[racket] is-empty? on region% with no device-context.

From: Patrick Mahoney (paddy.mahoney at gmail.com)
Date: Wed May 2 17:57:50 EDT 2012

#|is-empty? fails when called on a region produced without an associated
(define intersection
  (lambda (region-a region-b)
    (define new-region (new region%))
    (send new-region union region-a)
    (send new-region intersect region-b)


On 2 May 2012 17:55, Patrick Mahoney <paddy.mahoney at gmail.com> wrote:

> #|Hello All,
> I'm attempting to ensure that shapes drawn onto a canvas don't overlap. In
> order to do so, I attempted to use region% objects.| #
> #|Takes two rectangles, and tells whether they overlap as regions.|#
> (define overlaps?
>   (lambda (rect-a rect-b)
>     (define region-a (rectangle->region rect-a))
>     (define region-b (rectangle->region rect-b))
>     (define a-intersection-b (intersection dc region-a region-b))
>     (send a-intersection-b is-empty?)))
> #|Unfortunately this doesn't work-the is-empty? method only works on a
> region% with an associated drawing context. Is there any way to salvage
> this such that is-empty? can be used on region% objects unassociated with a
> dc object? I'm not sure why they would require one, as otherwise the
> region% interface seems fairly decoupled from the notion of
> device-contexts. However, I'm sure that there is a reason that I don't yet
> understand.|#
> (define intersection
>   (lambda (region-a region-b)
>     (define new-region (new region% [dc a-dc]))
>     (send new-region union region-a)
>     (send new-region intersect region-b)
>     new-region))
> #|The region documentation...
> http://docs.racket-lang.org/draw/region_.html?q=region&q=any-of%3F#%28meth._%28%28%28lib._racket/draw..rkt%29._region~25%29._in-region~3f%29%29<http://docs.racket-lang.org/draw/region_.html?q=region&q=any-of%3F#%28meth._%28%28%28lib._racket/draw..rkt%29._region%7E25%29._in-region%7E3f%29%29>
> I really just don't want to have to rewrite the overlaps? logic for my set
> of shapes. Thanks all. |#
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