Swift4 基础部分: Type Casting(类型转换)

本文是学习《The Swift Programming Language》整理的相关随笔，基本的语法不作介绍，主要介绍Swift中的一些特性或者与OC差异点。

系列文章：

• Swift4 基础部分:The Basics
• Swift4 基础部分:Basic Operators
• Swift4 基础部分:Strings and Characters
• Swift4 基础部分:Collection Types
• Swift4 基础部分:Control Flow
• Swift4 基础部分:Functions
• Swift4 基础部分:Closures
• Swift4 基础部分: Enumerations
• Swift4 基础部分: Classes and Structures
• Swift4 基础部分: Properties
• Swift4 基础部分: Methods
• Swift4 基础部分: Subscripts
• Swift4 基础部分: Inheritance
• Swift4 基础部分: Initialization
• Swift4 基础部分: Deinitialization
• Swift4 基础部分: Automatic Reference Counting(自动引用计数)
• Swift4 基础部分: Optional Chaining(可选链)
• Swift4 基础部分: Error Handling(错误处理)

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

• 类型转换可以用来检测实例的类型，或者将实例看成父类或子类的实例。

为类型转化定义一个类的层级(Defining a Class Hierarchy for Type Casting)

例子:

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")
];

检查类型(Checking Type)

Use the type check operator (is) to check whether an instance is of a 
certain subclass type.

• 利用检查操作符is去检查实例是否是一个特定的子类类型。

例子:

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");

执行结果:

Media library contains 2 movies and 3 songs

向下转换(Downcasting)

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 a type 
cast operator (as? or as!).

• 当一个常量或变量实际上是一个子类的实例时，可以使用as? or as向下转化。

例子：

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("Movie: \(song.name), by \(song.artist)");
    }
}

执行结果:

Media library contains 2 movies and 3 songs
Movie: Casablanca, dir. Michael Curtiz
Movie: Blue Suede Shoes, by Elvis Presley
Movie: Citizen Kane, dir. Orson Welles
Movie: The One And Only, by Chesney Hawkes
Movie: Never Gonna Give You Up, by Rick Astley

Any与AnyObject的类型转换(Type Casting for Any and AnyObject)

Swift provides two special types for working with nonspecific types:

Any can represent an instance of any type at all, including function 
types.

AnyObject can represent an instance of any class type.

• Any可以表示任何类型的实例包括函数类型。
• Anyobject可以表示任何类类型的实例。

例子:

var things = [Any]();
things.append(0);
things.append(0.1);
things.append((3.0,5.0));
things.append("hello");

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 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)")
        default:
            print("something else");
    }
}

执行结果:

zero as an Int
a positive double value of 0.1
an (x, y) point at 3.0, 5.0
a string value of "hello"

这里简单扩展一下NSObject,AnyObject,Any的区别。

例子

class Example{}
class ObjectExample:NSObject{}
var example:Example = Example();
var objectExample:ObjectExample = ObjectExample();
print(example is NSObject);
print(example is AnyObject);
print(objectExample is NSObject);
print(objectExample is AnyObject);

执行结果

false
true
true
true

• 1.每一个NSObject对象都是AnyObject,单并非每一个AnyObject都是NSObject。
• 2.NSObjectSwift中需要显示继承。