swift基础（十）：类型检查操作符is、类型转换操作符as(包括!及?的区别)、AnyObject和Any类型

参考文档：《The Swift programming
language 中文版 V1.2》

目录:

1.类型检查操作符is

2.类型转换操作符as(包括!及?的区别)

3.AnyObject类型

4.Any类型

//1.类型检查操作符:is

//用类型检查操作符( is )来检查一个实例是否属于特定子类型。若实例属于那个子类型,类型检查操作符返回true
,否则返回 false 。

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

    }

    else if item
is Song

    {

        ++songCount

    }

}

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

//"Media library contains 2 movies and 3 songs\n"

//2.类型转换操作符(as?
或 as!)

/*

    因为向下转型可能会失败,类型转型操作符带有两种不同形式。条件形式(conditional form) as?
返回一个 你试图向下转成的类型的可选值(optional value)。强制形式 as!
把试图向下转型和强制解包(force-unwra ps)结果作为一个混合动作。

1.当你不确定向下转型可以成功时,用类型转换的条件形式( as? )。条件形式的类型转换总是返回一个可选值(opt
ional value),并且若下转是不可能的,可选值将是 nil
。这使你能够检查向下转型是否成功。

2.只有你可以确定向下转型一定会成功时,才使用强制形式( as! )。当你试图向下转型为一个不正确的类型时,强
制形式的类型转换会触发一个运行时错误。

*/

for item in
library

{

    if let movie = item
as? Movie

    {

        print("Movie: '\(movie.name)', dir.
\(movie.director)")//2 times

    }

    else if
let song = item as?
Song

    {

        print("Song: '\(song.name)', by
\(song.artist)")//3 times

    }

}

// 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

//3.AnyObject类型转换

//当在工作中使用 Cocoa APIs,我们一般会接收一个 AnyObject类型的数组,或者说“一个任何对象类型的数组”。

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 object
in someObjects

{

    let movie = object
as! Movie

    print("Movie: '\(movie.name)', dir.
\(movie.director)")

}

// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick

// Movie: 'Moon', dir. Duncan Jones

// Movie: 'Alien', dir. Ridley Scott

for movie in
someObjects as! [Movie]//简化处理

{

    print("Movie: '\(movie.name)', dir.
\(movie.director)")

}

// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick

// Movie: 'Moon', dir. Duncan Jones

// Movie: 'Alien', dir. Ridley Scott

//4.Any 类型转换

//使用 Any
类型来和混合的不同类型一起工作,包括方法类型和非 class
类型

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"))

things.append({ (name:
String) -> String
in "Hello,
\(name)" })

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)',
dir. \(movie.director)")

    case let stringConverter
as String ->
String:

        print(stringConverter("Michael"))

    default:

        print("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

// Hello, Michael