swift单例

原理：利用静态成员自动满足dispatch_once规则

class Singleton {
    static let shared = Singleton()
    /*
     static let shared = {
     let instance = Singleton()
     return instance
     }()
     */
    private init(){}
}

利用全局变量自动满足dispatch_once规则

private let instance = Singleton()
final class Singleton {
    static var shared: Singleton {
        instance
    }
    fileprivate init() {}
}

参考：总结Swift5单例的几种写法和常见错误写法