To denote the current receiver, you use this expressions:
* In a member of a class, this refers to the current object of that class.
* In an extension function or a function literal with receiver this denotes the receiver parameter that is passed on the left-hand side of a dot.
If this has no qualifiers, it refers to the innermost enclosing scope. To refer to this in other scopes, label qualifiers are used:
Qualified this
this@label
如果有一个顶级函数签名与成员函数一样, 那么需要使用
this, 不然会调用到顶级函数.