Cocos2D-JS无法模拟物理

Question

我正在尝试在Cocos2D-JS中模拟物理，但我没有成功。我是Cocos2D-JS的新手。有人能告诉我为什么我的精灵没有下降，如果我的世界设置为重力为真？以及为什么DistanceJoint不起作用？非常感谢你。

var b2Vec2 = Box2D.Common.Math.b2Vec2
    , b2AABB = Box2D.Collision.b2AABB
    , b2BodyDef = Box2D.Dynamics.b2BodyDef
    , b2Body = Box2D.Dynamics.b2Body
    , b2FixtureDef = Box2D.Dynamics.b2FixtureDef
    , b2Fixture = Box2D.Dynamics.b2Fixture
    , b2World = Box2D.Dynamics.b2World
    , b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape
    , b2CircleShape = Box2D.Collision.Shapes.b2CircleShape
    , b2DebugDraw = Box2D.Dynamics.b2DebugDraw
    , b2MouseJointDef =  Box2D.Dynamics.Joints.b2MouseJointDef
    , b2DistanceJointDef =  Box2D.Dynamics.Joints.b2DistanceJointDef
    , b2Shape = Box2D.Collision.Shapes.b2Shape
    , b2_dynamicBody =  Box2D.Dynamics.b2_dynamicBody

var HelloWorldLayer = cc.Layer.extend({
    spriteSoccerball:null,
    spriteBasketball:null,
    ctor:function () {
        this._super();
        var size = cc.winSize;
        var gravity = new Box2D.Common.Math.b2Vec2(0, -10)
        world = new b2World(gravity,true);
        //--------------------------
        var helloLabel = new cc.LabelTTF("RoboArm", "Arial", 38);
        helloLabel.x = size.width / 2;
        helloLabel.y = size.height / 2 + 200;
        this.addChild(helloLabel, 5);
        //--------------------------
        spriteSoccerball = new cc.Sprite(res.soccerBall_png);
        spriteSoccerball.setPositionX(300);
        spriteSoccerball.setPositionY(300);
        spriteSoccerball.anchorX=0.5;
        spriteSoccerball.anchorY=0.5;
        spriteSoccerball.setScale(0.1, 0.1);
        this.addChild(spriteSoccerball, 0);
        //--------------------------
        spriteBasketball = new cc.Sprite(res.basketBall_png);
        spriteBasketball.setPositionX(200);
        spriteBasketball.setPositionY(200);
        spriteBasketball.anchorX=0.5;
        spriteBasketball.anchorY=0.5;
        spriteBasketball.setScale(0.1, 0.1);
        this.addChild(spriteBasketball, 0);
        //--------------------------
        a = createBody(world, spriteBasketball, spriteBasketball.getPositionX(), spriteBasketball.getPositionY(), 1, 1);
        b = createBody(world, spriteSoccerball, spriteSoccerball.getPositionX(), spriteSoccerball.getPositionY(), 1, 1);
        createDistanceJoint(world,a,b);
        //--------------------------
        var listener = cc.EventListener.create({
            event: cc.EventListener.TOUCH_ALL_AT_ONCE,
            onTouchBegan: function (touch, event) {
                return true;
            },
            onTouchesMoved: function (touch, event) {
                var pos = touch[0].getLocation();
                a.SetPosition(pos);
                return true;
            },
            onTouchEnded: function (touch, event) {
                return true;
            }
        });
        cc.eventManager.addListener(listener, this);
        this.scheduleUpdate();
    },
    update: function (dt) {
        spriteBasketball.setPosition(a.GetPosition().x ,
            a.GetPosition().y);
        var timeStep = 1.0/(fps * 0.8);
        //move the box2d world ahead
        world.Step(timeStep , 8 , 3);
        world.ClearForces();
        //call this function again after 1/60 seconds or 16.7ms
        setTimeout(this.step , 1000 / fps);
    }
});

//Create standard boxes of given height , width at x,y
function createBox(world, x, y, width, height, options)
{
    var body_def = new b2BodyDef();
    var fix_def = new b2FixtureDef();

    fix_def.shape = new b2PolygonShape();
    fix_def.shape.SetAsBox( width/2 , height/2 );

    body_def.position.Set(x , y);
    body_def.linearDamping = 0.5;
    body_def.angularDamping = 0.5;

    var b = world.CreateBody( body_def );
    var f = b.CreateFixture(fix_def);

    return b;
}

//Create standard boxes of given height , width at x,y
function createBody(world, sprite, x, y, width, height)
{
    var body_def = new b2BodyDef;
    body_def.type = b2_dynamicBody;
    body_def.angle = 1;
    body_def.gravityScale = 1.0;
    body_def.position.Set(x, y);
    body_def.linearVelocity.Set(1,1);
    body_def.angularVelocity = 2.0;
    body_def.linearDamping = 3.0;
    body_def.angularDamping = 4.0;
    body_def.inertiaScale = 3.0;
    body_def.active=true;

    var fix_def = new b2FixtureDef;
    fix_def.shape = new b2PolygonShape();
    fix_def.shape.SetAsBox( width/2 , height/2 );
    fix_def.density = 1.0;
    fix_def.friction = 0.3;

    var b = world.CreateBody(body_def);
    b.CreateFixture(fix_def);

    return b;
}

function createDistanceJoint(world, a,b)
{
    //create distance joint between b and c
    var joint_def = new b2DistanceJointDef;
    joint_def.bodyA = a;
    joint_def.bodyB = b;

    //connect the centers - center in local coordinate - relative to body is 0,0
    joint_def.localAnchorA = new b2Vec2(0.5,0.5);
    joint_def.localAnchorB = new b2Vec2(0.5,0.5);

    //difference in angle of each body
    joint_def.referenceAngle = 1 * Math.PI / 3;

    //add the joint to the world
    world.CreateJoint(joint_def);
}

var HelloWorldScene = cc.Scene.extend({
    onEnter:function () {
        this._super();
        var layer = new HelloWorldLayer();
        this.addChild(layer);
    }
});