Adding Canvas to TScrollBox - delphi

Question

I am trying to do simple thing: Add a Canvas property on the TScrollBox descendant. I have read this article
but my ScrollBox descendant simply does not draw on the canvas. May anybody tell me, what is wrong?
TfrmScrollContainer = class (TScrollBox)
private
FCanvas: TCanvas;
FControlState:TControlState;
procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
protected
procedure Paint;
procedure PaintWindow(DC: HDC); override;
property Canvas: TCanvas read FCanvas;
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
end;
... this is exact copy, how TWincontrol turns to TCustomControl (but it fails somewhere)
constructor TfrmScrollContainer.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
FCanvas := TControlCanvas.Create;
TControlCanvas(FCanvas).Control := Self;
end;
destructor TfrmScrollContainer.Destroy;
begin
FCanvas.Free;
inherited Destroy;
end;
procedure TfrmScrollContainer.WMPaint(var Message: TWMPaint);
begin
Include(FControlState, csCustomPaint);
inherited;
Exclude(FControlState, csCustomPaint);
end;
procedure TfrmScrollContainer.PaintWindow(DC: HDC);
begin
FCanvas.Lock;
try
FCanvas.Handle := DC;
try
TControlCanvas(FCanvas).UpdateTextFlags;
Paint;
finally
FCanvas.Handle := 0;
end;
finally
FCanvas.Unlock;
end;
end;
procedure TfrmScrollContainer.Paint; // this is not executed (I do not see any ellipse)
begin
Canvas.Brush.Color:=clRed;
Canvas.Ellipse(ClientRect);
end;
Thanx

Answer 1

You are not including csCustomPaint to ControlState, you're including it to the similarly named field you declared. Your field is not checked, the ascendant control does not know anything about it. To solve, replace
procedure TfrmScrollContainer.WMPaint(var Message: TWMPaint);
begin
Include(FControlState, csCustomPaint);
inherited;
Exclude(FControlState, csCustomPaint);
end;
with
procedure TfrmScrollContainer.WMPaint(var Message: TWMPaint);
begin
ControlState := ControlState + [csCustomPaint];
inherited;
ControlState := ControlState - [csCustomPaint];
end;
Alternatively your scroll box may parent any control for your custom painting to work. The inherited WM_PAINT handler checks to see the control count and if it's not 0 it calls the paint handler instead of delivering the message to the default handler.