Guided Practice 12.2 - Solution

The question was:

In flashing-balls.rkt, the code for add-to-scene in FlashingBall% duplicates some of the code for add-to-scene in Ball%. Eliminate this by using super.

Solution:

There are a number of ways to do this. Here's how I did it:

1. In Ball%, make add-to-scene look at a field called 'color' instead of referring to the constant "red".

       (field [color "red"])
   
       (define/public (add-to-scene s)
         (place-image
          (circle radius 
                  (if selected? "solid" "outline")
                  color)      ;;; <---- changed line
          x y s))
   

2. In FlashingBall%, inherit the field 'color.' Then have change-colors update the color field, and have add-to-scene call super instead of duplicating the code from the superclass:

   
       (inherit-field color)
   
       ;; -> Void
       ;; EFFECT: rotate the list of colors, and reset time-left
       (define (change-colors)
         (set! colors (append (rest colors) (list (first colors))))
         ;; put desired color into 'color' field
         (set! color (first colors))    ; <--- new line
         (set! time-left color-change-interval))
       
       ;; Scene -> Scene
       ;; RETURNS: a scene like the given one, but with the flashing ball
       ;; painted on it.
       ;; EFFECT: decrements time-left and changes colors if necessary
       (define/override (add-to-scene s)
         (begin
           (if (zero? time-left)
             (change-colors)
             (set! time-left (- time-left 1)))
           ;; (place-image
           ;;   (circle radius
           ;;     (if selected? "solid" "outline")
           ;;     (first colors))
           ;;   x y s)
           (super add-to-scene s)   ; <-- call to super replaces
                                    ;     duplicated code
           ))