Swift singleton && Lazy initializition

偷个懒，看到了个比较靠谱的答案，直接复制过来了。

原网址：http://stackoverflow.com/questions/24024549/dispatch-once-singleton-model-in-swift

From my short experience with Swift there are three approaches to implement the Singleton pattern that support lazy initialization and thread safety.

These approaches might change or become redundant as the language matures.

Global constant

private let _SingletonSharedInstance = Singleton()

class Singleton  {
    class var sharedInstance : Singleton {
        return _SingletonSharedInstance
    }
}

We use a global constant because class constants are not yet supported.

This approach supports lazy initialization because Swift lazily initializes global constants (and variables), and is thread safe by virtue of let.

我比较喜欢用这个Nested struct方式，和以前OC中风格一样~

Nested struct

class Singleton {
    class var sharedInstance : Singleton {
        struct Static {
            static let instance : Singleton = Singleton()
        }
        return Static.instance
    }
}

Unlike classes, structs do support static constants. By using a nested struct we can leverage its static constant as a class constant.

The nested struct is the approach I recommend until class constants are supported.

dispatch_once

The traditional Objective-C approach ported to Swift. I'm fairly certain there's no advantage over the nested struct approach but I'm putting it here anyway as I find the differences in syntax interesting.

class Singleton {
    class var sharedInstance : Singleton {
        struct Static {
            static var onceToken : dispatch_once_t = 0
            static var instance : Singleton? = nil
        }
        dispatch_once(&Static.onceToken) {
            Static.instance = Singleton()
        }
        return Static.instance!
    }
}

See this GitHub project for unit tests.