Hilt自定义作用域对象添加Lazy Provider包装报错的解决方法

问题场景 #

在基于Hilt的自定义作用域项目中，尝试给CustomComponent.Builder添加Provider包装实现懒加载时，触发以下Dagger绑定错误：

[Dagger/MissingBinding] dagger.internal.Provider<com.example.testcustomscopes.TestService> cannot be provided without an @Provides-annotated method.
  public abstract static class SingletonC implements MyApplication_GeneratedInjector,

现有可运行的基础代码如下：

interface TestService {
    fun doSomething(): String
}

@CustomScope
class TestServiceImpl @Inject constructor() : TestService {
    override fun doSomething(): String {
        return "TestServiceImpl"
    }
}

@Module
@InstallIn(CustomComponent::class)
object AppModule {

    @CustomScope
    @Provides
    fun provideScopedClass(): TestService {
        return TestServiceImpl()
    }
}

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

@CustomScope
@DefineComponent(parent = SingletonComponent::class)
interface CustomComponent {
    @DefineComponent.Builder
    interface Builder {
        fun build(): CustomComponent
    }
}

@InstallIn(CustomComponent::class)
@EntryPoint
interface CustomEntryPoint {
    fun getCustomDependency(): TestService
}

@Singleton
class CustomComponentManager @Inject constructor(
    private val customComponentBuilder: CustomComponent.Builder
) {
    private var customComponent: CustomComponent? = null

    fun createComponent() {
        customComponent = customComponentBuilder.build()
    }

    fun getComponent(): CustomComponent? {
        return customComponent
    }

    fun clearComponent() {
        customComponent = null
    }
}

@AndroidEntryPoint
class MainActivity : ComponentActivity() {

    @Inject
    lateinit var customComponentManager: CustomComponentManager

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        
        var customDependency: TestService? = null
        val customComponent = customComponentManager.getComponent()
        if (customComponent != null) {
            val entryPoint = EntryPoints.get(customComponent, CustomEntryPoint::class.java)
            customDependency = entryPoint.getCustomDependency()

            println(customDependency.doSomething())
        }
}

问题原因 #

Dagger Hilt不允许跨作用域直接请求子作用域对象的Provider实例：CustomComponentManager属于Singleton作用域，而TestService绑定在子作用域CustomComponent下，直接在父作用域中尝试获取子作用域对象的Provider会导致绑定缺失。同时，直接注入Provider<CustomComponent.Builder>不符合Hilt管理子组件创建的规范。

解决方案 #

1. 用ComponentFactory替代Provider<CustomComponent.Builder> #

Hilt提供ComponentFactory来规范子组件的创建，这是获取子组件Builder的标准方式，替换CustomComponentManager的注入参数：

@Singleton
class CustomComponentManager @Inject constructor(
    private val customComponentFactory: ComponentFactory<CustomComponent.Builder>
) {
    private var customComponent: CustomComponent? = null

    fun createComponent() {
        customComponent = customComponentFactory.create().build()
    }

    fun getComponent(): CustomComponent? {
        return customComponent
    }

    fun clearComponent() {
        customComponent = null
    }
}

2. 按需为TestService添加懒加载（可选） #

如果需要延迟TestService的实例化，修改CustomEntryPoint返回Provider<TestService>：

@InstallIn(CustomComponent::class)
@EntryPoint
interface CustomEntryPoint {
    fun getCustomDependency(): Provider<TestService>
}

使用时按需获取实例：

val customComponent = customComponentManager.getComponent()
if (customComponent != null) {
    val entryPoint = EntryPoints.get(customComponent, CustomEntryPoint::class.java)
    val testServiceProvider = entryPoint.getCustomDependency()
    // 需要时才创建实例
    val customDependency = testServiceProvider.get()
    println(customDependency.doSomething())
}

3. 移除冗余的Provider方法（优化） #

TestServiceImpl已经有@Inject构造函数，AppModule中的provideScopedClass方法属于冗余代码，可以直接删除，Hilt会自动通过构造函数注入TestServiceImpl：

@Module
@InstallIn(CustomComponent::class)
object AppModule {
    // 移除provideScopedClass方法
}

内容的提问来源于stack exchange，提问作者Rowan Gontier