Binding a SwiftUI Button to AnySubscriber like RxCocoa's button tap

Question

I use the following UIViewController and RxSwift/RxCocoa based piece of code to write a very simply MVVM pattern to bind a UIButton tap event to trigger some Observable work and listen for the result:

import UIKit
import RxSwift
import RxCocoa

class ViewController: UIViewController {

    @IBOutlet weak var someButton: UIButton!

    var viewModel: ViewModel!
    private var disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()
        viewModel = ViewModel()
        setupBindings()
    }

    private func setupBindings() {
        someButton.rx.tap
        .bind(to: self.viewModel.input.trigger)
        .disposed(by: disposeBag)

        viewModel.output.result
            .subscribe(onNext: { element in
            print("element is \(element)")
            }).disposed(by: disposeBag)
    }
}

class ViewModel {

    struct Input {
        let trigger: AnyObserver<Void>
    }

    struct Output {
        let result: Observable<String>
    }

    let input: Input
    let output: Output

    private let triggerSubject = PublishSubject<Void>()

    init() {
        self.input = Input(trigger: triggerSubject.asObserver())
        let resultObservable = triggerSubject.flatMap { Observable.just("TEST") }
        self.output = Output(result: resultObservable)
    }
}

It compiles and runs well. However, I need to Combin ify this pattern with SwiftUI , so I converted that code into the following:

import SwiftUI
import Combine

struct ContentView: View {
    var viewModel: ViewModel
    var subscriptions = Set<AnyCancellable>()

    init(viewModel: ViewModel) {
        self.viewModel = viewModel
        setupBindings()
    }

    var body: some View {

        Button(action: {
            // <---- how to trigger viewModel's trigger from here
        }, label: {
            Text("Click Me")
        })
    }

    private func setupBindings() {
        self.viewModel.output.result.sink(receiveValue: { value in
            print("value is \(value)")
            })
            .store(in: &subscriptions) // <--- doesn't compile due to immutability of ContentView
    }
}

class ViewModel {

    struct Input {
        let trigger: AnySubscriber<Void, Never>
    }

    struct Output {
        let result: AnyPublisher<String, Never>
    }

    let input: Input
    let output: Output

    private let triggerSubject = PassthroughSubject<Void, Never>()

    init() {
        self.input = Input(trigger: AnySubscriber(triggerSubject))

        let resultPublisher = triggerSubject
            .flatMap { Just("TEST") }
            .eraseToAnyPublisher()

        self.output = Output(result: resultPublisher)
    }
}

This sample doesn't compile due to two errors (commented in code):

(1) Problem 1 : How to trigger the publisher's work from the button's action closure like the case of RxSwift above ?

(2) Problem 2 is related somehow to architectural design rather than a compile error: the error says: ... Cannot pass immutable value as inout argument: 'self' is immutable ... , that's because SwiftUI views are structs, they are designed to be changed only through sorts of bindings ( @State , @ObservedObject , etc ...), I have two sub-questions related to problem 2:

[A]: is it considered a bad practice to sink a publisher in a SwiftUI View ? which may need some workaround to store the cancellable at the View 's struct scope ?

[B]: which one is better for SwiftUI/Combine projects in terms of MVVM architectural pattern: using a ViewModel with [ Input[Subscribers], Output[AnyPublishers] ] pattern, or a ObservableObject ViewModel with [ @Published properties] ?

Answer 1

I had same problem understanding best mvvm approach. Recommend also look into this thread Best data-binding practice in Combine + SwiftUI?

Will post my working example. Should be easy to convert to what you want.

SwiftUI View:

struct ContentView: View {
    @State private var dataPublisher: String = "ggg"
    @State private var sliderValue: String = "0"
    @State private var buttonOutput: String = "Empty"


    let viewModel: SwiftUIViewModel
    let output: SwiftUIViewModel.Output

    init(viewModel: SwiftUIViewModel) {
        self.viewModel = viewModel
        self.output = viewModel.bind(())
    }

    var body: some View {
        VStack {
            Text(self.dataPublisher)
            Text(self.sliderValue)
            Slider(value: viewModel.$sliderBinding, in: 0...100, step: 1)
            Button(action: {
                self.viewModel.buttonBinding = ()
            }, label: {
                Text("Click Me")
            })
            Text(self.buttonOutput)
        }
        .onReceive(output.dataPublisher) { value in
            self.dataPublisher = value
        }
        .onReceive(output.slider) { (value) in
            self.sliderValue = "\(value)"
        }
        .onReceive(output.resultPublisher) { (value) in
            self.buttonOutput = value
        }
    }
}

AbstractViewModel:

protocol ViewModelProtocol {
    associatedtype Output
    associatedtype Input

    func bind(_ input: Input) -> Output
}

ViewModel:

final class SwiftUIViewModel: ViewModelProtocol {
    struct Output {
        let dataPublisher: AnyPublisher<String, Never>
        let slider: AnyPublisher<Double, Never>
        let resultPublisher: AnyPublisher<String, Never>
    }

    typealias Input = Void

    @SubjectBinding var sliderBinding: Double = 0.0
    @SubjectBinding var buttonBinding: Void = ()

    func bind(_ input: Void) -> Output {
        let dataPublisher = URLSession.shared.dataTaskPublisher(for: URL(string: "https://www.google.it")!)
            .delay(for: 5.0, scheduler: DispatchQueue.main)
            .map{ "Just for testing - \($0)"}
            .replaceError(with: "An error occurred")
            .receive(on: DispatchQueue.main)
            .share()
            .eraseToAnyPublisher()

        let resultPublisher = _buttonBinding.anyPublisher()
            .dropFirst()
            .flatMap { Just("TEST") }
            .share()
            .eraseToAnyPublisher()

        return Output(dataPublisher: dataPublisher,
                      slider: _sliderBinding.anyPublisher(),
                      resultPublisher: resultPublisher)
    }
}

SubjectBinding property wrapper:

@propertyWrapper
struct SubjectBinding<Value> {
    private let subject: CurrentValueSubject<Value, Never>

    init(wrappedValue: Value) {
        subject = CurrentValueSubject<Value, Never>(wrappedValue)
    }

    func anyPublisher() -> AnyPublisher<Value, Never> {
        return subject.eraseToAnyPublisher()
    }

    var wrappedValue: Value {
        get {
            return subject.value
        }
        set {
            subject.value = newValue
        }
    }

    var projectedValue: Binding<Value> {
        return Binding<Value>(get: { () -> Value in
            return self.subject.value
        }) { (value) in
            self.subject.value = value
        }
    }
}

Answer 2

So I recently was also wondering how I would do this since we are not starting to write out views in SwiftUI.

I made a helper object the encapsulates the transition from a function call to a Publisher. I called it a Relay.

@available(iOS 13.0, *)
struct Relay<Element> {
    var call: (Element) -> Void { didCall.send }
    var publisher: AnyPublisher<Element, Never> { 
        didCall.eraseToAnyPublisher() 
    }

    // MARK: Private

    private let didCall = PassthroughSubject<Element, Never>()
}

In your case specifically, you would be able to declare a private Relay and use it like so;

    Button(action: relay.call, 
    label: {
        Text("Click Me")
    })

And then you can do whatever you like with.

relay.publisher