b2BuoyancyController 使用浮力

package{
    import Box2D.Collision.b2AABB;
    import Box2D.Collision.b2RayCastInput;
    import Box2D.Collision.b2RayCastOutput;
    import Box2D.Collision.Shapes.b2CircleShape;
    import Box2D.Collision.Shapes.b2PolygonShape;
    import Box2D.Collision.Shapes.b2Shape;
    import Box2D.Common.Math.b2Vec2;
    import Box2D.Dynamics.b2Body;
    import Box2D.Dynamics.b2BodyDef;
    import Box2D.Dynamics.b2Fixture;
    import Box2D.Dynamics.b2FixtureDef;
    import Box2D.Dynamics.b2World;
    import flash.display.Sprite;
    import flash.utils.getTimer;
    import Box2D.Dynamics.Joints.b2PulleyJoint;
    import Box2D.Dynamics.Joints.b2PulleyJointDef;
    import Box2D.Dynamics.Controllers.b2BuoyancyController;

    /**
    *
    */
    public class Main extends BaseMain{

        private var _bodies:Vector.<b2Body>=new Vector.<b2Body>();

        public function Main(){
            super(new b2Vec2(0,10));
        }

        override protected function init():void{
            createBodies();
            createBuoyancyController();
        }

        private function createBodies():void{
            for(var i:int=0;i<50;i++){
                var x:Number=stage.stageWidth*0.5;
                var y:Number=50;
                var body:b2Body=Math.random()>0.5?createBox(20,20,x,y):createCircle(10,x,y);
                _bodies.push(body);
            }
        }

        private function createBuoyancyController():void{
            var controller:b2BuoyancyController=new b2BuoyancyController();
            controller.angularDrag=0.5;
            controller.density=1.5;//设置水的密度,要大于水中刚体的密度才能上浮
            //controller.gravity=null;
            controller.linearDrag=0.5;
            controller.normal.Set(0,-1);//设置水面的法向量
            controller.offset=-200/_pixelToMeter;//水面的高度，默认在最顶部（y轴0的位置）,-100表示向下移动100
            //controller.useDensity=false;
            //controller.useWorldGravity=true;
            //controller.velocity.Set(0,0);
            _world.AddController(controller);
            for(var i:int=0;i<_bodies.length;i++){
                controller.AddBody(_bodies[i]);
            }

        }

        override protected function stepBefore():void{ 

        }

    };
}