swift-type-erasure-pattern.swift

protocol CatProtocol {
    var name: String { get set }
    func nya() -> String
    mutating func sleep()
    func clone() -> Self
    func cast<T>(to: T.Type) -> T?
}

class Cat : CatProtocol {
    required init() {}
    var name: String = "tama"
    func nya() -> String { return "nyaaaa" }
    func sleep() { print("zzz...") }
    func clone() -> Self {
        return type(of: self).init()
    }
    func cast<T>(to: T.Type) -> T? {
        return self as? T
    }
}

struct AnyCat1 : CatProtocol {
    init<X: CatProtocol>(_ x: X) {
        var x = x
        _getName = { x.name }
        _setName = { x.name = $0 }
        _nya = { x.nya() }
        _sleep = { x.sleep() }
        _clone = { AnyCat1(x.clone()) }
        _cast = { (to: Any.Type) -> Any? in
            print(type(of: to))
            return nil
            //return x.cast(to: to)
        }
    }
    
    var name: String {
        get { return _getName() }
        set { _setName(newValue) }
    }

    func nya() -> String {
        return _nya()
    }
    
    func sleep() { _sleep() }

    func clone() -> AnyCat1 {
        return _clone()
    }
    
    func cast<T>(to: T.Type) -> T? {
        return _cast(to) as? T
    }

    var _getName: () -> String
    var _setName: (String) -> Void
    var _nya: () -> String
    var _sleep: () -> Void
    var _clone: () -> AnyCat1
    var _cast: (Any.Type) -> Any?
}

let a = AnyCat1(Cat())
print(String(describing: a.cast(to: String.self)))