Kotlin Flow:emitAll 永远不会被收集 [英] Kotlin Flow: emitAll is never collected

问题描述

我正在尝试为 networkBoundResource 的 kotlin 版本编写一个单元测试，该版本可以在 serveral 上找到来源具有多项功能

I am trying to write a UnitTest for the kotlin-version of networkBoundResource that can be found on serveral sources with several features

这是我对以下问题的标记评论版本.

Here is my version of it with marker-comments for the following question.

inline fun <ResultType, RequestType> networkBoundResource(
    ...
    coroutineDispatcher: CoroutineDispatcher
) = flow {

    emit(Resource.loading(null)) // emit works!

    val data = queryDatabase().firstOrNull()

    val flow = if (shouldFetch(data)) {
        
        emit(Resource.loading(data)) // emit works!

        try {
            saveFetchResult(fetch())
            query().map { Resource.success(it) }
            
        } catch (throwable: Throwable) {
            onFetchFailed(throwable)
            query().map { Resource.error(throwable.toString(), it) }
            
        }
    } else {
        query().map { Resource.success(it) }
    }

    emitAll(flow) // emitAll does not work!

}.catch { exception ->
    emit(Resource.error("An error occurred while fetching data! $exception", null))

}.flowOn(coroutineDispatcher)

这是我对此代码的单元测试之一.对代码进行了一些编辑以专注于我的问题:

This is one of my UnitTests for this code. The code is edited a bit to focus on my question:

@get:Rule
val testCoroutineRule = TestCoroutineRule()

private val coroutineDispatcher = TestCoroutineDispatcher()

@Test
fun networkBoundResource_noCachedData_shouldMakeNetworkCallAndStoreUserInDatabase() = testCoroutineRule.runBlockingTest {
    ...

    // When getAuthToken is called
    val result = networkBoundResource(..., coroutineDispatcher).toList()

    result.forEach {
        println(it)
    }    
}

问题是 println(it) 只打印 Resource.loading(null) 排放.但是，如果您查看 flow {} 块的最后一行，您会发现应该有另一个 val flow 的发射.但是这种排放从未到达我的 UnitTest.为什么?

The problem is that println(it) is only printing the Resource.loading(null) emissions. But if you have a look at the last line of the flow {} block, you will see that there should be another emission of the val flow. But this emission never arrives in my UnitTest. Why?

推荐答案

正如@MarkKeen 所建议的，我现在创建了自己的实现，并且效果很好.与 SO 上的代码相比，这个版本现在注入了 coroutineDispatcher 以便于测试，它让流负责错误处理，它不包含嵌套流，而且更易于阅读和理解.将更新的数据存储到数据库中仍然存在副作用，但我现在太累了，无法解决这个问题.

As @MarkKeen suggested, I now created my own implementation and it works quite well. Compared to the code that is going around on SO, this version now injects the coroutineDispatcher for easier testing, it lets flow take care of error handling, it does not contain nested flows and is imho easier to read and understand, too. There is still the side-effect of storing updated data to the database, but I am too tired now to tackle this.

import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.flow.*

inline fun <ResultType, RequestType> networkBoundResource(
    crossinline query: () -> Flow<ResultType?>,
    crossinline fetch: suspend () -> RequestType,
    crossinline saveFetchResult: suspend (RequestType) -> Unit,
    crossinline shouldFetch: (ResultType?) -> Boolean = { true },
    coroutineDispatcher: CoroutineDispatcher
) = flow<Resource<ResultType>> {
    
    // check for data in database
    val data = query().firstOrNull()
    
    if (data != null) {
        // data is not null -> update loading status
        emit(Resource.loading(data))
    }
    
    if (shouldFetch(data)) {
        // Need to fetch data -> call backend
        val fetchResult = fetch()
        // got data from backend, store it in database
        saveFetchResult(fetchResult)        
    }

    // load updated data from database (must not return null anymore)
    val updatedData = query().first()

    // emit updated data
    emit(Resource.success(updatedData))    
    
}.onStart { 
    emit(Resource.loading(null))
    
}.catch { exception ->
    emit(Resource.error("An error occurred while fetching data! $exception", null))
    
}.flowOn(coroutineDispatcher)

此内联乐趣的一个可能的单元测试，用于 AuthRepsiory:

One possible UnitTest for this inline fun, which is used in an AuthRepsitory:

@ExperimentalCoroutinesApi
class AuthRepositoryTest {

    companion object {
        const val FAKE_ID_TOKEN = "FAkE_ID_TOKEN"
    }

    @get:Rule
    val testCoroutineRule = TestCoroutineRule()

    private val coroutineDispatcher = TestCoroutineDispatcher()

    private val userDaoFake = spyk<UserDaoFake>()

    private val mockApiService = mockk<MyApi>()

    private val sut = AuthRepository(
        userDaoFake, mockApiService, coroutineDispatcher
    )

    @Before
    fun beforeEachTest() {
        userDaoFake.clear()
    }

    @Test
    fun getAuthToken_noCachedData_shouldMakeNetworkCallAndStoreUserInDatabase() = testCoroutineRule.runBlockingTest {
        // Given an empty database
        coEvery { mockApiService.getUser(any()) } returns NetworkResponse.Success(UserFakes.getNetworkUser(), null, HttpURLConnection.HTTP_OK)

        // When getAuthToken is called
        val result = sut.getAuthToken(FAKE_ID_TOKEN).toList()

        coVerifyOrder {
            // Then first try to fetch data from the DB
            userDaoFake.get()

            // Then fetch the User from the API
            mockApiService.getUser(FAKE_ID_TOKEN)

            // Then insert the user into the DB
            userDaoFake.insert(any())

            // Finally return the inserted user from the DB
            userDaoFake.get()
        }
        
        assertThat(result).containsExactly(
            Resource.loading(null),
            Resource.success(UserFakes.getAppUser())
        ).inOrder()
    }
}

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