MVVM: what it is, the Model-View-ViewModel pattern in mobile development

// ViewModel with StateFlow — modern MVVM implementation
class UserViewModel(
    private val repository: UserRepository
) : ViewModel() {

    private val _state = MutableStateFlow<UserState>(UserState.Loading)
    val state: StateFlow<UserState> = _state.asStateFlow()

    fun loadUser(userId: Int) {
        viewModelScope.launch {
            _state.value = UserState.Loading
            repository.getUser(userId)
                .onSuccess { user ->
                    _state.value = UserState.Success(user)
                }
                .onFailure { e ->
                    _state.value = UserState.Error(e.message ?: "Unknown")
                }
        }
    }
}

sealed interface UserState {
    data object Loading : UserState
    data class Success(val user: User) : UserState
    data class Error(val message: String) : UserState
}

// View (Activity) subscribes to state
class UserActivity : AppCompatActivity() {
    private val viewModel: UserViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        viewModel.state.onEach { state ->
            when (state) {
                is UserState.Loading -> /* show loading */
                is UserState.Success -> /* display data */
                is UserState.Error -> /* show error */
            }
        }.launchIn(lifecycleScope)
        viewModel.loadUser(42)
    }
}

import SwiftUI
import Combine

// ViewModel — ObservableObject with @Published properties
final class UserViewModel: ObservableObject {
    @Published private(set) var state: UserState = .loading
    private let service: UserService

    init(service: UserService) {
        self.service = service
    }

    func loadUser(id: Int) {
        state = .loading
        service.fetchUser(id: id) { [weak self] result in
            guard let self else { return }
            switch result {
            case .success(let user):
                self.state = .success(user)
            case .failure(let error):
                self.state = .error(error.localizedDescription)
            }
        }
    }
}

enum UserState {
    case loading
    case success(User)
    case error(String)
}

// SwiftUI View — subscribes to ViewModel
struct UserView: View {
    @StateObject private var viewModel: UserViewModel

    var body: some View {
        switch viewModel.state {
        case .loading:
            ProgressView()
        case .success(let user):
            VStack {
                Text(user.name).font(.title)
                Text(user.email).font(.body)
            }
        case .error(let message):
            Text(message).foregroundColor(.red)
        }
    }
}

// Android ViewModel unit test with MockK
class UserViewModelTest {

    private val repository = mockk<UserRepository>()
    private val viewModel = UserViewModel(repository)

    @Test
    fun loadUser_success_updatesState() = runTest {
        val user = User(1, "John", "john@test.com")
        coEvery { repository.getUser(1) } returns Result.success(user)

        viewModel.loadUser(1)

        assertEquals(UserState.Success(user), viewModel.state.value)
    }

    @Test
    fun loadUser_error_updatesErrorState() = runTest {
        val error = RuntimeException("Network error")
        coEvery { repository.getUser(1) } returns Result.failure(error)

        viewModel.loadUser(1)

        val state = viewModel.state.value
        assertTrue(state is UserState.Error)
        assertEquals("Network error", (state as UserState.Error).message)
    }
}