KeyPaths.swift

struct User {
    var username: String
    private var age : Int = 1
    
    init(username : String) {
        self.username = username
    }
}

var player = User(username: "Mario")
player[keyPath: \User.username] = "Link"
//player[keyPath: \User.age] = 10 // Error
//player.username = "Link"

let nameKeyPath : WritableKeyPath<User, String> = \User.username
player[keyPath: nameKeyPath] = "Luigi"

//nested
struct Address {
    //let street: String
    var street: String {
        didSet {
            //
        }
    }
}
struct ShoppingUser {
    //let address: Address
    var address: Address
}
var x : KeyPath<ShoppingUser, Int> = \ShoppingUser.address.street.count
var street : WritableKeyPath<ShoppingUser, String> = \ShoppingUser.address.street
var mindy = ShoppingUser(address: Address(street: "gangnam"))
//print(mindy[keyPath: x])
mindy[keyPath: street] = "yongsan"


//Class
var pointer : AnyObject

class Student : NSObject {
    var address: Address
    @objc dynamic var age : Int = 5
    init(street: String) {
        self.address = Address(street: street)
    }
    func sleep() {
        age += 10
    }
}

var oowl = Student(street: "gangnam")
var oopath : ReferenceWritableKeyPath<Student, String> = \Student.address.street
//print(oowl.value(forKeyPath: "age"))
//oowl.setValue(10, forKey: "age")
pointer = oowl

class Teacher : NSObject {
    override init() {
        super.init()
//        oowl.addObserver(self, forKeyPath: "address.street", options: [.initial, .new, .old], context: nil)
        oowl.addObserver(self, forKeyPath: "age", options: [.new], context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print("observer = \(object) \(keyPath) \(change)")
    }
}
let teacher = Teacher.init()
oowl.sleep()

// Partial
struct GameUser {
    var name : String
    var age : Int
    var quote : Float
}
// String
let a: PartialKeyPath<GameUser> = \GameUser.name
// Int
let b: PartialKeyPath<GameUser> = \GameUser.age
// Float
let c: PartialKeyPath<GameUser> = \GameUser.quote

func acceptKeyPath (_ keyPath: PartialKeyPath<GameUser>) {
    //
}

// AnyKeyPath
let keyPaths: [AnyKeyPath] = [\User.username, \String.count]

//Composition

// ShoppingUser -> address
let addressKeyPath = \ShoppingUser.address
// Address -> street
let streetKeyPath = \Address.street
// User -> address -> street
let userStreetKeyPath = addressKeyPath.appending(path: streetKeyPath)