As I'm writing a real-world application in racket (and it's a great tool to do
so!) I just ran into another problem:
When a a panel% has a visible scrollbar and there are enough children to
actually use that scrollbar, then all children get flooded with on-paint
calls when the scrollbar is used.
To be precise, the problem is not that the children get lots of on-paint, but
that invisible children get that calls, too, which results in a pileup of
on-paint calls that block the application when the drawing operation is
expensive.
In my case I have several dozen graphs to present and even touching the
scrollbar results in a desaster.
Is there a workaround, say, can I find out if a child is visible at all?
Example code - just scroll around and watch how often the first child is
redrawn, even when invisible:
#lang racket
(require racket/gui)
(require plot)
(define counter 0)
(define frame (new frame% [label "on-paint flooding"] [min-height 800]
[min-width 800]))
(define panel (new vertical-panel% [parent frame] [style '(auto-hscroll
auto-vscroll)]))
(for-each
(λ (a)
(new canvas%
[parent panel] [min-height 150] [min-width 150]
[paint-callback (λ (c dc)
(when (= a 0) (set! counter (add1 counter)))
(define-values (w h) (send dc get-size))
; uncomment the following line to see the effect on
expensive rendering:
; (plot/dc (function (λ (x) (sin (* 4 x))) -1 1) dc
0 0 w h )
(send dc draw-text (format "~a" counter) 0 0))]))
'(0 1 2 3 4 5 6 7 8 9 10))
(send frame show #t)
Nik
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