滑轮关节实现悬挂物

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;

    /**
    * 
    */
    public class Main extends BaseMain{
        
        
        public function Main(){
            super(new b2Vec2(0,10));
        }
        
        override protected function init():void{
            var bodyA:b2Body=this.createBox(50,50,400,350);
            var bodyB:b2Body=_world.GetGroundBody();
            
            var jointDef:b2PulleyJointDef=new b2PulleyJointDef();
            
            //悬挂点
            var bindPos:b2Vec2=new b2Vec2(400/_pixelToMeter,300/_pixelToMeter);
            
            var anchorA:b2Vec2=bodyA.GetWorldCenter();
            var anchorB:b2Vec2=bodyB.GetWorldCenter();
            
            //距离必须大于滑轮关节的最小距离
            if(b2Vec2.Distance(anchorA,bindPos)>b2PulleyJoint.b2_minPulleyLength){
                var r:Number=0;
                jointDef.Initialize(bodyA,bodyB,bindPos,bindPos,anchorA,anchorB,r);
                var joint:b2PulleyJoint=_world.CreateJoint(jointDef) as b2PulleyJoint;
            }
            
        }
        
        override protected function stepBefore():void{ 
            
        }
        
        
    };
}