Mocking view model in swiftui with combine

Question

Is there a way to mock the viewmodels of an application that uses SwiftUI and Combine? I always find articles about mocking services used by the viewmodel but never mocking a viewmodel itself.

I tried to create protocols for each viewmodel. Problem: the @Published wrapper cannot be used in protocols. It seems like there are no solutions...

Thanks for your help

Answer 1

Using a protocol type as @ObservableObject or @StateObject would not work. Inheritance might be a solution (like Jake suggested), or you could go with a generic solution.

protocol ContentViewModel: ObservableObject {
    var message: String { get set }
}

Your view model would be simple.

final class MyViewModel: ContentViewModel {
    
    @Published var message: String
    init(_ message: String = "MyViewModel") {
        self.message = message
    }
}

On the other hand, your views would be more complex using a constrained generic.

struct ContentView<Model>: View where Model: ContentViewModel {
    @ObservedObject
    var viewModel: Model
    
    var body: some View {
        VStack {
            Text(viewModel.message)
            Button("Change message") {
                viewModel.message = "🥳"
            }
        }
    }
}

The disadvantage is that you have to define the generic concrete type when using the view --- inheritance could avoid that.

// your mock implementation for testing
final class MockViewModel: ContentViewModel {
    @Published var message: String = "Mock View Model"
}

let sut = ContentView<MockViewModel>(viewModel: MockViewModel())