Welcome-to-Swift-18类型转换（Type Casting）

 类型转换是一种检查类实例的方式，并且哦或者也是让实例作为它的父类或者子类的一种方式。

Type casting is a way to check the type of an instance, and/or to treat that instance as if it is a different superclass or subclass from somewhere else in its own class hierarchy.

类型转换在Swift中使用is 和 as操作符实现。这两个操作符提供了一种简单达意的方式去检查值的类型或者转换它的类型。

Type casting in Swift is implemented with the is and as operators. These two operators provide a simple and expressive way to check the type of a value or cast a value to a different type.

你也可以用来检查一个类是否实现了某个协议，就像在 Protocols》Checking for Protocol Conformance部分讲述的一样。

You can also use type casting to check whether a type conforms to a protocol, as described in Checking for Protocol Conformance.

定义一个类层次作为例子Defining a Class Hierarchy for Type Casting

你可以将它用在类和子类的层次结构上，检查特定类实例的类型并且转换这个类实例的类型成为这个层次结构中的其他类型。这下面的三个代码段定义了一个类层次和一个array包含了几个这些类的实例，作为类型转换的例子。

You can use type casting with a hierarchy of classes and subclasses to check the type of a particular class instance and to cast that instance to another class within the same hierarchy. The three code snippets below define a hierarchy of classes and an array containing instances of those classes, for use in an example of type casting.

第一个代码片段定义了一个新的基础类MediaItem。这个类为任何出现在数字媒体库的项提供基础功能。特别的，它声明了一个 String 类型的 name 属性，和一个init name初始化器。（它假定所有的媒体项都有个名称。）

The first snippet defines a new base class called MediaItem. This class provides basic functionality for any kind of item that appears in a digital media library. Specifically, it declares a name property of type String, and an init nameinitializer. (It is assumed that all media items, including all movies and songs, will have a name.)

class MediaItem {
    var name: String
    init(name: String) {
        self.name = name
    }
}

下一个代码段定义了 MediaItem 的两个子类。第一个子类Movie，在父类（或者说基类）的基础上增加了一个 director（导演） 属性，和相应的初始化器。第二个类在父类的基础上增加了一个 artist（艺术家） 属性，和相应的初始化器：

The next snippet defines two subclasses of MediaItem. The first subclass, Movie, encapsulates additional information about a movie or film. It adds a director property on top of the base MediaItem class, with a corresponding initializer. The second subclass, Song, adds an artist property and initializer on top of the base class:

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)
    }
}

最后一个代码段创建了一个array常量 library ，包含两个Movie实例和三个Song实例。library的类型是在它被初始化时根据它的array标记符和里面的内容（ps: literal： 标记符其实就是指“[”和“]”，虽然苹果官方的翻译里翻译为字面当总感觉不好理解，有点奇怪。不如翻译为标记符）推断来的。Swift的类型检测器能够演绎出Movie 和 Song 有共同的父类 MediaItem，所以它推断出 MediaItem[] 类作为 library 的类型。

The final snippet creates a constant array called library, which contains two Movie instances and three Song instances. The type of the library array is inferred by initializing it with the contents of an array literal. Swift’s type checker is able to deduce that Movie and Song have a common superclass of MediaItem, and so it infers a type of MediaItem[] for the library array:

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")
]
// the type of "library" is inferred to be MediaItem[]

在幕后library 里存储的项依然是 Movie 和 Song 类型的，但是，若你迭代它，取出的实例会是 MediaItem 类型的，而不是 Movie 和 Song 类型的。为了让它们作为它们本来的类型工作，你需要检查它们的类型或者向下转换它们的类型到其它类型，就像下面描述的一样。

The items stored in library are still Movie and Song instances behind the scenes. However, if you iterate over the contents of this array, the items you receive back are typed as MediaItem, and not as Movie or Song. In order to work with them as their native type, you need to check their type, or downcast them to a different type, as described below.

检查类型 Checking Type

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。类型检查操作符返回 true 若实例属于那个子类型，若不属于返回 false 。

Use the type check operator (is) to check whether an instance is of a certain subclass type. The type check operator returns true if the instance is of that subclass type and false if it is not.

