为什么我有我的子类的方法？

Question

class MediaItem {
    var name: String
    init(name: String) {
        self.name = name
    }
}

class Movie: MediaItem {
    var director: String
    init(name: String, director: String) {
        self.director = director
        super.init(name: name)
    }
}

class Song: MediaItem {
    var artist: String
    init(name: String, artist: String) {
        self.artist = artist
        super.init(name: name)
    }
}

var song : MediaItem = Song(name: "Mateo",artist: "Romeo")

我知道这首歌是 MediaItem 实例，但我正在使用 Song 实例对其进行初始化。

我的歌是什么类型的？是MediaItem 还是Song？

我应该使用哪种方法？

var song : MediaItem = Song(name: "Mateo",artist: "Romeo")

或

var song = Song(name: "Mateo",artist: "Romeo")

Answer 1

song 是您声明为 MediaItem 实例的 Song 实例。一首歌可能是一个媒体项目（通过继承），但一个媒体项目不一定是一首歌。

var song : MediaItem = Song(name: "Mateo",artist: "Romeo")

song.dynamicType // Song.Type
song is Song // true
song is MediaItem // true
song is Movie // false

song.name // "Mateo"
song.artist // Value of Type 'MediaItem' has no member 'artist'

将其声明为 MediaItem 的“陷阱”是，如果不强制向下转换 as explained on the Swift blog，则不能明确地向下转换 song，as explained on the Swift blog

song as Song // raises the error "'MediaItem' is not convertible to 'Song'; did you mean to use 'as!' to force downcast?"
song as! Song // forced downcast is allowed

类型推断是首选方法；它更具可读性和简洁性：

var song = Song(name: "Mateo",artist: "Romeo")