swift学习日记(十) 类型转换 类型嵌套

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其实这个昨天就写好了，今天才发

预告：这几天在写大作业，可能无法保持每天一更，尽量有空就看看

一，类型检查

这个可以说是借鉴java的了

语法非常简单

var num:Int = 1;
print(num is Int)

is可以看作是一个二元操作符，其返回true 或false

其作用是判断某个变量是否为某个类型

1，向下转型

as

看到这个语法，顾名思义，应该想得到这是把一个变量转换成其它类型来用，主要是用来转型成子类

语法有两种形式，as? 和 as

带?是可选形式，如果下转不成功，将返回nil，以便检查是否成功

as是强制下转，当不成功时，将触发runtime error

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

// Movie: 'Casablanca', dir. Michael Curtiz
// Song: 'Blue Suede Shoes', by Elvis Presley
// Movie: 'Citizen Kane', dir. Orson Welles
// Song: 'The One And Only', by Chesney Hawkes
// Song: 'Never Gonna Give You Up', by Rick Astley

2，Any和AnyObject的类型检查

AnyObject将可以指代任何类型的实例

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

for movie in someObjects as Movie[] {
println("Movie: '\(movie.name)', dir. \(movie.director)")
}
// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
// Movie: 'Moon', dir. Duncan Jones
// Movie: 'Alien', dir. Ridley Scott

与前一个不同，any可以指代任何一个类型，而不是该类型的实例

具体其实也没什么好解释的，看代码理解就行了，通俗易懂

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

// zero as an Int
// zero as a Double
// an integer value of 42
// a positive double value of 3.14159
// a string value of "hello"
// an (x, y) point at 3.0, 5.0
// a movie called 'Ghostbusters', dir. Ivan Reitman

在一个switch语句的case中使用强制形式的类型检查操作符（as, 而不是 as?）来检查和转换到一个明确的类型。在 switch case 语句的内容中这种检查总是安全的。

二，类型嵌套

这不是新的语法，只是一个新增的说明

swift中，我们可以在一个类/结构体/枚举类型中 嵌套 类/结构体/枚举类型

struct BlackjackCard {
// 嵌套定义枚举型Suit
enum Suit: Character {
case Spades = "♠", Hearts = "♡", Diamonds = "♢", Clubs = "♣"
}
// 嵌套定义枚举型Rank
enum Rank: Int {
case Two = 2, Three, Four, Five, Six, Seven, Eight, Nine, Ten
case Jack, Queen, King, Ace
struct Values {
let first: Int, second: Int?
}
var values: Values {
switch self {
case .Ace:
return Values(first: 1, second: 11)
case .Jack, .Queen, .King:
return Values(first: 10, second: nil)
default:
return Values(first: self.toRaw(), second: nil)
}
}
}
// BlackjackCard 的属性和方法
let rank: Rank, suit: Suit
var description: String {
var output = "suit is \(suit.toRaw()),"
output += " value is \(rank.values.first)"
if let second = rank.values.second {
output += " or \(second)"
}
return output
}
}

let heartsSymbol = BlackjackCard.Suit.Hearts.toRaw()