just some indication for the active tab
SetROP2(Message.DrawItemStruct.hDC, R2_NOTXORPEN);
if Bool(Message.DrawItemStruct.itemState and ODS_SELECTED) then
Ellipse(Message.DrawItemStruct.hDC, Rect.Left + 4, Rect.Top + 4,
Rect.Left + 12, Rect.Top + 12);
// we want to clip the DC so that the borders to be drawn are out of region
Rgn := CreateRectRgn(0, 0, 0, 0);
SelectClipRgn(Message.DrawItemStruct.hDC, Rgn);
DeleteObject(Rgn);
Message.Result := 1;
inherited;
end;
Here is how the above looks here: