ActionScript 3.0 Design Patterns

Example: Animating Composite Objects Using Inverse Kinematics | 227

Line 26 in theMainclass (Example 6-14) registers theonKeyPresshandler method to

respond toKey_DOWNevents. The left and right arrow keys make the snake head rotate

counter-clockwise and clockwise by 10 degrees. The up and down arrow keys make

the snake head move forward or back by 5 pixels. ThePoint.polarmethod comes in

handy here, as the snake needs to move in the direction that it’s facing (rotation

angle). ThePoint.polar()method returns a point that is the distance and angle from

the origin (0, 0). Adding the coordinates of this point (xandyproperties of thePoint

class) to the current location gives us the new location.

Before developing the body segments of the snake, we need to explore how a kine-

matic chain works.

23 addChild(snake);

24

25 // register with the stage to receive key press events

26 stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyPress);

27 }

28

29 private function onKeyPress(event:KeyboardEvent):void

30 {

31 switch (event.keyCode)

32 {

33 case Keyboard.LEFT :

34 snake.rotation -= 10;

35 break;

36 case Keyboard.RIGHT :

37 snake.rotation += 10;

38 break;

39 case Keyboard.UP :

40 var newFWLocOffset:Point = Point.polar(5, snake.rotation * Math.PI / 180);

41 snake.x += newFWLocOffset.x;

42 // move forward by the x offset

43 snake.y += newFWLocOffset.y;

44 // move forward by the y offset

45 break;

46 case Keyboard.DOWN :

47 var newBKLocOffset:Point = Point.polar(5, snake.rotation * Math.PI / 180);

48 snake.x -= newBKLocOffset.x;

49 // move back by the x offset

50 snake.y -= newBKLocOffset.y;

51 // move back by the y offset

52 break;

53 }

54 }

55 }

56 }

Example 6-14. Main.as