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20. Click Circle

Exercises 20.6.3 and 20.6.4.

A circle moves to a random position on the screen every so often. (20.6.3)
Clicking on the circle displays a win screen. (20.6.4) Note: I recommend using a special posn as the model to indicate when to display the win screen.

check-with

The clause (check-with posn?) makes sure that every handler returns a posn. You should add this clause to your big-bang whenever you work with posns. Colors have a similar check, color?.

Stop-when

There is a new clause in big-bang called stop-when. It works like the linked demo code. The big-bang looks like this:

(big-bang 0
          (on-tick add1 0.1)
          (on-draw draw-num 300 200)
          (stop-when above-ten? draw-done))

The above-ten? method takes in a model and produces a boolean value. The big-bang stops if this function returns true.

The draw-done method is a draw handler called when to produce the last frame of the animation - regarless of why the big-bang is ending.

Impossible State Trick

One way to decide that an animation is over is to change the model to something that is impossible under regular circumstances. For example, a posn with negative coordinates could not be created by the mouse-handler under normal circumstances, so it could be used to indicate that the animation should end.

(define MODEL-END-IT (make-posn -1001 999))
(define (stopping? model) 
  (posn=? model MODEL-END-IT))

Notes

Never place randomness in the draw handler. The draw handler can be called lots more frequently than you imagine (for example, when you drag the window and it needs to redraw).
Your model should not be an image any more. Use a better model.
Complete big-bang documentation so you can look up any unusual keys or functions that you need. For example: button-down.