Based on a list of ids I need to retrieve a list of users wi

Question

Based on a list of ids I need to retrieve a list of users with detailed informaiton of each user Below a simplification of my models and API ``` data class UserDTO val id String data class PetDTO val name String interface Api suspend fun users List lt UserDTO gt suspend fun pets userId String List lt PetDTO gt data class User val id String val pets List lt Pet gt data class Pet val name String class UsersRepository api Api val users Flow lt List lt User gt gt = TODO ``` In RxJava I would do something like this ``` val users Observable lt List lt User gt gt = api users flatMapIterable it concatMap userDto gt api pets userDto id map petsListDto gt User userDto id petsListDto map Pet it name toList ``` How can I implement `UsersRepository` and return the list of `User`s using Kotlin Flow

Nick Allen · Answer

Wrote and answer but actually I m a little confused why is UsersRespository users returning a Flow The underlaying API doesn t use any Flow

Nick Allen · Answer

Do you actually just want a suspend function ```class UsersRepository api Api suspend fun users List lt User gt = api users map userDto gt User userDto id api pets userDto id map petDto gt petDto name ```

Nick Allen · Answer

If you would use a Single or Completable in RxJava you just use a suspend method in kotlin coroutines

Nick Allen · Answer

For the RxJava snippet you wrote ```val users Observable lt List lt User gt gt ``` but I suspect you meant ```val users Single lt List lt User gt gt ``` based on the Api interface and based on you ending the expression with toList which returns a Single

Diego · Answer

The repository returns Flow because behind scenes I have to do polling to keep sending real time updates

Diego · Answer

I ended up writing the solution below it works but I m not sure if there is a better way of doing and I m saying this because it would be better if I can send requests in parallel to make everything faster

Diego · Answer

```class UsersRepository api Api val users Flow lt List lt User gt gt = flow val usersWithPets = api users map userDto gt val pets = api pets userDto id map petsListDto gt Pet petsListDto name User userDto id pets emit usersWithPets ```

gts13 · Answer

imo since your API doesn t return flow then your suggested solution is definitely fine But in case you also have a Room database and you first store the retrieved lists of pets and users then you could write some query that return the flow you need

Nick Allen · Answer

You only emit one item In this case I d really recommend just using a suspend method not a `Flow` You wouldn t return a `List` if you only have one result in a synchronous method so why would you return a `Flow` here If you then want to bring it into a `Flow` expression you can do `repository users asFlow `

Nick Allen · Answer

If you want to do tasks at the same time you need to use a method like `launch` or `async` Here s your code adapted to use async ```class UsersRepository api Api val users Flow lt List lt User gt gt = flow val usersWithPets = coroutineScope val userIdsWithPetsTasks = api users map userDto gt val petsTask = async api pets userDto id map petsListDto gt Pet petsListDto name userDto id to petsTask userIdsWithPetsTasks map userId petsTask gt User userId pets await emit usersWithPets ``` The `api users map ` starts a task to get the pets for each user `userIdsWithPetsTasks map ` waits for the results for each user Be careful how many `api pets` calls you are making at the same time if your underlying api doesn t limit concurrency somehow for you You can limit concurrency with a Semaphore or using <https kotlinlang org docs channels html fan out|Fan out> Side note `emit` is outside `coroutineScope` purposely you need `channelFlow ` to emit from a coroutine other that the one the lambda is running in

Diego · Answer

Thank you guys