Combine MVVM Demo Playground

CombineMVVMPlayground.swift

//: A UIKit based Playground for presenting user interface
 
/// Add an event publisher to UIControls
/// From: https://forums.swift.org/t/a-uicontrol-event-publisher-example/26215/19
extension UIControl {

    private class EventObserver {

        let control: UIControl
        let event: UIControl.Event
        let subject: PassthroughSubject<UIControl, Never>

        init(control: UIControl, event: UIControl.Event) {
            self.control = control
            self.event = event
            self.subject = .init()
        }

        func start() {
            control.addTarget(self, action: #selector(handleEvent(from:)), for: event)
        }

        func stop() {
            control.removeTarget(self, action: nil, for: event)
        }

        @objc
        func handleEvent(from sender: UIControl) {
            subject.send(sender)
        }
    }

    struct ControlEventPublisher: Publisher {

        typealias Output = UIControl
        typealias Failure = Never

        let control: UIControl
        let event: UIControl.Event

        init(control: UIControl, event: UIControl.Event) {
            self.control = control
            self.event = event
        }

        func receive<S>(subscriber: S)
            where S : Subscriber,
            S.Failure == Failure,
            S.Input == Output {
                let observer = EventObserver(control: control, event: event)
                observer
                    .subject
                    .handleEvents(
                        receiveSubscription: { _ in observer.start() },
                        receiveCancel: observer.stop
                    )
                    .receive(subscriber: subscriber)

        }
    }

    func eventPublisher(for event: UIControl.Event) -> ControlEventPublisher {
        return ControlEventPublisher(control: self, event: event)
    }
}


// Must be a class otherwise @Published is not supported
final class ViewModel {

    // Mark: Outputs
    @Published private(set) var outHeadlineText: String? = "Hello there!"
    @Published private(set) var outButtonText: String? = "Press Me!"

    // Mark: Inputs
    // Use a Void property as a small "trick" to be able to use 'assign' also for inputs
    var inTestAction: Void = () {
        didSet {
            outHeadlineText = "Button pressed!"
            outButtonText = "Press me again!"
        }
    }
}

// If you know RxSwift you know DisposeBag.
// This is to demonstrate that Combine uses the same concept.
typealias DisposeBag = Set<AnyCancellable>

final class ViewController: UIViewController {
    let viewModel = ViewModel()
    var label = UILabel()
    var button = UIButton(type: .system)

    var text: String = ""

    private var subscriptions = DisposeBag()

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        label.text = "Hello World!"
        label.textColor = .black
        label.textAlignment = .center
        view.addSubview(label)

        label.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            label.widthAnchor.constraint(equalToConstant: 200),
            label.heightAnchor.constraint(equalToConstant: 20),
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])

        view.addSubview(button)

        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.widthAnchor.constraint(equalToConstant: 200),
            button.heightAnchor.constraint(equalToConstant: 44),
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.topAnchor.constraint(equalTo: label.bottomAnchor, constant: 32)
        ])

        self.view = view
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        bindViewModel()
    }

    private func bindViewModel() {

        // How to bind method 1)
        // Syntax with sink is not that pretty...
        viewModel.$outHeadlineText.sink { [weak self] text in
            self?.text = text ?? ""
        }.store(in: &subscriptions)

        // How to bind method 2)
        // Using assign and a keypath makes for a quite streamlined syntax.
        // The published value and the assigned value have to match 100%
        // You can not assign a non-optional type to a keypath that
        // expects an optional type!
        // See also https://heckj.github.io/swiftui-notes/#reference-assign
        subscriptions.add([
            // Outputs
            viewModel.$outHeadlineText.assign(to: \.text, on: label),
            viewModel.$outButtonText.assign(to: \.defaultTitle, on: button),

            // Inputs
            button.eventPublisher(for: .touchUpInside).map({_ in ()}).assign(to: \.viewModel.inTestAction, on: self)
        ])
    }
}

// Some syntactic sugar to make adding multiple subscriptions less cumbersome
extension DisposeBag {
    mutating func add(_ subscriptions: [AnyCancellable]) {
        for item in subscriptions {
           insert(item)
        }
    }
}

// It seems there is no default way to set the title using combine, add a custom
// computed property to be able to use 'assign'.
extension UIButton {
    var defaultTitle: String? {
        set {
            setTitle(newValue, for: .normal)
        }
        get {
            title(for: .normal)
        }
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = ViewController()