returning GUI elements in function?

[Travis Hinkelman]

Hi All,

I was playing around with creating a progress bar. The basic idea was straightforward.

#lang racket/gui

(define frame (new frame%

                   [label "Progress Bar"]

                   [width 300]))

(define hpane (new horizontal-pane%

                   [parent frame]))

(define gauge (new gauge%

                   [label ""]

                   [parent hpane]

                   [range 100]))

(define msg (new message%

                 [parent hpane]

                 [auto-resize #t]

                 [label "0%"]))

(send frame show #t)

(for ([i (in-range 1 101)])

  (sleep 0.05)

  (send gauge set-value i)

  (send msg set-label (string-append (~a i) "%")))

When I tried to take the next step of wrapping this up into a couple of functions, I was completely lost. I don't know how to initialize a frame with child elements by calling a function because all of the elements of the frame only exist in the function and not at the top-level (probably butchering the jargon here; hopefully it makes sense). I wasn't really sure where to look to learn more about doing such a thing. Here is some pseudocode to try to further illustrate my confusion.

(define (initialize-progress-bar)

  ;; insert code that creates progress bar

  ;; i.e., from defining frame to sending frame in code above)

(define (update-progress-bar new-value)

  (send gauge set-value new-value)

  (send msg set-label (string-append (~a new-value) "%")))

(initialize-progress-bar)

(for ([i (in-range 1 101)])

  (sleep 0.05)

  (update-progress-bar i))

Thanks,

Travis

[Laurent]

There are several ways to solve this (including deriving classes), but the simplest for you right now is probably to have `initialize-progress-bar' return the gui widgets (in particular `gauge' and `msg'), assign the values to `the-gauge' and `the-msg' (say) from the return values of `(initialize-progress-bar)` call, and then pass these values to `update-progress-bar' so that `gauge' and `msg' have the correct values there.

Does that make sense?

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[Travis Hinkelman]

It's funny how just knowing that there is an easy solution to a problem makes the problem easier to solve. My first thought about returning objects (prior to sending initial email) was that I would return all of the objects unnamed but then I was confused about how to specify the parents of the child objects in the `initialize-progress-bar` function. The code below solves my problem. As always, I welcome people sharing alternative ways to write the same code. 

#lang racket/gui

(define (initialize-progress-bar)

  (define frame (new frame%

                     [label "Progress Bar"]

                     [width 300]))

  (define hpane (new horizontal-pane%

                     [parent frame]))

  (define gauge (new gauge%

                     [label ""]

                     [parent hpane]

                     [range 100]))

  (define msg (new message%

                   [parent hpane]

                   [auto-resize #t]

                   [label "0%"]))

  (send frame show #t)

  (values gauge msg))

(define (update-progress-bar new-value)

  (send the-gauge set-value new-value)

  (send the-msg set-label (string-append (~a new-value) "%")))

(define-values (the-gauge the-msg) (initialize-progress-bar))

(for ([i (in-range 1 101)])

  (sleep 0.05)

  (update-progress-bar i))

On Tuesday, July 9, 2019 at 1:05:03 AM UTC-7, Laurent wrote:

There are several ways to solve this (including deriving classes), but the simplest for you right now is probably to have `initialize-progress-bar' return the gui widgets (in particular `gauge' and `msg'), assign the values to `the-gauge' and `the-msg' (say) from the return values of `(initialize-progress-bar)` call, and then pass these values to `update-progress-bar' so that `gauge' and `msg' have the correct values there.

Does that make sense?

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[Simon Schlee]

Classes are really nice and easy for tasks like this, it took me a little bit to get used to the class syntax, but now I love them.

I found that it helps to look at real code that uses classes, because the documentation quickly goes into quite advanced/special use cases.

You can use them to wrap-up/encapsulate a bunch of smaller gui parts and then just expose some methods that handle the updates etc. 

As always, I welcome people sharing alternative ways to write the same code.

Here you go:

#lang racket/gui

(define progress-bar%
  (class horizontal-pane%
    (super-new)

    (define gauge (new gauge%
                       [label #f]
                       [parent this]
                       [range 100]))

    ;; initialize to 100% with no auto resize
    ;; this way the label has the right size
    ;; so that everything can be seen
    ;; and it does not "wobble" around while filling up
    (define msg (new message%
                     [parent this]
                     [label "100%"]))

    (define/public (set-value val)
      (send gauge set-value val)
      (send msg set-label (string-append (~a val) "%")))

    ;; set back to 0%
    (set-value 0)))


(module+ main

  (define frame (new frame%
                     [label "Progress Bar"]
                     [width 300]))

  (define progress (new progress-bar% [parent frame]))

  (send frame show #t)
  (for ([i (in-range 1 101)])
    (sleep 0.05)

    (send progress set-value i)))