9 Protecting Graphics Operations

▼ |  | 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

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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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