class Phone {
    var model: String?
}

class Employee {
    var firstName: String?
    var lastName: String?
    var personalPhone: Phone?
}

let iPhoneX = Phone()
iPhoneX.model = "iPhone X"

let petr = Employee()
petr.firstName = "Petr"
petr.lastName = "Ivanov"
petr.personalPhone = iPhoneX

let keyPath = \Employee.personalPhone?.model
var phoneModel = petr[keyPath: keyPath]
//phoneModel is "iPhone X"
print("phoneModel is \(phoneModel!)")

iPhoneX.model = "iPhone X 256"
phoneModel = petr[keyPath: keyPath]
//phoneModel is "iPhone X 256"
print("phoneModel is \(phoneModel!)")