Working with .fold

UsingFold.kt

data class User(val name: String)
data class UserWithInterest(val user: User, val interests: List<String>)

class MyRepository {
    private val interestsMap = mapOf(
        "Sarah" to listOf(
            "Skiing",
            "Bingo"
        ),
        "Tim" to listOf(
            "TV",
            "Video games"
        ),
        "John" to listOf(
            "Cars",
            "Movies",
            "Walking"
        )
    )

    fun fetchUsers(): Flow<List<User>> =
        flowOf(
            listOf(
                User("Sarah"),
                User("Tim"),
                User("John")
            )
        )

    // Simulate an API call
    fun fetchInterests(user: User): Flow<List<String>?> =
        flowOf(interestsMap[user.name])

}

// Version using .fold
class MyFoldUseCase {
    private val repository = MyRepository() // This would normally be injected!

    fun fetchUsers(): Flow<List<UserWithInterest>> =
        repository.fetchUsers()
            .flatMapLatest { users ->
                users.map { user ->
                    repository.fetchInterests(user).map { interests ->
                        user.toUserWithInterests(interests.orEmpty())
                    }
                }.fold(initial = flowOf(emptyList<UserWithInterest>())) { acc, flow ->
                    acc.combine(flow) { list, userWithInterest ->
                        list + userWithInterest
                    }
                }
            }

    private fun User.toUserWithInterests(interests: List<String>) =
        UserWithInterest(this, interests)

}

// Version with Async
class MyAsyncUseCase {
    private val repository = MyRepository() // This would normally be injected!

    fun fetchUsers(): Flow<List<UserWithInterest>> =
        repository.fetchUsers()
            .flatMapLatest { users ->
                flow {
                    coroutineScope {
                        val results = users.map { user ->
                            async {
                                val interests = repository.fetchInterests(user).first()
                                user.toUserWithInterests(interests.orEmpty())
                            }
                        }.awaitAll()
                        emit(results)
                    }
                }
            }

    private fun User.toUserWithInterests(interests: List<String>) =
        UserWithInterest(this, interests)

}