pro OpenGL ES for ios.pdf 非扫描版

class Pet { enum Kind { case dog case cat case guineaPig } let name: String let kind: Kind let favoriteToy: Toy? init(name: String, kind: Kind, favoriteToy: Toy? = nil) { self.name = name self.kind = kind self.favoriteToy = favoriteToy } } class Person { let pet: Pet? init(pet: Pet? = nil) { self.pe