如何在依赖于视图模型的可组合函数中获得预览? [英] How to get preview in composable functions that depend on a view model?

问题描述

我想在我的 HomeScreenPreview 预览函数中预览我的 HomeScreen 可组合函数.但是这是不可能的，因为我收到以下错误:

I would like to have the preview of my HomeScreen composable function in my HomeScreenPrevieiw preview function. However this is not being possible to do because I am getting the following error:

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:

This is the code for my preview function:

@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

我认为作为我的 HomeScreen 的默认参数传递的视图模型可能有问题，所以我决定这样做:

My failed attempt

I thought it might be a problem with the view model being passed as the default parameter of my HomeScreen and so I decided to do it this way:

@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(我一直收到同样的错误)，并且不利于测试，因为它会阻止我使用模拟视图模型测试我的 HomeScreen.

But it still doesn't work (I keep getting the same error), and it's not good for testing as it would prevent me from testing my HomeScreen with a mocked view model.

推荐答案

这正是视图模型传递默认值的原因之一.在预览中，可以传递一个测试对象:

This is exactly one of the reasons why the view model is passed with a default value. In the preview, you can pass a test object:

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

因为你有一个仓库，你可以像测试视图模型一样做同样的事情.

Since you have a repository, you can do the same thing you would do to test the view model.

1. 为CityRepository创建接口

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)
}

1. 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)
}

1. 创建 FakeCityRepository 用于测试目的:

1. Create FakeCityRepository for testing purposes:

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给你的视图模型:HomeViewModel(FakeCityRepository())

So you can pass it into your view model: HomeViewModel(FakeCityRepository())

您可以使用 AppDatabase 而不是存储库来做同样的事情，这完全取决于您的需要.查看有关Hilt 测试

You can do the same with AppDatabase instead of a repository, it all depends on your needs. Check out more about Hilt testing

附言我不确定这是否会成功，因为我没有你的一些课程，但你应该明白这个想法.

p.s. I'm not sure if this will build, since I don't have some of your classes, but you should have caught the idea.

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