How do I mock network layers in UIKit with Swift?

Mocking network layers in UIKit with Swift is essential for unit testing your application effectively. By creating mock objects, you can simulate network responses without making actual network calls, allowing for faster tests and avoiding reliance on the internet.

import Foundation

// A protocol that defines the methods for the network service
protocol NetworkService {
    func fetchData(completion: @escaping (Data?) -> Void)
}

// A real implementation of the network service
class RealNetworkService: NetworkService {
    func fetchData(completion: @escaping (Data?) -> Void) {
        // Networking code here
    }
}

// A mock implementation of the network service for testing
class MockNetworkService: NetworkService {
    var mockData: Data?
    
    func fetchData(completion: @escaping (Data?) -> Void) {
        completion(mockData)
    }
}

// Example of using the mock in a unit test
class ViewModelTests: XCTestCase {
    var viewModel: MyViewModel!
    var mockNetworkService: MockNetworkService!

    override func setUp() {
        super.setUp()
        mockNetworkService = MockNetworkService()
        viewModel = MyViewModel(networkService: mockNetworkService)
    }

    func testFetchData() {
        // Given
        let expectedData = Data("Mock Data".utf8)
        mockNetworkService.mockData = expectedData
        
        // When
        viewModel.fetchData()
        
        // Then
        XCTAssertEqual(viewModel.dataReceived, expectedData)
    }
}