Swift的单例模式写法

Approach B: nested struct

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

Approach C: dispatch_once

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