[Lazarus] Problems with painting controls on canvas

[Mattias Gaertner]

On Fri, 20 Apr 2012 11:51:14 +0200
Joost van der Sluis <joost at cnoc.nl> wrote:

> Hi,
> 
> I try to paint a widget with child-widgets on a canvas in an OnPaint
> handler. But I can not get it right. Below is my current code. It almost
> works, but when I resize the widgets in the designer, there are all kind
> of artefacts on the lower right part. It has something to do with the
> IntersectClipRect, but I do not understand what it does exactly. 
> 
> Can somebody help me with this?
> 
> This is my current code: (Adapted from the designnonlcl example)
> 
> procedure TMyWidgetMediator.Paint;
> 
>   procedure PaintWidget(AWidget: TMyWidget);
>   var
>     i: Integer;
>     Child: TMyWidget;
>   begin
>     with LCLForm.Canvas do
>       begin
>       if not (AWidget is TMyForm) then
>         begin
>         // fill background
>         Brush.Style:=bsSolid;
>         Brush.Color:=clLtGray;
>         // FillRect(0,0,AWidget.Width,AWidget.Height);
>         // outer frame
>         Pen.Color:=clRed;
>         Rectangle(0,0,AWidget.Width,AWidget.Height);
>         // caption
>         TextOut(5,2,AWidget.Caption);
>         end;
>       // children
>       if AWidget.ChildCount>0 then
>         begin
>         //SaveHandleState;
>         // clip client area
>         {if IntersectClipRect(Handle, AWidget.left, AWidget.Top,
> AWidget.left+AWidget.Width,
>                              AWidget.Height+AWidget.Top)<>NullRegion
>         then} begin

Why adding AWidget.Left/Top ?
This is already done via MoveWindowOrgEx.

See the original code.

>           for i:=0 to AWidget.ChildCount-1 do begin
>             SaveHandleState;
>             Child:=AWidget.Children[i];
>             // clip child area
>             MoveWindowOrgEx(Handle,Child.Left,Child.Top);
>             if
> IntersectClipRect(Handle,0,0,Child.Width,Child.Height)<>NullRegion then
>               PaintWidget(Child);
>             RestoreHandleState;
>           end;
>         end;
>         //RestoreHandleState;
>         end;
>       end;
>   end;
> 
> begin
>   PaintWidget(MyForm);
>   inherited Paint;
> end;


Mattias