MVVM-observable-property.swift

import Foundation
import UIKit

public class ObservableProperty<T> {
    public var value: T {
        didSet { notifyObservers() }
    }
    private var observers = [(T) -> Void]()

    public init(_ initialValue: T) {
        self.value = initialValue
    }

    public func set(_ newValue: T) {
        self.value = newValue
    }

    //note: in reality, this would likely be a #selector, not callback, so we can un-observe
    public func onNext(_ callback: @escaping (T) -> Void) {
        observers.append(callback)
        callback(value) //send initial value
    }

    private func notifyObservers() {
        for observer in observers {
            observer(value)
        }
    }
}

extension ObservableProperty: CustomStringConvertible {
    public var description: String {
        if let printable = value as? CustomStringConvertible {
            return printable.description
        } else {
            return "\(value)"
        }
    }
}

public class Person {
    public var firstName: ObservableProperty<String>
    public var middleInitial: ObservableProperty<String?>
    public var lastName: ObservableProperty<String>

    public init(firstName: String, middleInitial: String?, lastName: String) {
        self.firstName = ObservableProperty<String>(firstName)
        self.middleInitial = ObservableProperty<String?>(middleInitial)
        self.lastName = ObservableProperty<String>(lastName)
    }

    public func abbreviateName() {
        firstName.value.removeLast()
    }
}

public class PersonViewModel {
    // observable properties
    public var personNameText = ObservableProperty<String?>(nil)
    public var abbreviateNameButtonEnabled = ObservableProperty<Bool>(true)

    public init(person: Person?) {
        self.person = ObservableProperty<Person?>(person)
        bindPerson()
    }

    @objc public func didPressAbbreviateName() {
        person.value?.abbreviateName()
    }

    // MARK: observe changes to data models

    public var person: ObservableProperty<Person?>

    private func bindPerson() {
        self.person.onNext {[unowned self] (person) in
            guard let person = person else {
                self.abbreviateNameButtonEnabled.set(false)
                return
            }
            self.abbreviateNameButtonEnabled.set(true)
            person.firstName.onNext {[unowned self] (_) in
                self.updatePersonName()
            }
            person.middleInitial.onNext {[unowned self] (_) in
                self.updatePersonName()
            }
            person.lastName.onNext {[unowned self] (_) in
                self.updatePersonName()
            }
        }
    }

    private func updatePersonName() {
        if let person = person.value {
            self.personNameText.value = "\(person.firstName.value) \(person.lastName.value)"
        } else {
            self.personNameText.value = "no person"
        }
    }
}

class PersonView {
    var personNameLabel: UILabel
    var abbreviateNameButton: UIButton
    var backButton: UIButton
    private var viewModel: PersonViewModel

    init(person: Person?) {
        viewModel = PersonViewModel(person: person)
        personNameLabel = UILabel(frame: CGRect.zero)
        abbreviateNameButton = UIButton(frame: CGRect.zero)
        backButton = UIButton(frame: CGRect.zero)
        bindViewModel()
    }

    func changePerson(person: Person?) {
        viewModel.person.set(person)
    }

    private var observers = [NSKeyValueObservation]()
    func bindViewModel() {
        abbreviateNameButton.addTarget(viewModel, action: #selector(PersonViewModel.didPressAbbreviateName), for: .touchUpInside)
        viewModel.abbreviateNameButtonEnabled.onNext {(isEnabled) in
            self.abbreviateNameButton.isEnabled = isEnabled
        }
        viewModel.personNameText.onNext {(text) in
            self.personNameLabel.text = text
        }
    }
}

func main() {
    let me = Person(firstName: "Peter", middleInitial: nil, lastName: "C")
    let daisy = Person(firstName: "Daisy", middleInitial: nil, lastName: "J")

    let view = PersonView(person: nil)
    print("button enabled? \(view.abbreviateNameButton.isEnabled)")
    print(view.personNameLabel.text ?? "nil")
    view.changePerson(person: me)
    print("button enabled? \(view.abbreviateNameButton.isEnabled)")
    print(view.personNameLabel.text ?? "nil")
    view.abbreviateNameButton.sendActions(for: .touchUpInside)
    print(view.personNameLabel.text ?? "nil")
    view.changePerson(person: daisy)
    print(view.personNameLabel.text ?? "nil")
}

main()