MVI — 移动应用中 Model-View-Intent 模式的本质

// Intent — 用户意图
sealed interface UserIntent {
    data class LoadUser(val userId: Int) : UserIntent
    data object Refresh : UserIntent
}

// State — 屏幕的单一状态
sealed interface UserState {
    data object Idle : UserState
    data object Loading : UserState
    data class Success(val user: User) : UserState
    data class Error(val message: String) : UserState
}

// Reducer — 纯函数
object UserReducer {
    fun reduce(state: UserState, intent: UserIntent): UserState = when (intent) {
        is UserIntent.LoadUser -> UserState.Loading
        is UserIntent.Refresh -> UserState.Loading
    }
}

// 使用 MVI 的 ViewModel
class UserViewModel(
    private val repository: UserRepository
) : ViewModel() {

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

    fun process(intent: UserIntent) {
        val newState = UserReducer.reduce(_state.value, intent)
        _state.value = newState
        when (intent) {
            is UserIntent.LoadUser -> loadUser(intent.userId)
            is UserIntent.Refresh -> loadUser(/* 上一个 ID */)
        }
    }

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

// View 发送 Intent
fun UserScreen(viewModel: UserViewModel) {
    val state by viewModel.state.collectAsState()
    LaunchedEffect(Unit) { viewModel.process(UserIntent.LoadUser(42)) }
    when (state) {
        is UserState.Loading -> CircularProgressIndicator()
        is UserState.Success -> UserCard((state as UserState.Success).user)
        is UserState.Error -> ErrorView((state as UserState.Error).message)
        is UserState.Idle -> Text("Press to load")
    }
}

// State — 不可变结构
struct UserState: Equatable {
    var user: User?
    var isLoading = false
    var errorMessage: String?
}

// Intent — 带有意图的枚举
enum UserIntent {
    case loadUser(id: Int)
    case refresh
    case userLoaded(User)
    case loadFailed(Error)
}

// Reducer — 纯函数
func userReducer(state: UserState, intent: UserIntent) -> UserState {
    var newState = state
    switch intent {
    case .loadUser, .refresh:
        newState.isLoading = true
        newState.errorMessage = nil
    case .userLoaded(let user):
        newState.isLoading = false
        newState.user = user
    case .loadFailed(let error):
        newState.isLoading = false
        newState.errorMessage = error.localizedDescription
    }
    return newState
}

// Store — 拥有状态并管理效应
final class UserStore: ObservableObject {
    @Published private(set) var state = UserState()
    private let service: UserService

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

    func dispatch(_ intent: UserIntent) {
        // 1. Reducer 更新状态
        state = userReducer(state: state, intent: intent)
        // 2. 副作用（如果需要）
        switch intent {
        case .loadUser(let id), .refresh:
            service.fetchUser(id: id) { [weak self] result in
                switch result {
                case .success(let user):
                    self?.dispatch(.userLoaded(user))
                case .failure(let error):
                    self?.dispatch(.loadFailed(error))
                }
            }
        default: break
        }
    }
}