[plt-scheme] using inherit-field when making new gui widgets

From: Stephen De Gabrielle (spdegabrielle at gmail.com)
Date: Thu Oct 4 10:09:23 EDT 2007

Hi, I'm having a go at creating a new widget

But I am getting the error

timeline-selector.ss::158: class*: superclass does not provide field:
min-width for class: timeline-selector%
(note the character offset - rather than the line number/character position
reported in bug 8954)

I think I am using inherit field correctly, and the pane% I am using as a
superclass does have a 'min-width' field.

As I understand the manual, I need inherit field so I can access the field
in my subclass (in this case to make a canvas).

An inherit-field declaration makes a public field defined by a superclass
> directly accessible in the class expression. If the indicated field is not
> defined in the superclass, the exn:fail:object exception is raised when the
> class expression is evaluated. Every field in a superclass is present in a
> derived class, even if it is not declared with inherit-field in the derived
> class. The inherit-field clause does not control inheritance, but merely
> controls lexical scope within a class expression.

Can anyone let me know where I have gone wrong?

This is how I call the creation of my new class

  (define timeline-selector (new timeline-selector% (parent
                                 (min-width 100) (min-height 800)
                                 (stretchable-width #f) (stretchable-height

;  make new widget

  (define timeline-selector% (class pane% ()
                               (inherit-field  min-width
                                               stretchable-height )
                               (init-field parent )
                               ; Make the drawing area
                               (super-instantiate () (parent parent)) ;
instantiate this FIRST
                               ;;    [min-width min-width]
[stretchable-width stretchable-width]   ;; commented out as hopefully not
                               ;;  (min-height min-height)
[stretchable-height stretchable-height])

                               ;; internal data
                               (define list-of-points '()) ;; start with
empty list
                               (define canvas (new canvas% [parent this]
                                                   (style '(control-border))
                                                   (paint-callback (lambda
(canvas dc) (draw-chart)))
                                                   [min-width min-width]
[stretchable-width #f]
                                                   [min-height min-height]
[stretchable-height #f]
                               (define/public (get-dc) (send canvas get-dc))
; Get the canvas's drawing context
(... and so on)



Stephen De Gabrielle
s.degabrielle at ucl.ac.uk
Telephone +44 (0)20 7679 5242 (x45242)
Mobile                  079 851 890 45
University College London Interaction Centre
Remax House - 31/32 Alfred Place
London - WC1E 7DP
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