通过metamethod调用函数

通过metamethod调用函数普通函数，发现indexOfMethod总是返回-1，查看文档，发现下面：

Q_INVOKABLE
Apply this macro to definitions of member functions to allow them to be invoked via the meta-object system. The macro is written before the return type, as shown in the following example:

class Window : public QWidget
{
     Q_OBJECT

public:
     Window();
     void normalMethod();
     Q_INVOKABLE void invokableMethod();
};
The invokableMethod() function is marked up using Q_INVOKABLE, causing it to be registered with the meta-object system and enabling it to be invoked using QMetaObject::invokeMethod(). Since normalMethod() function is not registered in this way, it cannot be invoked using QMetaObject::invokeMethod().

这个语句作用是把这个方法注册到meta-object system ，才能用