Graphics: Legacy Library
 
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
Version: 4.0.2
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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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