Thanks Matthias and Sean It's often helpful when people asking questions ask them clearly ! :) 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 cheers and thanks again Chris On 2 October 2014 11:57, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > > 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) > > > > > 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 > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > > -- 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
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