Sending Event from SwiftUI View to View using Combine

SwiftUIViewToViewEvent.swift

import SwiftUI
import Combine
import PlaygroundSupport

struct ContentView: View {
    var emitter = PassthroughSubject<(), Never>()
    var body: some View {
        VStack {
        Text("Hello, world!")
            .padding()
            Button("Send Event") {
                emitter.send(())
            }
            EventReceiver(event: emitter.eraseToAnyPublisher())
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct EventReceiver: View {
    var event: AnyPublisher<(), Never>
    @State var count = 0
    var body: some View {
        Text("Received \(count)")
            .onReceive(event) { _ in
                count += 1
            }
    }
}

PlaygroundPage.current.setLiveView(ContentView())

UIKitEventToAnotherTab.swift

///
/// Subscribing custom event publisher from a different view controller
///
import UIKit
import Combine
import PlaygroundSupport


class State {
    @Published var counter: Int = 0
}

let state = State()

// Main View Controller: state.counter += 1
let vcMain = Tab1ViewController(state: state)
// Detail View Controller: state.assign(to: \.lastCounter, on: self)
// - But avoiding retain-cycle
// - Take action whenever lastCounter value changes: label update or query
let vcDetail = Tab2ViewController(detailTapped: state.$counter.eraseToAnyPublisher())
let vcTab = UITabBarController()
vcTab.viewControllers = [vcMain, vcDetail]
vcTab.tabBar.backgroundColor = .systemMint
vcTab.selectedIndex = 0

PlaygroundPage.current.liveView = vcTab

// MARK: -

class Tab1ViewController: UIViewController {
    let state: State
    var subscriptions: [AnyCancellable] = []

    init(state: State) {
        self.state = state
        super.init(nibName: nil, bundle: nil)
    }
    
    internal lazy var button: UIButton = {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        button.setTitleColor(.darkText, for: .normal)
        button.setTitle("Detail", for: .normal)
        button.addTarget(self, action: #selector(didTapButton), for: .touchUpInside)
        return button
    }()
    
    override func viewDidLoad() {

        tabBarItem = .init(UITabBarItem(tabBarSystemItem: .search, tag: 0))
        
        view.addSubview(button)
        
        NSLayoutConstraint.activate([

            button.widthAnchor.constraint(equalToConstant: 80),
            button.heightAnchor.constraint(equalToConstant: 30),
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
    
    // MARK: -
    @objc func didTapButton() {
        state.counter += 1                                                         // 1. Send out event as sequence number
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class Tab2ViewController: UIViewController {
    let detailTapped: AnyPublisher<Int, Never>
    var subscriptions: [AnyCancellable] = []
    var lastCounter: Int = 0 {
        didSet {
            button.setTitle("Show detail \(lastCounter)", for: .normal)      // 3. Handle event since its a new event
            print("counter: \(lastCounter)")
        }
    }
    
    init(detailTapped: AnyPublisher<Int, Never>) {
        self.detailTapped = detailTapped
        super.init(nibName: nil, bundle: nil)
    }
    
    internal lazy var button: UIButton = {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        button.setTitleColor(.darkText, for: .normal)
        button.setTitle("Detail", for: .normal)
        return button
    }()
    
    override func viewDidLoad() {

        detailTapped.sink { [weak self] counter in
            self?.lastCounter = counter
        }.store(in: &subscriptions)                                           // 2. Subscribe and update local `lastCounter`
        
        tabBarItem = .init(UITabBarItem(tabBarSystemItem: .search, tag: 0))
        
        view.addSubview(button)
        
        NSLayoutConstraint.activate([

            button.widthAnchor.constraint(equalToConstant: 80),
            button.heightAnchor.constraint(equalToConstant: 30),
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class SampleViewController: UIViewController {

    internal lazy var button: UIButton = {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        button.setTitleColor(.darkText, for: .normal)
        button.setTitle("Detail", for: .normal)
        button.addTarget(self, action: #selector(didTapButton), for: .touchUpInside)
        return button
    }()
    
    init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    override func viewDidLoad() {

        tabBarItem = .init(UITabBarItem(tabBarSystemItem: .search, tag: 0))
        
        view.addSubview(button)
        
        NSLayoutConstraint.activate([

            button.widthAnchor.constraint(equalToConstant: 80),
            button.heightAnchor.constraint(equalToConstant: 30),
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
    
    @objc func didTapButton() {
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}