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    <p>You can probably write a container that delegates most of the work off to the contained dc%. You could do something like this:</p> <pre><code>#lang racket (require racket/gui/base) (define my-dc% (class* object% (dc&lt;%&gt;) (init-field delegate) (super-new) (define/public (cache-font-metrics-key) (send delegate cache-font-metrics-key)) (define/public (clear) (send delegate clear)) (define/public (copy x y width height x2 y2) (send delegate copy x y width height x2 y2)) (define/public (draw-arc x y width height start-radians end-radians) (send delegate draw-arc x y width height start-radians end-radians)) ;; FILL ME IN... )) </code></pre> <p>going through all the methods listed in the <a href="http://docs.racket-lang.org/draw/dc___.html" rel="nofollow"><code>dc&lt;%&gt;</code></a> interface. This approach is admittedly pretty brute force, but it should work. Then you could add whatever extra methods you wanted to this class, since it's yours.</p> <p>Here is a complete example, using some macros to reduce a bunch of the copy-and-pasting I'd otherwise do:</p> <pre><code>#lang racket (require racket/gui/base) ;; Defines a dc&lt;%&gt; implementation that can wrap around ;; another dc. ;; ;; Can also be found at: https://gist.github.com/dyoo/5025445 ;; ;; The test code near the bottom shows an example ;; of how to use the delegate. (define wrapped-dc% (class* object% (dc&lt;%&gt;) (init-field delegate) (super-new) ;; This bit of code tries to generate the delegate method ;; given the method signature. It's not quite perfect ;; yet because I'm having trouble capturing the re-write rule ;; for set-pen and set-brush. (define-syntax (write-delegate-method stx) (syntax-case stx () [(_ (name args ...)) (with-syntax ([(arg-ids ...) (for/list ([arg (syntax-&gt;list #'(args ...))]) (syntax-case arg () [(id default) #'id] [id #'id]))]) #'(define/public (name args ...) (send delegate name arg-ids ...)))])) (define-syntax-rule (write-delegate-methods sig ...) (begin (write-delegate-method sig) ...)) (write-delegate-methods (cache-font-metrics-key) (clear) (copy x y width height x2 y2) (draw-arc x y width height start-radians end-radians) (draw-bitmap source dest-x dest-y (style 'solid) (color (send the-color-database find-color "black")) (mask #f)) (draw-bitmap-section source dest-x dest-y src-x src-y src-width src-height [style 'solid] [color (send the-color-database find-color "black")] [mask #f]) (draw-ellipse x y width height) (draw-line x1 y1 x2 y2) (draw-lines points [xoffset 0] [yoffset 0]) (draw-path path [xoffset 0] [yoffset 0] [fill-style 'odd-even]) (draw-point x y) (draw-polygon points [xoffset 0] [yoffset 0] [fill-style 'odd-even]) (draw-rectangle x y width height) (draw-rounded-rectangle x y width height [radius -0.25]) (draw-spline x1 y1 x2 y2 x3 y3) (draw-text text x y [combine #f] [offset 0] [angle 0]) (end-doc) (end-page) (erase) (flush) (get-alpha) (get-background) (get-brush) (get-char-height) (get-char-width) (get-clipping-region) (get-device-scale) (get-font) (get-gl-context) (get-initial-matrix) (get-origin) (get-pen) (get-rotation) (get-scale) (get-size) (get-smoothing) (get-text-background) (get-text-extent string [font #f] [combine? #f] [offset 0]) (get-text-foreground) (get-text-mode) (get-transformation) (glyph-exists? c) (ok?) (resume-flush) (rotate angle) (scale x-scale y-scale) (set-alpha opacity) (set-background color) ;(set-brush brush) ;; fixme: this is not quite right (set-clipping-rect x y width height) (set-clipping-region rgn) (set-font font) (set-initial-matrix m) (set-origin x y) ;(set-pen pen) ;; fixme: this is not quite right (set-rotation angle) (set-scale x-scale y-scale) (set-smoothing mode) (set-text-background color) (set-text-foreground color) (set-text-mode mode) (set-transformation t) (start-doc message) (start-page) (suspend-flush) (transform m) (translate dx dy) (try-color try result)) ;; We'll manually write the methods for set-brush and set-pen ;; because they're case-lambdas and a bit unusual, rather ;; than complicate the macro any further. (public set-brush) (define set-brush (case-lambda [(brush) (send delegate set-brush brush)] [(color style) (send delegate set-brush color style)])) (public set-pen) (define set-pen (case-lambda [(pen) (send delegate set-pen pen)] [(color width style) (send delegate set-pen color width style)])))) (module+ test (define bm (make-bitmap 100 100)) (define my-dc (new wrapped-dc% [delegate (send bm make-dc)])) (send my-dc draw-rectangle 10 10 30 50) (print bm) (newline) (define extended-dc% (class wrapped-dc% (super-new) (inherit draw-arc) (define/public (draw-circle x y radius) (draw-arc (- x radius) (- y radius) (* 2 radius) (* 2 radius) 0 (* 2 pi))))) (define bm2 (make-bitmap 100 100)) (define my-new-dc (new extended-dc% [delegate (send bm2 make-dc)])) (send my-new-dc set-smoothing 'aligned) (send my-new-dc draw-circle 50 50 30) (print bm2)) </code></pre> <p>The <code>test</code> module at the end here shows that we can wrap a <code>dc</code> and extend it as we want.</p> <p>Racket is supposed to have some built-in support for this in the <a href="http://docs.racket-lang.org/reference/Surrogates.html" rel="nofollow"><code>surrogate</code></a> form, though I have to admit I haven't tried it yet. If I have time, I'll try to cook up an example and revise this answer if it's an improvement over the above.</p>
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    1. COSurrogates sure look interesting. I think -- it's hard to make sense of the reference pages of the Racket documentation when you're not already an experienced Schemer.
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    2. COIt's not just you. Frankly, I'm having a hard time myself with this specific part of the docs. I think it needs an example or two to figure out what it's actually doing. :)
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    3. COI've updated the answer above with a hand-coded delegate for dc<%>. I hope this helps!
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