如何在依赖于视图模型的可组合函数中进行预览？

问题描述

我想要预览我的HomeScreen我的可组合函数HomeScreenPrevieiw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)
    ...

My code

这是我的HomeScreen code:

@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 = {})
}

我的视图模型：

@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)

}

应用程序数据库：

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

我失败的尝试

我认为这可能是视图模型作为我的默认参数传递的问题HomeScreen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（我不断收到相同的错误），并且它不利于测试，因为它会阻止我测试我的HomeScreen带有模拟视图模型。

这正是视图模型传递默认值的原因之一。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视图模型相同的操作。

1. 创建接口用​​于CityRepository

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)
}

2. 实施它是为了CityRepository:

@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)
}

3. Create FakeCityRepository出于测试目的：

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视图模型中：HomeViewModel(FakeCityRepository())

你可以用同样的方法AppDatabase而不是存储库，这完全取决于您的需求。查看更多有关刀柄测试 https://developer.android.com/training/dependency-injection/hilt-testing

附注我不确定这是否会建立，因为我没有你的一些课程，但你应该已经明白了这个想法。