1 定义一个类层次作为例子 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 class MediaItem { var name: String init(name: String) { self.name = name } } class Movie: MediaItem { var director: String init(name: String, director: String) { self.director