You can draw on the canvas from the outside using `get-dc`. Possibly, though, you're seeing your drawing erased when the canvas receives a refresh event from the window system.
See http://docs.racket-lang.org/gui/canvas___.html for more about `on-paint` and the impermanence of drawing from the outside. If you need to draw from the outside and there's not a good way to reconstruct the drawing at any time within `paint-callback`, then one common strategy is: Draw to a bitmap, copy the bitmap to the canvas whenever the bitmap content is changed, and also have `on-paint` copy the bitmap to the canvas. At Tue, 27 Jan 2015 20:08:17 +0200, Alexandr M wrote: > Thanks, > > I copied your example and it seems it doesn't work .. > > If I comment the code inside [paint-callback .. it also doesn't draw > anything. > > In general I need to send 'messages' from 'outside' to the canvas object > to visualize numerical experiment performed by separate function. > > I can implement experiment itself inside [pain-callback (lambda ... > functions ) under definition of canvas, but it looks strange and clumsy. > Sometimes I need just to run computations, but sometimes I need also to do > visualization with animation. > > How could I achieve this? > > Best regards, > Alex > > On 27 January 2015 at 19:54, Matthias Felleisen <[email protected]> > wrote: > > > > > Do you want something like this: > > > > #lang racket/gui > > > > (define frame (new frame% [label ""] [width 700] [height 500])) > > (define canvas > > (new canvas% > > [parent frame] > > [paint-callback > > (lambda (c dc) > > (send dc draw-line 1 1 100 100))])) > > (send frame show #true) > > > > (define (my-draw) > > (send (send canvas get-dc) draw-line 100 100 0 200)) > > > > > > -- Matthias > > > > > > > > On Jan 27, 2015, at 12:09 PM, Alexandr M wrote: > > > > > Hello, > > > > > > How can I draw the line (for example) in the canvas object defined as: > > > > > > (define frame (new frame% [label ""] [width 700] [height 500])) > > > (define canvas (new canvas% [parent frame] > > > ;;;; - call from outside: [paint-callback (lambda (c dc) (send dc > > draw-line 1 1 10 10) )] > > > )) > > > > > > by calling a function like > > > > > > (define (MyDraw canvas-obj) .... (draw-line 1 1 10 10)) > > > > > > ? > > > > > > -- > > > Best regards, > > > Alex > > > ____________________ > > > Racket Users list: > > > http://lists.racket-lang.org/users > > > > > > > -- > Best regards, > Alexander Maslov > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
