Swift 中的类型转换是什么？如何使用 is、as、as? 和 as！？

Swift 中的类型转换是什么？如何使用 is、as、as? 和 as! 进行类型转换？

Swift 中的类型转换用于检查实例的类型，或者将其视为超类或子类。类型转换在处理多态和继承层次结构时非常重要。

is 操作符：

• 检查实例是否是特定类型的实例
• 返回布尔值
• 示例：

  swift
  class Vehicle {}
  class Car: Vehicle {}
  class Truck: Vehicle {}
  
  let vehicle = Car()
  print(vehicle is Car)      // true
  print(vehicle is Vehicle)   // true
  print(vehicle is Truck)     // false

as 操作符：

• 用于向上转换（子类到父类）
• 总是成功
• 示例：

  swift
  let car = Car()
  let vehicle = car as Vehicle

as? 操作符：

• 用于向下转换（父类到子类）
• 返回可选类型
• 转换失败时返回 nil
• 示例：

  swift
  let vehicle: Vehicle = Car()
  if let car = vehicle as? Car {
      print("This is a car")
  }
  
  if let truck = vehicle as? Truck {
      print("This is a truck")
  } else {
      print("Not a truck")
  }

as! 操作符：

• 用于强制向下转换
• 转换失败时触发运行时错误
• 只在确定转换会成功时使用
• 示例：

  swift
  let vehicle: Vehicle = Car()
  let car = vehicle as! Car
  
  // 危险：可能导致崩溃
  // let truck = vehicle as! Truck

类型转换的实际应用：

swift
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)
    }
}

let library = [
    Movie(name: "Casablanca", director: "Michael Curtiz"),
    Song(name: "Blue Suede Shoes", artist: "Elvis Presley"),
    Movie(name: "Citizen Kane", director: "Orson Welles"),
    Song(name: "The One And Only", artist: "Chesney Hawkes"),
    Song(name: "Never Gonna Give You Up", artist: "Rick Astley")
]

var movieCount = 0
var songCount = 0

for item in library {
    if item is Movie {
        movieCount += 1
    } else if item is Song {
        songCount += 1
    }
}

print("Media library contains \(movieCount) movies and \(songCount) songs")

for item in library {
    if let movie = item as? Movie {
        print("Movie: \(movie.name), dir. \(movie.director)")
    } else if let song = item as? Song {
        print("Song: \(song.name), by \(song.artist)")
    }
}

Any 和 AnyObject 类型转换：

swift
var things = [Any]()

things.append(0)
things.append(0.0)
things.append(42)
things.append(3.14159)
things.append("hello")
things.append((3.0, 5.0))
things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))

for thing in things {
    switch thing {
    case 0 as Int:
        print("zero as an Int")
    case 0 as Double:
        print("zero as a Double")
    case let someInt as Int:
        print("an integer value of \(someInt)")
    case let someDouble as Double where someDouble > 0:
        print("a positive double value of \(someDouble)")
    case is Double:
        print("some other double value that I don't want to print")
    case let someString as String:
        print("a string value of \"\(someString)\"")
    case let (x, y) as (Double, Double):
        print("an (x, y) point at \(x), \(y)")
    case let movie as Movie:
        print("a movie called \(movie.name)")
    default:
        print("something else")
    }
}

最佳实践：

1. 使用 is 检查类型
2. 使用 as? 安全地进行类型转换
3. 只在确定转换会成功时使用 as!
4. 使用 switch 处理多种类型
5. 避免过度使用 Any 和 AnyObject