下面的例子定义了连个变量，movieCount 和 songCount，用来计算数组library 中 Movie 和 Song 类型的实例数量。

The example below defines two variables, movieCount and songCount, which count the number of Movie and Songinstances in the library array:

var movieCount = 0
var songCount = 0

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

println("Media library contains (movieCount) movies and (songCount) songs")
// prints "Media library contains 2 movies and 3 songs"

示例迭代了数组 library 中的所有项。每一次， for-in 循环设置 item 常量的值为数组中的下一个 MediaItem。

This example iterates through all items in the library array. On each pass, the for-in loop sets the item constant to the next MediaItem in the array.

若当前 MediaItem 是一个 Movie 类型的实例， item is Movie 返回 true，相反返回 false。同样的，item is Song检查item是否为Song类型的实例。在循环末尾，movieCount 和 songCount的值就是被找到属于各自的类型的实例数量。

item is Movie returns true if the current MediaItem is a Movie instance and false if it is not. Similarly, item is Songchecks whether the item is a Song instance. At the end of the for-in loop, the values of movieCount and songCountcontain a count of how many MediaItem instances were found of each type.

向下转型（简称下转） Downcasting

某类型的一个常量或变量可能在幕后实际上属于一个子类。你可以相信，上面就是这种情况。你可以尝试向下转到它的子类型，用类型转换操作符(as)

A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with the type cast operator (as).

因为向下转型可能会失败，类型转换操作符带有两种不同形式。可选形式（ optional form） as? 返回一个你试图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-unwraps）结果作为一个混合动作。 

Because downcasting can fail, the type cast operator comes in two different forms. The optional form, as?, returns an optional value of the type you are trying to downcast to. The forced form, as, attempts the downcast and force-unwraps the result as a single compound action.

当你不确定下转可以成功时，用类型转换的可选形式(as?)。可选形式的类型转换总是返回一个可选值（optional value），并且若下转是不可能的，可选值将是 nil 。这使你能够检查下转是否成功。

Use the optional form of the type cast operator (as?) when you are not sure if the downcast will succeed. This form of the operator will always return an optional value, and the value will be nil if the downcast was not possible. This enables you to check for a successful downcast.

只有你可以确定下转一定会成功时，才使用强制形式。当你试图下转为一个不正确的类型时，强制形式的类型转换会触发一个runtime error。

Use the forced form of the type cast operator (as) only when you are sure that the downcast will always succeed. This form of the operator will trigger a runtime error if you try to downcast to an incorrect class type.

下面的例子，迭代了library里的每一个 MediaItem ，并打印出适当的描述。要这样做，item需要真正作为Movie 或 Song的类型来使用。不仅仅是作为 MediaItem。为了能够使用Movie 或 Song的 director 或 artist属性，这是必要的。

The example below iterates over each MediaItem in library, and prints an appropriate description for each item. To do this, it needs to access each item as a true Movie or Song, and not just as a MediaItem. This is necessary in order for it to be able to access the director or artist property of a Movie or Song for use in the description.

在这个示例中，数组中的每一个item可能是 Movie 或 Song。 事前你不知道每个item的真实类型，所以这里使用可选形式的类型转换 (as?)去检查循环里的每次下转。

In this example, each item in the array might be a Movie, or it might be a Song. You don’t know in advance which actual class to use for each item, and so it is appropriate to use the optional form of the type cast operator (as?) to check the downcast each time through the loop:

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

示例首先试图将 item 下转为 Movie。因为 item 是一个 MediaItem 类型的实例，它可能是一个Movie；同样，它可能是一个 Song，或者仅仅是基类 MediaItem。因为不确定，as?形式试图下转时返还一个可选值。 item as Movie 的返回值是Movie?类型或 “optional Movie”。

The example starts by trying to downcast the current item as a Movie. Because item is a MediaItem instance, it’s possible that it might be a Movie; equally, it’s also possible that it might a Song, or even just a base MediaItem. Because of this uncertainty, the as? form of the type cast operator returns an optional value when attempting to downcast to a subclass type. The result of item as Movie is of type Movie?, or “optional Movie”.

当应用在两个Song实例时，下转为 Movie 失败。为了处理这种情况，上面的实例使用了可选绑定（optional binding）来检查optional Movie真的包含一个值（这个是为了判断下转是否成功。）可选绑定是这样写的“if let movie = item as? Movie”,可以这样解读：

Downcasting to Movie fails when applied to the two Song instances in the library array. To cope with this, the example above uses optional binding to check whether the optional Movie actually contains a value (that is, to find out whether the downcast succeeded.) This optional binding is written “if let movie = item as? Movie”, which can be read as:

“尝试将 item 转为 Movie类型。若成功，设置一个新的临时常量 movie 来存储返回的optional Movie”

“Try to access item as a Movie. If this is successful, set a new temporary constant called movie to the value stored in the returned optional Movie.”

