StateFlow .combine usage with .asLiveData

I’m using StateFlow under the hood, but still exposing LiveData in my API. I’m currently experiencing some unexpected behavior with regard to calling combine to join two instances of MutableStateFlow, and subsequently using the combined value as LiveData.

    private val _read = MutableStateFlow<ChannelUserRead?>(null)
    private val _messages = MutableStateFlow<Map<String, Message>>(emptyMap())

    ...

    private val _unreadCount: StateFlow<Int?> =
        _read.combine(_messages) { channelUserRead: ChannelUserRead?, messagesMap: Map<String, Message> ->
            computeUnreadCount(domainImpl.currentUser, channelUserRead, messagesMap.values.toList())
        }.stateIn(domainImpl.scope, SharingStarted.Eagerly, 0)

    override val unreadCount: LiveData<Int?> = _unreadCount.asLiveData()

I would expect this to run the combine transformation any time _read or _messages value changes, and also notify any observers of the unreadCount. This seems to work in the acutal app, but previously passing tests now fail when trying to validate unreadCount.value. It seems that, when under test, the combine transformation is called once on instantiation, and not when _messages.value is updated, and so

            channelController.unreadCount.getOrAwaitValue().let { unreadCount ->
                Truth.assertThat(unreadCount).isEqualTo(1)
            }

always fails with actual being 0.

[elizarov]

It is not expected, but it may, indeed, change the behavior, just like putting delay makes it work in this particular test case. This is definitely not a general solution and does not guarantee anything if you write a longer test. Data propagation through a flow is, in general, asynchronous. On the other hand, putting advanceUntilIdle() after updates guarantees that all data propagation had finished and you can safely assert the derived values.

[zsmb13]

Makes sense, thanks for the explanations! I think we can close this 😄