Kotlin Flow in repository pattern

Question

I would like to use a Flow as a return type for all functions in my repository. For ex:

suspend fun create(item:T): Flow<Result<T>>

This function should call 2 data sources: remote(to save data on the server) and local(to save returned data from the server locally). The question is how I can implement this scenario:

try to save data with RemoteDataSource
if 1. fails - try it N times with M timeout
if data has finally returned from the server - same them locally with LocalDataSource
return flow with locally saved data

RemoteDataSource and LocalDataSource both have fun create with the same signature:

suspend fun create(item:T): Flow<Result<T>>

So they both return flow of data. If you have any ideas about how to implement it, I will be grateful.

------ Update #1 ------

a part of a possible solution:

suspend fun create(item:T): Flow<T> {
 // save item remotely
 return remoteDataSource.create(item)
  // todo: call retry if fails
  // save to local a merge two flows in one
  .flatMapConcat { remoteData ->
   localDataSource.create(remoteData)
  }
  .map {
   // other mapping
  }
}

Is it a working idea?

CyrilFind CyrilFind · Accepted Answer · 2020-07-01T13:49:57

I think you have the right idea but you are trying to do everything at once.

What I found works best (and easily) is to have:

an exposed flow of data coming from your local datasource (easy with Room)
one or more exposed suspend functions like create or refresh that operate on the remote data source and save to the local one (if there is no error)

For ex I have a repository that fetches vehicles in my project (the isCurrent info is only local and isLeft/isRight is because I use Either but any error handling applies):

class VehicleRepositoryImpl(
    private val localDataSource: LocalVehiclesDataSource,
    private val remoteDataSource: RemoteVehiclesDataSource
) : VehicleRepository {

    override val vehiclesFlow = localDataSource.vehicleListFlow
    override val currentVehicleFlow = localDataSource.currentVehicleFLow

    override suspend fun refresh() {
        remoteDataSource.getVehicles()
            .fold(
                ifLeft = { /* handle errors, retry, ... */ },
                ifRight = { reset(it) }
            )
    }

    private suspend fun reset(vehicles: List<VehicleEntity>) {
        val current = currentVehicleFlow.first()
        localDataSource.reset(vehicles)
        if (current != null) localDataSource.setCurrentVehicle(current)
    }

    override suspend fun setCurrentVehicle(vehicle: VehicleEntity) =
        localDataSource.setCurrentVehicle(vehicle)

    override suspend fun clear() = localDataSource.clear()
}

Hope this helps and you can adapt it to your case :)

Kotlin Flow in repository pattern

1 Answers