若下转成功，然后movie的属性将用于打印一个Movie实例的描述，包括它的导演的名字director。当Song被找到时，一个相近的原理被用来检测 Song 实例和打印它的描述。

If the downcasting succeeds, the properties of movie are then used to print a description for that Movie instance, including the name of its director. A similar principle is used to check for Song instances, and to print an appropriate description (including artist name) whenever a Song is found in the library.

注意

转换没有真的改变实例或它的值。潜在的根本的实例保持不变；只是简单地把它作为它被转换成的类来使用。

NOTE

Casting does not actually modify the instance or change its values. The underlying instance remains the same; it is simply treated and accessed as an instance of the type to which it has been cast.

Any和AnyObject的转换 Type Casting for Any and AnyObject

Swift为不确定类型提供了两种特殊类型别名：

• AnyObject可以代表任何class类型的实例。

• Any可以表示任何类型，除了方法类型（function types）。

  Swift provides two special type aliases for working with non-specific types:

• AnyObject can represent an instance of any class type.

• Any can represent an instance of any type at all, apart from function types.

  注意
  
  只有当你明确的需要它的行为和功能时才使用Any和AnyObject。在你的代码里使用你期望的明确的类型总是更好的。
  
  NOTE
  
  Use Any and AnyObject only when you explicitly need the behavior and capabilities they provide. It is always better to be specific about the types you expect to work with in your code.
  

AnyObject类型

当需要在工作中使用Cocoa APIs，它一般接收一个AnyObject[]类型的数组，或者说“一个任何对象类型的数组”。这是因为OC没有明确的类型化数组。但是，你常常可以确定包含在仅从你知道的API信息提供的这样一个数组中的对象的类型。

When working with Cocoa APIs, it is common to receive an array with a type of AnyObject[], or “an array of values of any object type”. This is because Objective-C does not have explicitly typed arrays. However, you can often be confident about the type of objects contained in such an array just from the information you know about the API that provided the array.

在这些情况下，你可以使用强制形式的类型转换(as)来下转在数组中的每一项到比 AnyObject 更明确的类型，不需要可选解包（optional unwrapping）。

In these situations, you can use the forced version of the type cast operator (as) to downcast each item in the array to a more specific class type than AnyObject, without the need for optional unwrapping.

下面的示例定义了一个 AnyObject[] 类型的数组并填入三个Movie类型的实例：

The example below defines an array of type AnyObject[] and populates this array with three instances of the Movie class:

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

因为知道这个数组只包含 Movie 实例，你可以直接用(as)下转并解包到不可选的Movie类型（ps：其实就是我们常用的正常类型，这里是为了和可选类型相对比）。

Because this array is known to contain only Movie instances, you can downcast and unwrap directly to a non-optional Movie with the forced version of the type cast operator (as):

for object in someObjects {
    let movie = object 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

为了变为一个更短的形式，下转someObjects类型成功 Movie[] 类型代替下转每一项。

For an even shorter form of this loop, downcast the someObjects array to a type of Movie[] instead of downcasting each item:

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

Any类型

这里有个示例，使用 Any 类型来和混合的不同类型一起工作，包括非class类型。它创建了一个可以存储Any类型的数组 things。

Here’s an example of using Any to work with a mix of different types, including non-class types. The example creates an array called things, which can store values of type Any:

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 数组包含两个 Int 值，2个 Double 值，1个 String 值，一个元组 (Double, Double) ，Ivan Reitman导演的电影“Ghostbusters”。

The things array contains two Int values, two Double values, a String value, a tuple of type (Double, Double), and the movie “Ghostbusters”, directed by Ivan Reitman.

你可以在 switch cases里用is 和 as 操作符来发觉只知道是 Any 或 AnyObject的常量或变量的类型。 下面的示例迭代 things数组中的每一项的并用switch语句查找每一项的类型。这几种switch语句的情形绑定它们匹配的值到一个规定类型的常量，让它们可以打印它们的值：

You can use the is and as operators in a switch statement’s cases to discover the specific type of a constant or variable that is known only to be of type Any or AnyObject. The example below iterates over the items in the things array and queries the type of each item with a switch statement. Several of the switch statement’s cases bind their matched value to a constant of the specified type to enable its value to be printed:

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