On this page:
1.7.1 Drawing on a Canvas
start
start/  cartesian-plane
stop
draw-circle
draw-solid-disk
draw-solid-rect
draw-solid-line
draw-solid-string
sleep-for-a-while
clear-circle
clear-solid-disk
clear-solid-rect
clear-solid-line
clear-solid-string
clear-all
1.7.2 Interactions with Canvas
wait-for-mouse-click
get-key-event
big-bang
on-key-event
on-tick-event
end-of-time

1.7 Simple Drawing: "draw.rkt" 🔗

 (require htdp/draw) package: htdp-lib

The teachpack provides two sets of functions: one for drawing into a canvas and one for reacting to canvas events.

NOTE: This library is deprecated; use 2htdp/image (probably in conjunction with 2htdp/universe), instead. You may continue to use the library for solving exercises from How To Design Programs, First Edition but do consider switching to How To Design Programs, Second Edition instead.

1.7.1 Drawing on a Canvas 🔗

DrawColor: (and/c symbol? (one-of/c 'white 'yellow 'red 'blue 'green 'black)) These six colors are definitely provided. If you want other colors, guess! For example, 'orange works, but 'mauve doesn’t. If you apply the function to a symbol that it doesn’t recognize as a color, it raises an error.

procedure

(start width height)  true

  width : number?
  height : number?
Opens a width x height canvas.

procedure

(start/cartesian-plane width height)  true

  width : number?
  height : number?
Opens a width x height canvas and draws a Cartesian plane.

procedure

(stop)  true

Closes the canvas.

procedure

(draw-circle p r c)  true

  p : posn?
  r : number?
  c : DrawColor
Draws a c circle at p with radius r.

procedure

(draw-solid-disk p r c)  true

  p : posn?
  r : number?
  c : DrawColor
Draws a c disk at p with radius r.

procedure

(draw-solid-rect ul width height c)  true

  ul : posn?
  width : number?
  height : number?
  c : DrawColor
Draws a width x height, c rectangle with the upper-left corner at ul.

procedure

(draw-solid-line strt end c)  true

  strt : posn?
  end : posn?
  c : DrawColor
Draws a c line from strt to end.

procedure

(draw-solid-string p s)  true

  p : posn?
  s : string?
Draws s at p.

procedure

(sleep-for-a-while s)  true

  s : number?
Suspends evaluation for s seconds.

The teachpack also provides clear- functions for each draw- function:

procedure

(clear-circle p r c)  true

  p : posn?
  r : number?
  c : DrawColor
clears a c circle at p with radius r.

procedure

(clear-solid-disk p r c)  true

  p : posn?
  r : number?
  c : DrawColor
clears a c disk at p with radius r.

procedure

(clear-solid-rect ul width height c)  true

  ul : posn?
  width : number?
  height : number?
  c : DrawColor
clears a width x height, c rectangle with the upper-left corner at ul.

procedure

(clear-solid-line strt end c)  true

  strt : posn?
  end : posn?
  c : DrawColor
clears a c line from strt to end.

procedure

(clear-solid-string p s)  true

  p : posn?
  s : string?
clears s at p.

procedure

(clear-all)  true

clears the entire screen.

1.7.2 Interactions with Canvas 🔗

procedure

(wait-for-mouse-click)  posn?

Waits for the user to click on the mouse, within the canvas.

DrawKeyEvent: (or/c char? symbol?) A DrawKeyEvent represents keyboard events:
  • char?, if the user pressed an alphanumeric key;

  • symbol?, if the user pressed, for example, an arror key: 'up 'down 'left 'right

procedure

(get-key-event)  (or/c false DrawKeyEvent)

Checks whether the user has pressed a key within the window; false if not.

DrawWorld: For proper interactions, using the teachpack requires that you provide a data definition for DrawWorld . In principle, there are no constraints on this data definition. You can even keep it implicit, even if this violates the Design Recipe.

The following functions allow programs to react to events from the canvas.

procedure

(big-bang n w)  true

  n : number?
  w : DrawWorld
Starts the clock, one tick every n (fractal) seconds; w becomes the first “current” world.

procedure

(on-key-event change)  true

  change : (-> DrawKeyEvent DrawWorld DrawWorld)
Adds change to the world. The function reacts to keyboard events and creates a new DrawWorld.

procedure

(on-tick-event tock)  true

  tock : (-> DrawWorld DrawWorld)
Adds tock to the world. The function reacts to clock tick events, creating a new current world.

procedure

(end-of-time)  DrawWorld

Stops the world; returns the current world.