Swift 2 学习笔记 22.类型检查和类型转换

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类型检查和类型转换

• 类型检查

let library: [MediaItem] = [
    Movie(name: "Zootopia", genre: "Animation"),
    Music(name: "Hello", artistName: "Adele"),
    Game(name: "LIMBO", developer: "Playdead"),
    Music(name: "See you agian", artistName: "Wiz Khalifa"),
    Game(name: "The Bridge", developer: "The Quantum Astrophysicists Guild")
]

for mediaItem in library{
    if mediaItem is Movie{
        movieCount += 1
    }
    else if mediaItem is Music{
        musicCount += 1
    }
    else if mediaItem is Game{
        gameCount += 1
    }
}

• 向下类型转换

//let item0 = library[0] as? Movie
//let item0 = library[0] as? Music
//let item0 = library[0] as! Movie
//let item0 = library[0] as! Music

for mediaItem in library{
    if let movie = mediaItem as? Movie{
        print("Movie:", movie.name, "Genre:", movie.genre)
    }
    else if let music = mediaItem as? Music{
        print("Music:", music.name, "Artist:", music.artist)
    }
    else if let game = mediaItem as? Game{
        print("Game:", game.name, "Developer(s):", game.developer)
    }

• 检查协议遵守

protocol Shape{
    var name: String{get}
}

protocol HasArea{
    func area() -> Double
}


struct Point: Shape{
    let name: String = "point"
    var x: Double
    var y: Double
}

struct Rectangle: Shape, HasArea{
    let name: String = "rectangle"
    var origin: Point
    var width: Double
    var height: Double
    
    func area() -> Double{
        return width * height
    }
}

struct Circle: Shape, HasArea{
    let name = "circle"
    var center: Point
    var radius: Double
    
    func area() -> Double{
        return Double.pi * radius * radius
    }
}

struct Line: Shape{
    let name = "line"
    var a: Point
    var b: Point
}

let shapes:[Shape] = [
    Rectangle(origin: Point(x:0.0,y:0.0), width: 3.0, height: 4.0),
    Point(x: 0.0, y: 0.0),
    Circle(center: Point(x:0.0,y:0.0), radius: 1.0),
    Line(a: Point(x:1.0,y:1.0), b: Point(x:5.0,y:5.0))
]

for shape in shapes{
    
    if shape is HasArea{
        print("\(shape.name) has area.")
    }
    else{
        print("\(shape.name) has no area.")
    }
}

print("==========")

for shape in shapes{
    
    if let areaShape = shape as? HasArea{
        print("The area of \(shape.name) is \(areaShape.area()).")
    }
    else{
        print("\(shape.name) has no area.")
    }
}