Programmatically close a window made with `racket/gui` (to stop a `timer%`)

问题

Racket programs that use racket/gui run until all of the windows are closed. This makes it easy to write a program like:

#lang racket/gui
(define window (new frame% [label "Hello"] [width 100] [height 100]))
(send window show #t)

And now the program will continue to run until the window is closed.

However, it sometimes makes sense to close a window programmatically, for example, if I want a countdown that will close the window and finish after the countdown is done.

As far as I can tell though, the only way to 'close' a window is the show method:

(send window show #f)

This however only stops the window from being displayed, but does not actually close the window. Normally, this is good enough and the program exits, like in this example:

#lang racket/gui
(define window (new frame% [label "hello"]))
(send window show #f)

However, if the program has a timer, it will not exit until the timer finishes. You can set a callback in the window on-close, but this is only called when the window is actually closed, not when its hidden with show. For example, this program will not get stuck:

#lang racket/gui
(define window
  (new (class frame%
         (super-new [label "hello"])
         (define timer
           (new timer%
                [interval 1000]
                [notify-callback (λ x (displayln "ding"))]))
         (define/augment (on-close)
           (send timer stop)))))

(send window show #f)

So, is there a way to either figure out when a window is hidden (via the show) function or programmatically close the window? If neither is the case, is overwriting the show method to stop the timer myself a bad idea?

回答1:

Since you are subclassing the frame% class anyway, you can override the show method¹ to stop the timer whenever the window is closed. (Do remember to restart it when the window reopens if that is important to you.)

(define/override (show show?)
  (unless show?
    (send timer stop))
  (super show show?))

Making your overall class look something like:

#lang racket/gui
(define window
  (new (class frame%
         (super-new [label "hello"])
         (define timer
           (new timer%
                [interval 1000]
                [notify-callback (λ x (displayln "ding"))]))
         (define/augment (on-close)
           (send timer stop))
         (define/override (show show?)
           (unless show?
             (send timer stop))
           (super show show?)))))

(send window show #f)

Now your program will terminate.

_{¹There is a on-superwindow-show method, but it does not seem to always run when show is called.}

来源：https://stackoverflow.com/questions/43877936/programmatically-close-a-window-made-with-racket-gui-to-stop-a-timer