Basic KVC implementation.

kvc.swift

//: Playground - noun: a place where people can play

import Cocoa

struct KVCTypeInfo : Hashable {
    let key: String
    let type: Any.Type
    
    // Terrible hash value, just FYI.
    var hashValue: Int { return key.hashValue &* 3 }
}

func ==(lhs: KVCTypeInfo, rhs: KVCTypeInfo) -> Bool {
    return lhs.key == rhs.key && lhs.type == rhs.type
}

protocol KeyValueCodable : _KeyValueCodable {
    mutating func setValue<T>(value: T, forKey: String) throws
    func getValue<T>(forKey: String) throws -> T
}

protocol _KeyValueCodable {
    static var _codables: [KVCTypeInfo] { get }
    var _kvcstore: Dictionary<String, Any> { get set }
}

extension KeyValueCodable {
    mutating func setValue<T>(value: T, forKey: String) {
        for codable in Self._codables {
            if codable.key == forKey {
                if value.dynamicType != codable.type {
                    fatalError("The stored type information does not match the given type.")
                }
                
                _kvcstore[forKey] = value
                return
            }
        }
        
        fatalError("Unable to set the value for key: \(forKey).")
    }
    
    func getValue<T>(forKey: String) -> T {
        guard let stored = _kvcstore[forKey] else {
            fatalError("The property is not set; default values are not supported.")
        }
        
        guard let value = stored as? T else {
            fatalError("The stored value does not match the expected type.")
        }
        
        return value
    }
}

struct Person : KeyValueCodable {
    static var _codables: [KVCTypeInfo] { return [ _idKey, _fnameKey ]}
    var _kvcstore: Dictionary<String, Any> = [:]
}

extension Person {
    private static let _idKey = KVCTypeInfo(key: "id", type: Int.self)
    private static let _fnameKey = KVCTypeInfo(key: "fname", type: String.self)
    
    init(id: Int, fname: String) {
        self.id = id
        self.fname = fname
    }

    var id: Int {
        get { return getValue("id") as Int }
        set { setValue(newValue, forKey: "id") }
    }
    
    var fname: String {
        get { return getValue("fname") as String }
        set { setValue(newValue, forKey: "fname") }
    }
}


var p = Person(id: 123, fname: "David")
p.id
p.fname

let id: Int = p.getValue("id")
let fname: String = p.getValue("fname")

p.setValue(21, forKey: "id")
p.setValue("Sally", forKey: "fname")

let id1: Int = p.getValue("id")
let fname1: String = p.getValue("fname")

Pretty cool. Even works in swift 4