如何在类型上使用属性包装器来抽象单例？

@propertyWrapper
struct Singleton<Value> {
    private var value: Value?
    
    init() {
        self.value = nil
    }
    
    var wrappedValue: Value {
        get {
            if let value = value {
                return value
            } else {
                let newValue = Value()
                value = newValue
                return newValue
            }
        }
        set {
            value = newValue
        }
    }
}

class MySingletonClass {
    @Singleton
    static var shared = MySingletonClass()
    
    private init() { }
    
    func doSomething() {
        // 单例操作
    }
}