再用3.0rc来开发碰撞检测的问题时，在原来的beta2之前可以调用的方式
auto contactListener = EventListenerPhysicsContact::create();
contactListener->onContactBegin = CC_CALLBACK_2(GameWorld::onContactBegin, this);
this->getEventDispatcher()->addEventListenerWithSceneGraphPriority(contactListener, this);
```
第二行无法编译。查看源码并对比后发现，函数声明出现了变化
下面是beta2的：
public:
/*
* @brief it will called at two shapes start to contact, and only call it once.
*/
std::function onContactBegin;
```
下面是rc的
public:
/*
* @brief it will called at two shapes start to contact, and only call it once.
*/
std::function onContactBegin;
```
可以看到rc0中参数少了一个。那我该如何使用碰撞检测呢？？新手求解
