Deterministic, succinct Swift/ObjC data binding without a reactive framework.

playground.swift

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

import UIKit

var str = "Hello, playground"

class Person: NSObject {
    dynamic var firstName: String
    dynamic var lastName: String
    init(_ fn: String, _ ln: String) {
        firstName = fn
        lastName = ln
    }
}

class ViewModel: NSObject {
    weak var delegate: ViewModelProtocol!
    var person = Person("Lee","Fastenau")
    var fullName: String { return "\(person.firstName) \(person.lastName)" }
    init(_ delegate: ViewModelProtocol) {
        super.init()
        self.delegate = delegate
        person.addObserver(self, forKeyPath: #keyPath(Person.firstName), options: [.new, .initial], context: nil)
        person.addObserver(self, forKeyPath: #keyPath(Person.lastName), options: [.new, .initial], context: nil)
    }
    deinit {
        person.removeObserver(self, forKeyPath: #keyPath(Person.firstName))
        person.removeObserver(self, forKeyPath: #keyPath(Person.lastName))
    }
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        delegate.fullNameChanged(self)
    }
}

protocol ViewModelProtocol: class {
    func fullNameChanged(_ viewModel: ViewModel)
}

class ViewController: ViewModelProtocol {
    let label = UILabel()
    var vm: ViewModel!
    init() {
        vm = ViewModel(self)
    }
    func fullNameChanged(_ viewModel: ViewModel) {
        label.text = viewModel.fullName
    }
}

let vc = ViewController()
print(vc.label.text!)
vc.vm.person.firstName = "Jack"
print(vc.label.text!)