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))))