scheme,lisp,racket , Racket: using big bang and on-click


Racket: using big bang and on-click

Question:

Tag: scheme,lisp,racket

I've somehow been able to draw a small basic image in racket (a house). Now, what I'm stuck with is, using big-bang to draw the image -- and then being able to click the roof of the house and have it change to another color. This is what I have so far:

#lang racket
(require 2htdp/image)
(require 2htdp/universe)



(text "---Small Shack---" 18 "brown")

(define door (rectangle 30 50 "solid" "white"))

(define shack (above (triangle 100 "solid" "red")
                     (rectangle 120 80 "solid" "grey")))

(define doorhandle (overlay/align "right" "center" (circle 5 "solid" "blue") door))

(define house-view (overlay/align "center" "bottom" doorhandle shack))

house-view

I've tried a million different things to get it to work with big-bang but nothing worked. Any help getting big-bang and it changing color of the roof when clicked?


Answer:

The big-bang form works by passing a "state" throughout handlers, which is then handed off to the to-draw handler to paint the picture that will be flushed to the screen.

For starters, you should lift your drawing code into big-bang's to-draw clause:

(big-bang 
 null
 (to-draw
  (λ (state)
    (text "---Small Shack---" 18 "brown")
    (define door (rectangle 30 50 "solid" "white"))
    (define shack (above (triangle 100 "solid" "red")
                         (rectangle 120 80 "solid" "grey")))
    (define doorhandle (overlay/align "right" "center" (circle 5 "solid" "blue") door))
    (define house-view (overlay/align "center" "bottom" doorhandle shack))
    house-view)))

This will display your house as the result of each draw call. You'll also need some way to manage your world state. The conventional way to do this is by using a struct.

(struct world (roof-color))

The first parameter you provide to big-bang is the initial state, so we should initialize the state to use "red" as the roof color. Then we can use the current state's color in our to-draw handler:

(big-bang 
 (world "red")
 (to-draw
  (λ (state)
    (text "---Small Shack---" 18 "brown")
    (define door (rectangle 30 50 "solid" "white"))
    (define shack (above (triangle 100 "solid" (world-roof-color state))
                         (rectangle 120 80 "solid" "grey")))
    (define doorhandle (overlay/align "right" "center" (circle 5 "solid" "blue") door))
    (define house-view (overlay/align "center" "bottom" doorhandle shack))
    house-view)))

Finally, you can implement a on-mouse clause to handle mouse clicks. Here's a simple implementation that makes the roof green when the user clicks.

(big-bang 
 (world "red")

 (on-mouse
  (λ (state x y event)
    (case event
      [("button-up")
       (world "green")]
      [else state])))

 (to-draw
  (λ (state)
    (text "---Small Shack---" 18 "brown")
    (define door (rectangle 30 50 "solid" "white"))
    (define shack (above (triangle 100 "solid" (world-roof-color state))
                         (rectangle 120 80 "solid" "grey")))
    (define doorhandle (overlay/align "right" "center" (circle 5 "solid" "blue") door))
    (define house-view (overlay/align "center" "bottom" doorhandle shack))
    house-view)))

For more information, see the relevant documentation in the HtDP 2e section of the Racket documentation. You should be able to figure out how to tweak this to your needs.


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