How do I mock network layers in Combine with Swift?

In Swift, when using Combine for network requests, it's often useful to mock network layers for testing purposes. This allows you to simulate network responses and behaviors without relying on actual network calls. Here's an example of how to achieve this:


        import Combine
        import Foundation

        // Network service protocol
        protocol NetworkService {
            func fetchData() -> AnyPublisher
        }

        // Real network service implementation
        class RealNetworkService: NetworkService {
            func fetchData() -> AnyPublisher {
                let url = URL(string: "https://api.example.com/data")!
                return URLSession.shared.dataTaskPublisher(for: url)
                    .map(\.data)
                    .eraseToAnyPublisher()
            }
        }

        // Mock network service implementation
        class MockNetworkService: NetworkService {
            func fetchData() -> AnyPublisher {
                let mockData = Data("{\"key\":\"value\"}".utf8)
                return Just(mockData)
                    .setFailureType(to: Error.self)
                    .eraseToAnyPublisher()
            }
        }

        // ViewModel using the NetworkService
        class ViewModel: ObservableObject {
            private var cancellables = Set()
            private let networkService: NetworkService
            
            @Published var data: String = ""
            
            init(networkService: NetworkService) {
                self.networkService = networkService
                fetchData()
            }
            
            private func fetchData() {
                networkService.fetchData()
                    .decode(type: [String: String].self, decoder: JSONDecoder())
                    .map { $0["key"] ?? "" }
                    .receive(on: RunLoop.main)
                    .sink(receiveCompletion: { completion in
                        // Handle completion
                    }, receiveValue: { [weak self] value in
                        self?.data = value
                    })
                    .store(in: &cancellables)
            }
        }

How do I mock network layers in Combine with Swift?

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How do I mock network layers in Combine with Swift?

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