[danielbernal.co] Protocols and Combine: Using @published in your Protocol declaration - https://danielbernal.co/combine-and-protocols-in-swift/

AnimalGenerator.2.swift

class AnimalGenerator: Generator, ObservableObject {

    @Published private(set) var name: String = ""
    var namePublished: Published<String> { _name }
    var namePublisher: Published<String>.Publisher { $name }

     let animals = ["Cat", "Dog", "Crow", "Horse", "Iguana", "Cow", "Racoon"]

    init() {
        generate()
    }

    func generate() {
        self.name = animals.randomElement() ?? ""
    }
}

AnimalGenerator.swift

class AnimalGenerator: Generator, ObservableObject {

     @Published private(set) var name: String = ""
     let animals = ["Cat", "Dog", "Crow", "Horse", "Iguana", "Cow", "Racoon"]

    init() {
        generate()
    }

    func generate() {
        self.name = animals.randomElement() ?? ""
    }
}

Generator.2.swift

protocol Generator {
    // Wrapped value  
    var name: String { get }

    // (Published property wrapper)
    var namePublished: Published<String> { get }

     // Publisher
    var namePublisher: Published<String>.Publisher { get }
    func generate()
}

Generator.swift

protocol Generator {
    @Published var name: String { get }
    func generate()
}

InitialExample.swift

import PlaygroundSupport
import SwiftUI
import Combine

class AnimalGenerator: ObservableObject {

     @Published private(set) var name: String = ""
     let animals = ["Cat", "Dog", "Crow", "Horse", "Iguana", "Cow", "Racoon"]

    init() {
        generate()
    }

    func generate() {
        self.name = animals.randomElement() ?? ""
    }
}

class TheViewModel: ObservableObject {

    @Published var displayData: String = "No data"
    var generator: AnimalGenerator = AnimalGenerator()
    var cancellable: AnyCancellable?

    init() {
        cancellable =
            generator
                .$name
                .receive(on: RunLoop.main)
                .sink { [weak self] data in
                    self?.displayData = data
            }
    }

    func generate() {
        generator.generate()
    }
}

struct TheView: View {

    @ObservedObject var viewModel: TheViewModel = TheViewModel()

    var body: some View {
        VStack {
            Text(viewModel.displayData).font(.system(.largeTitle)).padding()
            Button("Tap me", action: { self.viewModel.generate() })
        }
    }
}

PlaygroundPage.current.setLiveView(TheView())

PersonGenerator.swift

class PersonGenerator: Generator, ObservableObject {

    var namePublished: Published<String> { _name }
    var namePublisher: Published<String>.Publisher { $name }

     @Published private(set) var name: String = ""
     let persons = ["John", "Jane", "Carlos", "Daniel", "Helen", "David", "Bill"]

    init() {
        generate()
    }

    func generate() {
        self.name = persons.randomElement() ?? ""
    }
}

let viewModel = TheViewModel(generator: PersonGenerator())

PlaygroundPage.current.setLiveView(
    TheView(viewModel: viewModel)
)

TheView.2.swift

struct TheView: View {

    @ObservedObject var viewModel: TheViewModel

    var body: some View {
        VStack {
            Text(viewModel.textToShow).font(.system(.largeTitle)).padding()
            Button("Tap me", action: { self.viewModel.generate() })
        }
    }
}

TheView.swift

struct TheView: View {

    @ObservedObject var viewModel: TheViewModel = TheViewModel(generator: AnimalGenerator())

    var body: some View {
        VStack {
            Text(viewModel.textToShow).font(.system(.largeTitle)).padding()
            Button("Tap me", action: { self.viewModel.generate() })
        }
    }
}

TheViewModel.swift

class TheViewModel: ObservableObject {

    @Published var textToShow: String = "No data"
    var generator: Generator
    var cancellable: AnyCancellable?

    init(generator: Generator) {
        self.generator = generator
        cancellable =
            generator
                .namePublisher
                .receive(on: RunLoop.main)
                .sink { [weak self] data in
                    self?.textToShow = data
            }
    }

    func generate() {
        generator.generate()
    }
}

TheViewModelInstance.swift

let viewModel: TheViewModel = TheViewModel(generator: AnimalGenerator())

PlaygroundPage.current.setLiveView(
    TheView(viewModel: viewModel)
)