You should learn about Model-View-Controller architecture (for example at <a href="http://en.m.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller">http://en.m.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller</a>). Briefly, the model is a data structure that represents the information you are working with. The view is able to produce a visual representation of the model. A controller is used to update the model in response to input. In your example, the canvas is the view. Your on-draw method needs to draw the model. Your buttons will send a message to your controller, and the controller will update the model accordingly. Then, when on-draw is called, it will draw the updated model. Justin <div class="gmail_quote">On Oct 1, 2014 10:17 PM, "Chris Wright" <<a href="mailto:cawright.99@gmail.com">cawright.99@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div><div>Thanks Matthias and Sean </div> </div>It's often helpful when people asking questions ask them clearly ! :) </div>I'll now attempt a clarification... say I have a button in the window, and when that button is pressed, I want to draw on the canvas - and later, another button is pressed, and I might want to draw something else somewhere else on the canvas. I think I'm wanting to get hold of the dc outside of the definition of on-paint in the initialisation code </div>cheers and thanks again Chris </div><div class="gmail_extra"> <div class="gmail_quote">On 2 October 2014 11:57, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu" target="_blank">matthias@ccs.neu.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Does this help? #lang racket/gui (define frame (new frame% [label "Example"] [width 300] [height 300])) (define top-canvas (new (class canvas% (inherit get-dc) (super-new [parent frame]) (define/override (on-paint) (define dc (get-dc)) (send dc draw-rectangle 0 10 ; Top-left at (0, 10), 10 pixels down from top-left 30 10) ; 30 pixels wide and 10 pixels high (send dc draw-line 0 0 ; Start at (0, 0), the top-left corner 30 30) ; and draw to (30, 30), the bottom-right corner (send dc draw-line 0 30 ; Start at (0, 30), the bottom-left corner 30 0) ; and draw to (30, 0), the top-right corner )))) (send frame show #t) <div><div> On Oct 1, 2014, at 9:51 PM, Chris Wright wrote: > I would like to draw on a canvas in a window at various times during program execution. > My first attempt was to combine two examples: > > #lang racket/gui > > (define frame (new frame% > [label "Example"] > [width 300] > [height 300])) > (define top-canvas (new canvas% [parent frame])) > > (send frame show #t) > > (define dc (send top-canvas get-dc)) > > (send dc draw-rectangle > 0 10 ; Top-left at (0, 10), 10 pixels down from top-left > 30 10) ; 30 pixels wide and 10 pixels high > (send dc draw-line > 0 0 ; Start at (0, 0), the top-left corner > 30 30) ; and draw to (30, 30), the bottom-right corner > (send dc draw-line > 0 30 ; Start at (0, 30), the bottom-left corner > 30 0) ; and draw to (30, 0), the top-right corner > > > > The cross and box are drawn, but "instantly" over-written by a blank canvas. I suppose this is because on-paint is triggered? (not sure by what..) > If I put the (send frame...) form at the end of the code, the cross and box aren't seen. > > I am sure this is due to me not understanding the model properly - I'd be grateful for some help... > > many thanks > > Chris </div></div>> ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div> -- A/Prof Chris Wright MBBS, FRACP, FCICM, GradDipiSc(Physics) Academic Coordinator, Years III - V Central MBBS Intensive Care Specialist Faculty of Medicine, Nursing and Health Sciences, Monash University Clayton VIC </div> ____________________ Racket Users list: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div>