Hi ! I have a design problem with `Flow`. I have a source flow the emits the current result of a req...

# coroutines

[tseisel]

Hi ! I have a design problem with Flow. I have a source flow the emits the current result of a request in database whenever it changes (like a Flow returned by Android Room). I decided to expose the result of those request with a repository.

interface DataSource {
    val flow: Flow<List<Foo>>
}

class Repository(
    private val source: DataSource
)
    val diff: Flow<Diff<Foo>> get() = TODO()
    suspend fun getLatest(): List<Foo> = TODO()
}

How can I transform the source flow to adapt it to the needs of the Repository ? (The Diff is calculated from the latest and the previous list of `Foo`s.)
Is it possible to cache the latest value just like Android LiveData, to avoid costly database queries when repeateadly reading the latest values ?

[octylFractal]

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/conflate.html ?

[tseisel]

The conflate operator may be part of the solution, but it does not cache the latest value, each call to getLatest() still triggers a database query

[gildor]

There is no direct Flwo analogue of BehaviorSubject/LiveData. It in development now (there is a PR, search for DataFlow)

But you can use ConflatedBroadcastChannel in coroutines, there is asFlow() extension, so you can expose such channel as Flow

[tseisel]

Good to know ! I'm very enthousiastic about that DataFlow . @gildor Do you know how it behaves in case of an exception in the upstream ?
In my case, the source flow throws a specific exception if some permission is not denied. Will DataFlow rethrow that exception or re-collect the upstream ? I'd really like option 2.

[gildor]

But DataFlow or ConflatedBroadcastChannel do not have upstream, that is the whole point

[gildor]

Exactly the same as MutableLiveData

[gildor]

They are observable data holders