• 私有化构造方法（Swift不支持）
• 使用一个静态变量保存实例的引用 
• 提供一个全局的访问方法

 三、 Swift语言下的实现

class SwiftSingleton {
    class var shared: SwiftSingleton {
        dispatch_once(&Inner.token) {
            Inner.instance = SwiftSingleton()
        }
        return Inner.instance!
    }
    struct Inner {
        static var instance: SwiftSingleton?
        static var token: dispatch_once_t = 0
    }
    
}

class SwiftSingletonTest: XCTestCase {
    
    
    func testSingleton() {
        let singleton1 = SwiftSingleton.shared
        let singleton2 = SwiftSingleton.shared
        assert(singleton1 === singleton2, "pass")
    }
    
}

class SwiftSingleton {
    class var shared: SwiftSingleton {
        return Inner.instance
    }
    
    struct Inner {
        static let instance = SwiftSingleton()
    }
}

struct SwiftSingleton {
    var name: String = "1"
    static let shared = SwiftSingleton()
}


var single1 = SwiftSingleton.shared
var single2 = SwiftSingleton.shared

single2.name = "2"

println("------->\(single1.name)")
println("------->\(single2.name)")

------->1
------->2
Program ended with exit code: 0