Poor man's State and Binding without SwiftUI

State+Binding.swift

@dynamicMemberLookup
class Variable<T> {
    var value: T {
        get { sub.value }
        set { sub.value = newValue }
    }
    
    var stream: AnyPublisher<T, Never> {
        return sub.eraseToAnyPublisher()
    }
    
    subscript<P>(dynamicMember keyPath: WritableKeyPath<T, P>) -> P {
        get { sub.value[keyPath: keyPath] }
        set { sub.value[keyPath: keyPath] = newValue }
    }
    
    fileprivate let sub: CurrentValueSubject<T, Never>
    
    init(_ sub: CurrentValueSubject<T, Never>) {
        self.sub = sub
    }
}

class State<T>: Variable<T> {
    var binding: Binding<T> {
        return Binding(self)
    }

    init(_ value: T) {
        super.init(CurrentValueSubject(value))
    }

}

class Binding<T>: Variable<T> {
    init(_ state: State<T>) {
        super.init(state.sub)
    }
}

Usage.swift

import UIKit

struct Story {
    var text: String
}

class TextEditViewController: UIViewController, UITextViewDelegate {

    private let story: Binding<Story>

    init(story: Binding<Story>) {
        self.story = story
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    let textVw = UITextView(frame: .zero)

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(textVw)
        textVw.text = story.text
        textVw.delegate = self
        title = "Story Editor"
        view.backgroundColor = .white
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        textVw.frame = view.bounds
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        textVw.becomeFirstResponder()
    }

    func textViewDidChange(_ textView: UITextView) {
        story.text = textView.text
    }
}

class RootViewController: UIViewController {
    
    let label = UILabel(frame: .zero)
    let story = State(Story(text: ""))
    var cancellables = Set<AnyCancellable>()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(label)
        label.backgroundColor = .white
        label.textAlignment = .center
        view.backgroundColor = .gray
        title = "Story Viewer"
        
        story.stream.sink { [weak self] nextStory in
            print("recv: \(nextStory.text)")
            assert(Thread.isMainThread)
            self?.label.text = nextStory.text
        }.store(in: &cancellables)
                
        navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .edit, target: self, action: #selector(onEdit))
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        label.frame = CGRect(origin: .zero, size: CGSize(width: view.bounds.width - 40, height: 100))
        label.center = view.center
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        label.text = story.value.text
    }
    
    @objc func onEdit() {
        let textEditVwCtrl = TextEditViewController(story: story.binding)
        navigationController?.pushViewController(textEditVwCtrl, animated: true)
    }
}