[plt-scheme] A question about PLT Graphics Toolkit

At Thu, 5 Mar 2009 11:38:01 +0100, Ziboon wrote:
>  #lang scheme/gui
>  (define f (new frame%
>               (label "test") ))
> 
> 
> (define c (new canvas%
>                (parent f)
>                (min-width 200)
>                (paint-callback (λ (canvas dc)
>                                  (send dc clear)))))
> 
> 
>  (define dc (send c get-dc))
> 
> (send dc set-background (make-object color% "red"))
> 
> (send dc draw-rectangle 0 0 50 50)
> 
> (send f show #t )*
> 
> ;--------------------------------------------------
> -------------------------------------
> 
> Why set-background work :  I see my canvas red; but draw-rectangle doesn't
> work  there are no rectangle in my canvas.
> 
> I try to  delete set-background line but I don't see my rectangle.
> 
> so I know if I put  *(send dc draw-rectangle 0 0 50 50)    *in
> paint-callback  it will work .
> 
> But i want draw a rectangle not directly in my canvas class.

Images drawn onto a canvas are not persistent. When the canvas is
hidden or shown by the windowing system, then the canvas is re-painted
by calling its paint callback.

Your paint callback always clears the canvas. If you move the
`draw-rectangle' call into the paint callback after the clear, then
you'll see a rectangle.

Here's a revised version of your program:

----------------------------------------

 #lang scheme/gui
 (define f (new frame%
                (label "test")))

 (define c (new canvas%
                (parent f)
                (min-width 200)
                (paint-callback 
                 (λ (canvas dc)
                   (send dc set-background (make-object color% "red"))
                   (send dc clear)
                   (send dc draw-rectangle 0 0 50 50)))))

 (send f show #t )

----------------------------------------