Option Pattern 在 Swift 中使用，用于改善可选项的 API 设计，向已有类型中以类型安全的方式添加“存储”属性

OptionPattern.swift

protocol PersonOption {
    associatedtype Value
    static var defaultValue: Value { get }
}

class Person {
    enum Color: PersonOption {
        case red
        case blue
        
        static var defaultValue: Color = .red
    }
    var age = 0
    
    private var option: [ObjectIdentifier: Any] = [ObjectIdentifier: Any]()
 
    subscript<T: PersonOption>(option type: T.Type) -> T.Value {
        get { option[ObjectIdentifier(type)] as? T.Value ?? T.defaultValue }
        set { option[ObjectIdentifier(type)] = newValue }
    }
}

extension Int: PersonOption {
    static var defaultValue: Int = 0
}

extension Person {
    var color: Color {
        get { self[option: Color.self] }
        set { self[option: Color.self] = newValue }
    }
    
    var height: Int {
        get { self[option: Int.self] }
        set { self[option: Int.self] = newValue }
    }
}

extension Person.Color {
    var uiColor: UIColor {
        switch self {
        case .red:
            return .red
        case .blue:
            return .blue
        }
    }
}