1.8 Managing Control Arrows: "arrow.ss"

The teachpack implements a controller for moving shapes across a canvass. A shape is a class of data for which move and draw operations can be drawn.

(control-left-right shape n move draw) → true

shape : Shape

n : number?

move : (-> number? Shape Shape)

draw : (-> Shape true)

Moves shape n pixels left (negative) or right (positive).

(control-up-down shape n move draw) → true

shape : Shape

n : number?

move : (-> number? Shape Shape)

draw : (-> Shape true)

Moves shape n pixels up (negative) or down (positive).

(control shape n move-lr move-ud draw) → true

shape : Shape

n : number?

move-lr : (-> number? Shape Shape)

move-ud : (-> number? Shape Shape)

draw : (-> Shape true)

Moves shape N pixels left or right and up or down, respectively.

Example:

; A shape is a structure:

; (make-posn num num)

; RAD : the radius of the simple disk moving across a canvas

(define RAD 10)

; move : number shape -> shape or false

; to move a shape by delta according to translate

; effect: to redraw it

(define (move delta sh)

(cond

[(and (clear-solid-disk sh RAD)

(draw-solid-disk (translate sh delta) RAD))

(translate sh delta)]

[else false]))

; translate : shape number -> shape

; to translate a shape by delta in the x direction

(define (translate sh delta)

(make-posn (+ (posn-x sh) delta) (posn-y sh)))

; draw-it : shape -> true

; to draw a shape on the canvas: a disk with radius

(define (draw-it sh)

(draw-solid-disk sh RAD))

; RUN:

; this creates the canvas

(start 100 50)

; this creates the controller GUI

(control-left-right (make-posn 10 20) 10 move draw-it)