Plain publisher

publisher.swift

import UIKit

struct Books: Codable {}
struct Movies: Codable {}

// MARK: - Observer

protocol BooksObserver {
    
    func received(_ data: Books)
    //func subscribe(toService service: BooksService)
    
}

extension BooksObserver {
    
    func subscribe(toService service: BooksService) {
        service.add(self)
    }
    
}

protocol MoviesObserver {
    
    func received(_ data: Movies)
    //func subscribe(toService service: MoviesService)
    
}

extension MoviesObserver {
    
    func subscribe(toService service: MoviesService) {
        service.add(self)
    }
    
}

// MARK: - Service

class BooksService {
    
    var data: Books?
    
    //var observers: [BooksObserver] = .init()
    
    func publish() {
        print()
        //observers.forEach({ $0.received(data!)} )
    }
    
    func add(_ observer: BooksObserver) {
        //observers.append(observer)
    }
}

class MoviesService {
    
    var data: Movies?
    
    //var observers: [MoviesObserver] = .init()
    
    func publish() {
        //observers.forEach({ $0.received(data!)} )
    }
    
    func add(_ observer: MoviesObserver) {
        //observers.append(observer)
    }
    
}

// MARK: - Controllers

class ViewController: UIViewController {
    
    let bookService = BooksService()
    let movieService = MoviesService()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        subscribe(toService: bookService)
        subscribe(toService: movieService)
    }
    
}

extension ViewController: BooksObserver {
    
    func received(_ data: Books) {
        print(data)
    }
    
}

extension ViewController: MoviesObserver {
    
    func received(_ data: Movies) {
        print(data)
    }
    
}

///
/// A possible solution came to my mind.
///
/// Class in between publishers and observers, taking capability of publishing and information what are observers away from publisher.
///

class Coordinator {
    
    private var bookObservers: [BooksObserver] = .init()
    private var movieObservers: [MoviesObserver] = .init()
    
    // will be called from observers - viewControllers
    func add(_ observer: UIViewController) {
        if let observer = observer as? BooksObserver { bookObservers.append(observer) }
        if let observer = observer as? MoviesObserver { movieObservers.append(observer) }
    }
    
    // will be called from publishers 
    func publish(_ data: Codable) {
        if let data = data as? Books { bookObservers.forEach({ $0.received(data)} )}
        if let data = data as? Movies { movieObservers.forEach({ $0.received(data)} )}
    }
    
}