box2d多个fixture的位置问题

Cocos2d-x
#1

Dear ：
我用box2d创建多边形时，有多个fixture、PolygonShape。
在更新物理世界时，需要获取该body的position，但是取到的却是（0，0），而不是屏幕位置。
请问正确获取位置的姿势是什么？

// 创建多边形：

b2Body* createPoly(
 b2World* world,
 const std::vector<std::vector<Point> >& trian,  // 坐标是像素
 void *userData,
 bool isStatic
 
 )
{
 b2BodyDef bodyDef;
    bodyDef.type = isStatic ? b2_staticBody : b2_dynamicBody;
    bodyDef.position.Set(0, 0);
 b2Body* body = world->CreateBody(&bodyDef);
 if (userData) body->SetUserData(userData);


 b2FixtureDef fixtureDef;
 fixtureDef.density = 3.f;
 fixtureDef.restitution = 0.2f;
 fixtureDef.friction = 0.3f;


 for (size_t i = 0; i < trian.size(); ++i)
    {
 const std::vector<Point> &v = trian;
  b2PolygonShape pshape;
  b2Vec2* ar = new b2Vec2;
  int count = (int)points2VecArray(v, ar);   // 这里做了像素和米的转换
  pshape.Set(ar, count);
  fixtureDef.shape = &pshape;
        body->CreateFixture(&fixtureDef);
 delete] ar;
    }
 return body;
}

// 更新物理世界：


for (auto b = m_world->GetBodyList(); false && b; b = b->GetNext())
    {
 if (b->GetType() == b2_dynamicBody)
        {
 if (b->GetUserData())
            {
 auto s = (Node*)b->GetUserData();
                s->setPosition(b->GetPosition().x * PTM_RATIO,  //  做自由落体时 x都是0？
                               b->GetPosition().y * PTM_RATIO);
                s->setRotation(b->GetAngle());
            }
        }
    }