19.類型轉換

　　類型轉換在 Swift 中使用 is 和 as 操作符實作。is用于判斷執行個體的類型，as用于轉換執行個體的類型。

1.從一個示例談起 

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")
];

//檢查類型：用類型檢查操作符（is）來檢查一個執行個體是否屬于特定子類型。若執行個體屬于那個子類型，類型檢查操作符傳回 true，否則傳回 false。
var movieCount = 0;
var songCount = 0;
for item in library
{
    if item is Movie
    {
        movieCount += 1;  //執行兩次
    }
    else if item is Song
    {
        songCount += 1;  //執行三次
    }
}
print("Media library contains \(movieCount) movies and \(songCount) songs");  //"Media library contains 2 movies and 3 songs\n"

//向下轉型：某類型的一個常量或變量可能在幕後實際上屬于一個子類。當确定是這種情況時，你可以嘗試向下轉到它的子類型，用類型轉換操作符（as? 或 as!）。
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)");
    }
}
/*
 列印結果：
 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 的類型轉換

　　Swift 為不确定類型提供了兩種特殊的類型别名：

1. AnyObject：可以表示任何類類型的執行個體。
2. Any：可以表示任何類型，包括函數類型。

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);
    }
}

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

/*
 列印結果：
 Movie: '2001: A Space Odyssey', dir. Stanley Kubrick
 Movie: 'Moon', dir. Duncan Jones
 Movie: 'Alien', dir. Ridley Scott
 */

var things = [Any]();

things.append(0);
things.append(0.0);
things.append("Hello World");

print("\(things)");  //[0, 0.0, "Hello World"]      

無善無惡心之體，

有善有惡意之動，

知善知惡是良知，

為善去惡是格物。