android - 如何在依赖于 View 模型的可组合函数中获得预览？

Question

问题描述
我想预览我的HomeScreen我的 HomeScreenPrevieiw 中的可组合函数预览功能。但是，这是不可能的，因为我收到以下错误:

java.lang.IllegalStateException: ViewModels creation is not supported in Preview
    at androidx.compose.ui.tooling.ComposeViewAdapter$FakeViewModelStoreOwner$1.getViewModelStore(ComposeViewAdapter.kt:709)
    at androidx.lifecycle.ViewModelProvider.<init>(ViewModelProvider.kt:105)
    at androidx.lifecycle.viewmodel.compose.ViewModelKt.get(ViewModel.kt:82)
    at androidx.lifecycle.viewmodel.compose.ViewModelKt.viewModel(ViewModel.kt:72)
    at com.example.crud.ui.screens.home.HomeScreenKt.HomeScreen(HomeScreen.kt:53)
    at com.example.crud.ui.screens.home.HomeScreenKt.HomeScreenPreview(HomeScreen.kt:43)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    ...

我的代码
这是我的 HomeScreen代码:

@Composable
fun HomeScreen(
    viewModel: HomeViewModel = hiltViewModel(),
    navigateToDetailsAction: () -> Unit,
    openCardDetailsAction: (Int) -> Unit
) {
    val cities = viewModel.cities.observeAsState(listOf())
    Scaffold(
        topBar = { HomeAppBar() },
        floatingActionButton = { HomeFab(navigateToDetailsAction) }
    ) {
        HomeContent(cities) { id -> openCardDetailsAction(id) }
    }
}

这是我的预览功能的代码:

@Preview
@Composable
private fun HomeScreenPreview() {
    HomeScreen(navigateToDetailsAction = {}, openCardDetailsAction = {})
}

我的 View 模型:

@HiltViewModel
class HomeViewModel @Inject constructor(repository: CityRepository) : ViewModel() {
    val cities: LiveData<List<City>> = repository.allCities.asLiveData()
}

存储库:

@ViewModelScoped
class CityRepository @Inject constructor(appDatabase: AppDatabase) {
    private val dao by lazy { appDatabase.getCityDao() }

    val allCities by lazy { dao.getAllCities() }

    suspend fun addCity(city: City) = dao.insert(city)

    suspend fun updateCity(city: City) = dao.update(city)

    suspend fun deleteCity(city: City) = dao.delete(city)

    suspend fun getCityById(id: Int) = dao.getCityById(id)

}

应用数据库:

@Database(entities = [City::class], version = 2, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
    abstract fun getCityDao() : CityDao
}

我失败的尝试
我认为这可能是 View 模型作为我的 HomeScreen 的默认参数传递的问题。所以我决定这样做:

@Composable
fun HomeScreen(
    navigateToDetailsAction: () -> Unit,
    openCardDetailsAction: (Int) -> Unit
) {
    val viewModel: HomeViewModel = hiltViewModel()
    val cities = viewModel.cities.observeAsState(listOf())
    Scaffold(
        topBar = { HomeAppBar() },
        floatingActionButton = { HomeFab(navigateToDetailsAction) }
    ) {
        HomeContent(cities) { id -> openCardDetailsAction(id) }
    }
}

但它仍然不起作用(我不断收到同样的错误)，而且它不适合测试，因为它会阻止我测试我的 HomeScreen使用模拟 View 模型。

Answer 1

这正是使用默认值传递 View 模型的原因之一。在预览中，您可以传递一个测试对象:

@Preview
@Composable
private fun HomeScreenPreview() {
    val viewModel = HomeViewModel()
    // setup viewModel as you need it to be in the preview
    HomeScreen(viewModel = viewModel, navigateToDetailsAction = {}, openCardDetailsAction = {})
}

由于您有一个存储库，因此您可以执行与测试 View 模型相同的操作。

为 CityRepository 创建接口(interface)

interface CityRepositoryI {
    val allCities: List<City>

    suspend fun addCity(city: City)
    suspend fun updateCity(city: City)
    suspend fun deleteCity(city: City)
    suspend fun getCityById(id: Int)
}

为 CityRepository 实现它:

@ViewModelScoped
class CityRepository @Inject constructor(appDatabase: AppDatabase) : CityRepositoryI {
    private val dao by lazy { appDatabase.getCityDao() }

    override val allCities by lazy { dao.getAllCities() }

    override suspend fun addCity(city: City) = dao.insert(city)

    override suspend fun updateCity(city: City) = dao.update(city)

    override suspend fun deleteCity(city: City) = dao.delete(city)

    override suspend fun getCityById(id: Int) = dao.getCityById(id)
}

创建 FakeCityRepository用于测试目的:

class FakeCityRepository : CityRepositoryI {
    // predefined cities for testing
    val cities = listOf(
        City(1)
    ).toMutableStateList()

    override val allCities by lazy { cities }

    override suspend fun addCity(city: City) {
        cities.add(city)
    }

    override suspend fun updateCity(city: City){
        val index = cities.indexOfFirst { it.id == city.id }
        cities[index] = city
    }

    override suspend fun deleteCity(city: City) {
        cities.removeAll { it.id == city.id }
    }

    override suspend fun getCityById(id: Int) = cities.first { it.id == id }
}

所以你可以将它传递到你的 View 模型中: HomeViewModel(FakeCityRepository())你可以用 AppDatabase 做同样的事情而不是存储库，这完全取决于您的需求。查看更多关于 Hilt testing 的信息
p.s.我不确定这是否会建立，因为我没有你的一些类(class)，但你应该已经明白了。