Using a flow builder and asFlow error that should call from suspend function

Issue

I have the following interface to fetch recipes

interface FoodService {
    @GET(EndPoint.COMPLEX_SEARCH)
    suspend fun fetchComplexSearch(@Query("apiKey") apiKey: String): ResultModel
}

This works find using a flow builder

override fun complexSearch(apiKey: String): Flow<List<ComplexSearchEntity>> {
    return flow {
        val resultModel = foodService.fetchComplexSearch(apiKey)

        val listOfComplexSearchEntity = resultModel.results.map { complexSearchModel ->
            mapperDataToDomain.map(complexSearchModel)
        }

        emit(listOfComplexSearchEntity)
    }
}

However, I get an error when I use the extension asFlow

override fun complexSearch(apiKey: String): Flow<List<ComplexSearchEntity>> {
    return foodService.fetchComplexSearch(apiKey).results.map {
        mapperDataToDomain.map(it)
    }.asFlow()
}

Error 1

Required:
Flow<List<ComplexSearchEntity>>
Found:
Flow<ComplexSearchEntity>

Error 2

Suspend function 'fetchComplexSearch' should be called only from a coroutine or another suspend function

Solution

It doesn't do the same thing at all, hence the error.

In the first case, complexSearch immediately returns a cold flow without doing any work. No code from the lambda is executed, so no need to suspend anything. The collector of this cold flow will provide a coroutine environment for the collection and the suspension. Each time the collector asks for a new element, the content of your flow { ... } lambda will proceed until the next emit().

In the second case, you're performing all the work immediately to build a list, and only then convert that list into a flow before returning it. So all your asynchronism happens immediately and needs to be executing in a coroutine. The flow is just a wrapper around the list in this case.

Also, I'm not sure the code is doing what you want, because you're returning a flow of a single element (at least in the first case). This is usually better served by a suspend function rather than a Flow, which also better informs the users that this is a one-shot call. An API returning a Flow of a single element is misleading because the users of this API would likely expect to susbcribe to this flow and get updates.

Answered By - Joffrey