2023年Android Hilt注入辅助参数至ViewModel的更优方案问询

在2023年，除了已知的那种方式外，有没有更优或更少样板代码的方案，能将Assisted参数注入到ViewModel的构造方法中？

当前ViewModel实现代码：

class PlantDetailViewModel @AssistedInject constructor(
    plantRepository: PlantRepository,
    @Assisted private val savedStateHandle: SavedStateHandle,
    @Assisted private val plantId: String
) : ViewModel() {

    @AssistedFactory
    interface PlantDetailViewModelFactory {
        fun create(handle: SavedStateHandle, plantId: String): PlantDetailViewModel
    }

    companion object {
        fun provideFactory(
            assistedFactory: PlantDetailViewModelFactory,
            owner: SavedStateRegistryOwner,
            defaultArgs: Bundle? = null,
            plantId: String
        ): AbstractSavedStateViewModelFactory = object : AbstractSavedStateViewModelFactory(owner, defaultArgs) {
            @Suppress("UNCHECKED_CAST")
            override fun <T : ViewModel?> create(key: String, modelClass: Class<T>, handle: SavedStateHandle): T {
                return assistedFactory.create(handle, plantId) as T
            }
        }
    }
}

Fragment中的调用代码：

@AndroidEntryPoint
class PlantDetailFragment : Fragment() {

    private val args: PlantDetailFragmentArgs by navArgs()

    @Inject
    lateinit var plantDetailViewModelFactory: PlantDetailViewModelFactory

    private val plantDetailViewModel: PlantDetailViewModel by viewModels {
        PlantDetailViewModel.provideFactory(plantDetailViewModelFactory, this, arguments, args.plantId)
    }
}

方案1：利用Hilt的@HiltViewModel配合SavedStateHandle传递参数 #

如果项目已经接入Hilt，可以直接借助SavedStateHandle传递Assisted参数，完全省去手动创建工厂类的样板代码：

1. 修改ViewModel：

@HiltViewModel
class PlantDetailViewModel @Inject constructor(
    plantRepository: PlantRepository,
    private val savedStateHandle: SavedStateHandle
) : ViewModel() {
    // 直接从SavedStateHandle中获取plantId
    private val plantId: String = savedStateHandle.get<String>("plantId")!!
}

2. Fragment中调用：

@AndroidEntryPoint
class PlantDetailFragment : Fragment() {
    private val args: PlantDetailFragmentArgs by navArgs()
    
    // 通过默认创建扩展传递参数到SavedStateHandle
    private val plantDetailViewModel: PlantDetailViewModel by hiltViewModel(
        extras = defaultViewModelCreationExtras.apply {
            setSavedStateHandleArgs(mapOf("plantId" to args.plantId))
        }
    )
}

方案2：用Kotlin扩展函数简化工厂创建逻辑 #

如果必须保留@AssistedInject的方式，可以把通用的工厂创建逻辑抽成扩展函数，减少ViewModel内的重复代码：

1. 定义通用扩展函数：

inline fun <reified T : ViewModel> createAssistedSavedStateFactory(
    owner: SavedStateRegistryOwner,
    defaultArgs: Bundle? = null,
    crossinline create: (SavedStateHandle) -> T
): AbstractSavedStateViewModelFactory {
    return object : AbstractSavedStateViewModelFactory(owner, defaultArgs) {
        @Suppress("UNCHECKED_CAST")
        override fun <VM : ViewModel?> create(key: String, modelClass: Class<VM>, handle: SavedStateHandle): VM {
            return create(handle) as VM
        }
    }
}

2. 修改ViewModel（移除companion内的provideFactory方法）：

class PlantDetailViewModel @AssistedInject constructor(
    plantRepository: PlantRepository,
    @Assisted private val savedStateHandle: SavedStateHandle,
    @Assisted private val plantId: String
) : ViewModel() {

    @AssistedFactory
    interface PlantDetailViewModelFactory {
        fun create(handle: SavedStateHandle, plantId: String): PlantDetailViewModel
    }
}

3. Fragment中调用：

@AndroidEntryPoint
class PlantDetailFragment : Fragment() {
    private val args: PlantDetailFragmentArgs by navArgs()
    
    @Inject
    lateinit var plantDetailViewModelFactory: PlantDetailViewModelFactory

    private val plantDetailViewModel: PlantDetailViewModel by viewModels {
        createAssistedSavedStateFactory(this, arguments) { handle ->
            plantDetailViewModelFactory.create(handle, args.plantId)
        }
    }
}

方案3：借助AssistedInject的Kotlin委托库简化调用 #

引入AssistedInject的Kotlin扩展库，可以进一步简化Fragment中的ViewModel初始化：

先添加依赖：

implementation "com.squareup.inject:assisted-inject-ktx:0.7.0"

然后Fragment中直接使用委托：

@AndroidEntryPoint
class PlantDetailFragment : Fragment() {
    private val args: PlantDetailFragmentArgs by navArgs()
    
    @Inject
    lateinit var factory: PlantDetailViewModelFactory

    private val viewModel: PlantDetailViewModel by assistedViewModels {
        factory.create(savedStateHandle, args.plantId)
    }
}

内容的提问来源于stack exchange，提问作者Nexussim Lements