1、扩展中无法继承重写已有函数，不能添加函数。
Extensions can add new functionality to a type, but they cannot override existing functionality.
https://docs.swift.org/swift-book/LanguageGuide/Extensions.html
2、扩展函数的可见性：全局可见
If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined.
3、扩展中函数可以有条件被继承
dynamic 可以让声明在 extension 里面的函数能够被 override.
https://www.jianshu.com/p/91bfe3f11eec
4、扩展只可以添加计算属性：
Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.
https://docs.swift.org/swift-book/LanguageGuide/Extensions.html