package
  {
          import flash.display.Sprite;
          import flash.events.Event;
          import flash.events.KeyboardEvent;
          import flash.ui.Keyboard;
          
          public class @ax-animation-ch05-Gravity extends Sprite
          {
                  private var ball:animation_ch05_Ball;
                  private var vx:Number = 0;
                  private var vy:Number = 0;
                  private var ax:Number = 0
                  private var ay:Number = 0
                  private var gravity:Number = 0.1;
                  
                  public function @ax-animation-ch05-Gravity()
                  {
                          init();
                  }
                  
                  private function init():void
                  {
                          ball = new animation_ch05_Ball();
                          addChild(ball);
                          ball.x = stage.stageWidth / 2;
                          ball.y = stage.stageHeight / 2;
                          addEventListener(Event.ENTER_FRAME, onEnterFrame);
                          stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
                          stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUp);
                  }
                  
                  private function onKeyDown(event:KeyboardEvent):void
                  {
                          switch(event.keyCode)
                          {
                                  case Keyboard.LEFT :
                                  ax = -0.2;
                                  break;
                                  
                                  case Keyboard.RIGHT :
                                  ax = 0.2;
                                  break;
                                  
                                  case Keyboard.UP :
                                  ay = -0.2;
                                  break;
                                  
                                  case Keyboard.DOWN :
                                  ay = 0.2;
                                  break;
                                  
                                  default :
                                  break;
                          }
                  }
                  
                  private function onKeyUp(event:KeyboardEvent):void
                  {
                          ax = 0;
                          ay = 0;
                  }
                  
                  private function onEnterFrame(event:Event):void
                  {
                          vx += ax;
                          vy += ay;
                          vy += gravity;
                          ball.x += vx;
                          ball.y += vy;
                  }
          }
  }