Clean Architecture + DI + SwiftUI + Combine.swift

import Foundation
import SwiftUI
import Combine

// MARK: - Presentation Layer

struct MyView: View {
    @ObservedObject var viewModel: MyViewModel
    
    var body: some View {
        Text(viewModel.data)
    }
}

// MARK: - Domain Layer

class MyViewModel: ObservableObject {
    @Published var data: String = ""
    private let dataFetcher: DataFetcher
    private var cancellable: AnyCancellable?
    
    init(dataFetcher: DataFetcher) {
        self.dataFetcher = dataFetcher
    }
    
    func fetchData() {
        // Fetch data from the network or a local database
        // using the data access layer
        cancellable = dataFetcher.fetchData()
            // Handle the result of the fetch
            .receive(on: RunLoop.main)
            .sink { error in
                if case let .failure(error) = error {
                    print(error.localizedDescription)
                }
            } receiveValue: { (value: String) in
                self.data = value
            }
    }
}

// MARK: - Data Access Layer

protocol DataFetcher {
    func fetchData() -> AnyPublisher<String, Error>
}

class NetworkDataFetcher: DataFetcher {
    func fetchData() -> AnyPublisher<String, Error> {
        // Fetch data from the network and call the completion handler
        Just("Hello")
            .setFailureType(to: Error.self)
            .eraseToAnyPublisher()
    }
}

class DatabaseDataFetcher: DataFetcher {
    func fetchData() -> AnyPublisher<String, Error> {
        // Fetch data from the local database and call the completion handler
        Just("Mom")
            .setFailureType(to: Error.self)
            .eraseToAnyPublisher()
    }
}