Dependency Injection Mastery

Instructions

當問題涉及 Hilt 進階模式、scope 設計、跨模組存取、測試替換時載入。Dagger 2 → Hilt 的「遷移步驟」由 @techstackmigration 為唯一權威來源，本 skill 不重複（去重）；本 skill 處理 Hilt 已就位後的進階使用。

When to Use

• Scenario A：新專案 DI 架構建立
• Scenario C：舊專案模組拆分，Hilt 已存在
• Scenario F：KMP 共用模組與 Android 端 DI 整合
• 設計 User Session / Tenant Session 等 Custom Scope
• 多模組間以 EntryPoint 安全存取服務
• 用 Multi-binding 做 plugin 架構

When NOT to Use

• Dagger 2 → Hilt 遷移步驟 → @techstackmigration
• Hilt plugin 安裝、KSP 設定（Convention Plugin）→ @project_bootstrapping
• 平台升級造成的 KSP 版本問題 → @platformmodernization2026

Example Prompts

• 「ViewModel 要吃導航參數又要用 Hilt，怎麼接 Assisted？」
• 「想做 User Session Scope，登入後的服務都該綁這個 scope」
• 「多種支付方式想用 plugin 架構，Multi-binding 怎麼設計？」
• 「跨 module 想取得 feature 的服務，又不想直接 import，EntryPoint 怎麼用？」
• 「unit test 想 fake repository，@TestInstallIn 怎麼配？」

Workflow

1. 盤生命週期：列出實際 scope（App / User / Activity / Screen），對應到 Hilt 內建 component 或 Custom Component。
2. 設計 Module 邊界：依模組（network / database / repository / usecase）分檔，避免一檔過大。
3. Qualifier：相同型別不同實例（IO/Default/Main dispatcher、Auth/NoAuth client）用 qualifier 區分。
4. 跨模組存取：用 @EntryPoint 而非全域 singleton；遠離 applicationContext.applicationContext as App。
5. 測試：@TestInstallIn(replaces = [RealModule::class]) + HiltAndroidRule，可重組整個 graph。

Practical Notes (2026-04)

Minimal Template

目標: <例：登入後的服務綁 User Session>
Scope 對應:
  - SingletonComponent: Network, Database, Analytics
  - ActivityRetainedComponent: NavigationRouter
  - ViewModelComponent: 表單 state holder
  - UserSessionComponent (custom): UserPreferences, FeatureFlags
Module 結構:
  - core:network/di/NetworkModule
  - core:database/di/DatabaseModule
  - feature:checkout/di/CheckoutBindModule
Qualifier: @IoDispatcher, @AuthenticatedClient
測試替換: @TestInstallIn 在 androidTest/
驗收: Quick Checklist

Assisted Injection

Compose ViewModel 接導航參數

@HiltViewModel(assistedFactory = DetailViewModel.Factory::class)
class DetailViewModel @AssistedInject constructor(
    @Assisted private val productId: String,
    @Assisted savedStateHandle: SavedStateHandle,
    private val repository: ProductRepository,
) : ViewModel() {
    @AssistedFactory
    interface Factory {
        fun create(productId: String, savedStateHandle: SavedStateHandle): DetailViewModel
    }
}

@Composable
fun DetailScreen(productId: String) {
    val viewModel = hiltViewModel<DetailViewModel, DetailViewModel.Factory> { factory ->
        factory.create(productId, createSavedStateHandle())
    }
}

WorkManager

@HiltWorker
class SyncWorker @AssistedInject constructor(
    @Assisted context: Context,
    @Assisted params: WorkerParameters,
    private val repository: Repository,
) : CoroutineWorker(context, params) {
    override suspend fun doWork(): Result = runCatching { repository.sync() }
        .map { Result.success() }
        .getOrElse { Result.retry() }
}

Custom Hilt Components — 決策矩陣

User Session 範例

@Scope
@Retention(AnnotationRetention.RUNTIME)
annotation class UserSessionScope

@DefineComponent(parent = SingletonComponent::class)
@UserSessionScope
interface UserSessionComponent

@DefineComponent.Builder
interface UserSessionComponentBuilder {
    fun setUser(@BindsInstance user: User): UserSessionComponentBuilder
    fun build(): UserSessionComponent
}

@EntryPoint
@InstallIn(UserSessionComponent::class)
interface UserSessionEntryPoint {
    fun userPreferences(): UserPreferences
    fun featureFlagSource(): FeatureFlagSource
}

@Singleton
class UserSessionManager @Inject constructor(
    private val componentBuilder: Provider<UserSessionComponentBuilder>,
) {
    private var component: UserSessionComponent? = null

    fun login(user: User) {
        component = componentBuilder.get().setUser(user).build()
    }

    fun logout() {
        component = null
    }

    inline fun <reified T> entryPoint(extract: UserSessionEntryPoint.() -> T): T {
        val c = checkNotNull(component) { "Not logged in" }
        return EntryPoints.get(c, UserSessionEntryPoint::class.java).extract()
    }
}

Multi-binding（Plugin 架構）

Set

interface PaymentProcessor { fun supports(type: String): Boolean; fun process(amount: Long): Result<Unit> }

@Module
@InstallIn(SingletonComponent::class)
abstract class PaymentBindingModule {
    @Binds @IntoSet abstract fun creditCard(impl: CreditCardProcessor): PaymentProcessor
    @Binds @IntoSet abstract fun paypal(impl: PayPalProcessor): PaymentProcessor
}

class PaymentService @Inject constructor(
    private val processors: Set<@JvmSuppressWildcards PaymentProcessor>,
) {
    fun process(type: String, amount: Long): Result<Unit> =
        processors.first { it.supports(type) }.process(amount)
}

