依赖注入：什么是依赖注入，iOS 和 Android 中的依赖注入

// 应用类
@HiltAndroidApp
class MyApp : Application()

// 模块 —— 定义如何创建依赖
@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
    @Provides
    @Singleton
    fun provideOkHttpClient(): OkHttpClient = OkHttpClient.Builder().build()

    @Provides
    @Singleton
    fun provideApiService(client: OkHttpClient): ApiService {
        return Retrofit.Builder()
            .baseUrl("https://api.example.com")
            .client(client)
            .build()
            .create(ApiService::class.java)
    }
}

// ViewModel 通过构造函数接收依赖
@HiltViewModel
class MainViewModel @Inject constructor(
    private val apiService: ApiService
) : ViewModel() {
    private val _state = MutableStateFlow(MainState.Loading)
    val state: StateFlow<MainState> = _state.asStateFlow()

    fun loadData() {
        viewModelScope.launch {
            _state.value = MainState.Success(apiService.getData())
        }
    }
}

// Activity —— @AndroidEntryPoint 启用 DI
@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    private val viewModel: MainViewModel by viewModels()
}

import Swinject

// Assembly —— 注册组
class NetworkAssembly: Assembly {
    func assemble(container: Container) {
        container.register(NetworkServiceProtocol.self) { _ in
            NetworkService()
        }.inObjectScope(.container) // singleton

        container.register(UserRepositoryProtocol.self) { r in
            UserRepository(
                networkService: r.resolve(NetworkServiceProtocol.self)!
            )
        }
    }
}

// ViewModel 通过构造函数注入
class ProfileViewModel: ObservableObject {
    private let repository: UserRepositoryProtocol

    init(repository: UserRepositoryProtocol) {
        self.repository = repository
    }

    @Published var user: User?
    func loadUser() {
        repository.fetchUser { [weak self] user in
            self?.user = user
        }
    }
}

// 在 AppDelegate 或 App 中配置 DI
let assembler = Assembler([NetworkAssembly(), ServiceAssembly()])
let viewModel = assembler.resolver.resolve(ProfileViewModel.self)!

// UIKit ViewController 的属性注入
container.register(UserViewController.self) { r in
    let vc = UserViewController()
    vc.viewModel = r.resolve(ProfileViewModel.self)
    return vc
}

// ❌ 不好：6 个依赖 —— God Object
class ProfileViewModel @Inject constructor(
    private val api: ApiService,
    private val db: Database,
    private val analytics: Analytics,
    private val prefs: Preferences,
    private val location: LocationProvider,
    private val notification: NotificationManager
)

// ✅ 好：具有 1-2 个依赖的 Use Cases
class ProfileViewModel @Inject constructor(
    private val loadProfileUseCase: LoadProfileUseCase,
    private val trackAnalyticsUseCase: TrackAnalyticsUseCase
)