//Declaration
struct App : public TopWindow{
	typedef App CLASSNAME; //Needed by THISBACK macro
	Canvas canvas;	       //Prepair your brushes
	
	App();
	//Same signatur as canvas.paint(Draw& w)
	void OnCanvasPaint(Draw& w); //Want to paint on canvas
	//Paint on Click see below
	void OnCanvasMouseLeft(Point p, dword keyflags);
};

//Definition
App::App(){
	.
	.
	// The magic assignment:-)
	canvas.OnCanvasPaint=THISBACK(OnCanvasPaint);
	// The left mousebutton click is now routed to the canvas 
	canvas.OnMouseLeft=THISBACK(OnCanvasMouseLeft);
}  

void App::OnCanvasPaint(Draw& w)
{// Your paintings on Canvas goes here:
  Rect r=canvas.GetSize();
 // Draw a Line
  w.DrawLine(0,0,r.Width(),r.height(),1,Black());
}
/*
And not still enough, Canvas offers a way to paint not only
in a Paint Callback. For this purpose you can use the DrawSheet()
function. To make it short here is an Example:
*/
void App::OnCanvasMouseLeft(Point p, dword keyflags)
{ //Paint an ugly ellipse:-)
  Rect r=canvas.GetSize();
  DrawingDraw& ds=canvas.DrawSheet(); 
  ds.DrawEllipse(p.x,p.y,20,20,Red(),5,Green());
  canvas.EndSheet();//draw it now
}