Map（按 key 取）

@MapKey annotation class PaymentTypeKey(val value: PaymentType)

@Module
@InstallIn(SingletonComponent::class)
abstract class PaymentMapModule {
    @Binds @IntoMap @PaymentTypeKey(PaymentType.CREDIT_CARD)
    abstract fun bindCreditCard(impl: CreditCardProcessor): PaymentProcessor
}

class PaymentService @Inject constructor(
    private val processors: Map<PaymentType, @JvmSuppressWildcards Provider<PaymentProcessor>>,
) {
    // Provider 延遲建構，未使用的 processor 不會被生成
    fun process(type: PaymentType, amount: Long) = processors.getValue(type).get().process(amount)
}

EntryPoint：跨模組安全存取

當 feature:a 想用 feature:b 的服務，不要直接 import。改放在 core:domain 介面 + feature:b 實作 + feature:a 透過 EntryPoint 取。

@EntryPoint
@InstallIn(SingletonComponent::class)
interface AnalyticsEntryPoint {
    fun analytics(): Analytics
}

// 非 Hilt class（如某 Util object 或第三方 callback）需要服務時
class LegacyCallback(private val context: Context) {
    private val analytics by lazy {
        EntryPointAccessors
            .fromApplication(context, AnalyticsEntryPoint::class.java)
            .analytics()
    }
}

Qualifier 與 Dispatcher 注入

@Qualifier @Retention(AnnotationRetention.BINARY) annotation class IoDispatcher
@Qualifier @Retention(AnnotationRetention.BINARY) annotation class DefaultDispatcher
@Qualifier @Retention(AnnotationRetention.BINARY) annotation class MainDispatcher

@Module
@InstallIn(SingletonComponent::class)
object DispatcherModule {
    @Provides @IoDispatcher fun io(): CoroutineDispatcher = Dispatchers.IO
    @Provides @DefaultDispatcher fun default(): CoroutineDispatcher = Dispatchers.Default
    @Provides @MainDispatcher fun main(): CoroutineDispatcher = Dispatchers.Main.immediate
}

注入：

class Repository @Inject constructor(
    @IoDispatcher private val io: CoroutineDispatcher,
    private val api: Api,
) {
    suspend fun load() = withContext(io) { api.fetch() }
}

測試替換策略（@TestInstallIn / @UninstallModules）

// androidTest/.../FakeRepositoryModule.kt
@Module
@TestInstallIn(
    components = [SingletonComponent::class],
    replaces = [RepositoryModule::class],
)
abstract class FakeRepositoryModule {
    @Binds abstract fun fake(impl: FakeUserRepository): UserRepository
}

@HiltAndroidTest
class LoginScreenTest {
    @get:Rule val hiltRule = HiltAndroidRule(this)
    @Inject lateinit var repository: UserRepository

    @Before fun setup() = hiltRule.inject()
}

單一測試需更精細替換時用 @UninstallModules(RealModule::class) + @BindValue：

@UninstallModules(RepositoryModule::class)
@HiltAndroidTest
class CheckoutFlowTest {
    @BindValue val userRepository: UserRepository = mockk(relaxed = true)
}

Module Organization（多模組）

core/network/di/NetworkModule.kt           # OkHttp, Retrofit base
core/network/di/AuthInterceptorModule.kt
core/database/di/DatabaseModule.kt          # Room
core/data/di/RepositoryModule.kt            # Binds Repository impls
core/domain/di/UseCaseModule.kt             # 通常不需要，UseCase 用 constructor inject
feature/checkout/di/CheckoutModule.kt       # feature-local providers
app/di/AppModule.kt                         # Application-level

API/Impl 分離：

core:network:api    -> 介面與 DTO
core:network:impl   -> Retrofit 實作 + Hilt module（@InstallIn(SingletonComponent::class)）
feature:* 只 implementation core:network:api
app implementation core:network:impl

Cross-Skill References

• @techstackmigration：Dagger 2 → Hilt 遷移的唯一權威。本 skill 不重複，遇到遷移問題請載入它。
• @project_bootstrapping：Convention Plugin 套 Hilt + KSP；本 skill 是 Hilt 已就位後的進階。
• @platformmodernization2026：KSP 版本與 Kotlin 版本的對齊。
• @kotlin_multiplatform：KMP 端 commonMain 的 expect/actual + 平台 DI 接合。
• @testinglegacystrategies：Hilt test rule 在舊代碼覆寫的策略。

Quick Checklist

• kapt 完全移除，全用 KSP 2.x
• ViewModel 動態參數一律用 @AssistedInject + hiltViewModel<VM, Factory>
• Custom Component 僅在「真有獨立生命週期」時建立，並有對應的 lifecycle owner
• 跨模組存取走 @EntryPoint 或 core 介面，禁止直接 import 對方 impl
• Qualifier 區分同型別不同實例
• Dispatcher 一律用 @IoDispatcher 等 qualifier 注入，不直接寫 Dispatchers.IO
• 測試用 @TestInstallIn 替換整個 module；單一 case 用 @UninstallModules + @BindValue
• 無 circular dependency；用 Lazy<T> 或 Provider<T> 打破
• Multi-binding 使用 Provider<T> 避免不必要的初始化