swift类型检查

/**

1.定义一个类层次作为例子

2.检查类型

3.向下转型（Downcasting）

4.Any和AnyObject的类型检查

类型检查是一种检查类实例的方式，并且或者也是让实例作为它的父类或者子类的一种方式

类型检查在swift中使用is和as操作符实现，这连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

也可以用来检查一类是够实现了，某个协议

*/

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: "万万没想到", director:
"叫兽易小星"),

Movie(name: "屌丝男士", director:
"大鹏"),

Song(name: "一次就好", artist:
"杨宗纬"),

Song(name: "如果没有如果", artist:
"林俊杰")

];

//检查类型

/**

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，若不属于返回false

*/

var movieCount = 0;

var songCount = 0;

//var mediaCount = 0;

for item in
library {

if item is
Movie {

movieCount++;

}else if item
is Song {

songCount++;

}

// if item is MediaItem {

// mediaCount++;

// }

}

print("movieCount is
\(movieCount)\nsongCount is
\(songCount)");

//向下转型

/**

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，尝试向下转到它的子类型，用类型检查操作符（as）

因为向下转型可能会失败，类型检查操作符带有两种不同形式。可选形式as?返回一个你试图下转成功的类型的可选值。强制形式as把试图向下转型和强制解包结果作为一个混合动作

详见资料

let movie = item as? Movie
尝试将item转为Movie类型，若成功，设置一个新的临时常量movie来存储返回的可选Movie

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

*/

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("Song: '\(song.name)', by
\(song.artist)");

}

}

//Any和AnyObject的类型检查

let someObject: [AnyObject] = [

Movie(name: "港囧", director:
"徐峥"),

Movie(name: "老炮", director:
"管虎"),

Movie(name: "煎饼侠", director:
"大鹏")

];

for object in
someObject {

let movie = object
as! Movie;

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

// if let movie = object as? Movie{

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

// }

}

//为了变为一个更短的形式，下转someObjects数组为Movie[]类型来代替下转每一项方式

for movie in
someObject as! [Movie] {

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

}

//Any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:"sfs", director:
"gerg"));

for thing in
things {

switch thing {

case 0
as Int:

print("zero as an Int");

case 0
as Double:

print("zero as an 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)");

default:

print("something else");

}

}

/**

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

*/