9 Protecting Graphics Operations

To guarantee the proper closing of viewports in cases of errors, especially when a program manages several viewports simultaneously, a programmer should use dynamic-wind:

(let ([w (open-viewport "hello" 100 100)])
  (dynamic-wind
    ; what we want to happen first: nothing
    void
    ; the main program (errors constrained to this piece)
    (lambda () (draw-pixel 13))  ; an error
    ; what we would like to happen, whether the main program
    ; finishes normally or not
    (lambda () (close-viewport w))))

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1	Basic Commands
2	Position Operations
3	Color Operations
4	Draw, Clear, and Flip Operations
5	World Operations
6	Miscellaneous Operations
7	An Example
8	A More Complicated Example
9	Protecting Graphics Operations
10	Mouse Operations
11	Keyboard Operations
12	Flushing
13	Graphics Library as a Unit