Hello, I am trying to concat two flows in a way that I get results sequentially when I collect on it...

# coroutines

[Abhishek Bansal]

Hello, I am trying to concat two flows in a way that I get results sequentially when I collect on it

flowOf(
 remoteDataSource.getDataFromCache() // suspending function returning Flow<Data>
  .catch { error -> Timber.e(error) },
 remoteDataSource.getDataFromServer() // suspending function returning Flow<Data>
).flattenConcat().collect {
Timber.i("Response Received")
}

my expectation here was that I will get first result first and then second after sometime as response comes from server. But the problem here is collect only gets called after server returns result.

[louiscad]

Hello @Abhishek Bansal That is expected behavior, because flowOf cannot be called before its parameters are evaluated, which here, suspends. You probably want to use flow { emit(data) } instead.

[Abhishek Bansal]

Hello @louiscad, Thanks for your reply! I understand, since I do not have data directly here whats the best way to concatenate two flows directly? Something that behaves like Observable.concat() ?

[louiscad]

@Abhishek Bansal I'm not familiar with RxJava, but if you just describe in plain English what you want your code to do exactly, I can show you how to do it with coroutines and flows.

[Abhishek Bansal]

ah okay, I wanted to implement cache then network strategy. What I want is a final flow where I get data from cache if and when its available and then data from network if when its available. I do not want to wait for any of these events. I was able to solve it with this code

flow {
   remoteDataSource.getDataFromCache()
   .catch { error -> Timber.e(error) }
   .onCompletion {
       remoteDataSource.getDataFromServer()
            .collect {
                 emit(it)
            }
    }.collect { emit(it) }
}

[Abhishek Bansal]

I wonder if a clean concat like operator available in Flow for this.