20. Chase

In this animation, a 20x20 square moves toward the mouse whenever the mouse does something. If the square touches the mouse, then the game is over.

• model
• what model will we use to indicate that the game is over?
• draw handler
• stop-when: decide when the game is over
• lose screen draw handler: put “game over” on the screen

Square moves toward the mouse

You want the square to move toward the mouse, but not too fast or nobody will want to play the game.

Ideas:

• When you add posns A + B, that moves posn A in direction B
• When you want to limit your motion, scale the motion posn so that its length is (say) 5.

Basic: we will make the “1/20 of the distance” chase

The next-change function will tell how to adjust the current position of the square based on the goal position of the mouse.

next-change: posn(goal) posn(current) -> posn

Advanced: make the “speed = 3” chase

A variation on this is to make the square chase the mouse at a “constant speed” of 3 pixels every mouse motion. This done by making a helper function scale-to-3: posn -> posn that adjusts the posn so its length is always 3 (assume the posn is not (0,0)).