Question

Apple给我们提供了一个向下转换同类型对象的示例：

let someObjects: [AnyObject] = [
    Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),
    Movie(name: "Moon", director: "Duncan Jones"),
    Movie(name: "Alien", director: "Ridley Scott")
]

然后我们可以通过创建一个抽象变量＆＃34; object＆＃34;来访问每个单独的属性。并按照我们的预期（电影）投射它：

for object in someObjects {
    let movie = object as! Movie
    print("Movie: '\(movie.name)', dir. \(movie.director)")
}

但是如果我们进入下一个级别，例如我们有电影的子类：

无声电影

喜剧

动作

每个都具有相同的属性 - 名称＆amp;导演

我试图以与上面作为电影＆＃34;因为我假设是一个父类，它能够识别它的子类，但当然这没有用，否则这个问题就不存在了！

Answer 1

我理解你的问题是正确的。因此，转换对子类的工作方式与示例相同。也许你错过了一些东西（提示在代码注释中）：

// here the subclass SilentFilm
class SilentFilm:Movie {

}

// Here the array with SilentFilm objects
let someOtherObjects: [AnyObject] = [
    SilentFilm(name: "silentname", director: "silentdirector"),
    SilentFilm(name: "silentname2", director: "silentdirector2")
]

for object2 in someOtherObjects {
    // Here the cast to Movie like in your example
    let movie = object2 as! Movie
    // or cast to SilentFilm works also
    // let movie = object2 as! SilentFilm
    print("Movie: '\(movie.name)', dir. \(movie.director)")
}

// Expected Output
// Movie: 'silentname', dir. silentdirector
// Movie: 'silentname2', dir. silentdirector2

如果我错过了你的问题，请纠正我！

Answer 2

为什么不创建具有父Movie类的类层次结构，并在迭代数组时检查电影的类型？如果您指定数组包含AnyObject个对象，并且所有子类型都继承自Movie，则无需强制从Movie进行强制转换。

class Movie {
    var name: String!
    var director: String!

    init(name: String, director: String) {
        self.name = name
        self.director = director
    }
}

class Comedy: Movie {
    var laughTrack: Bool!

    init(name: String, director: String, laughTrack: Bool) {
        super.init(name: name, director: director)
        self.laughTrack = laughTrack
    }
}

let someObjects: [Movie] = [
    Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),
    Movie(name: "Moon", director: "Duncan Jones"),
    Movie(name: "Alien", director: "Ridley Scott"),
    Comedy(name: "Caddyshack", director: "Harold Ramis", laughTrack: false)
]

for movie in someObjects {
    if movie is Comedy {
        print("\(movie.name) is a comedy")
    } else {
        print("\(movie.name) is a movie")
    }
}

输出：

2001：太空漫游是一部电影

月亮是一部电影

外星人是一部电影

Caddyshack是一部喜剧

如何向下转换可以是多种类型的对象？

2 个答案: