Hi I m new to use Flow so I confused when to use regular `Fl

Question

Hi I m new to use Flow so I confused when to use regular `Flow` or `StateFlow` Many codes expose `Flow` in repository and convert to `StateFlow` in ViewModels like this ```val userState StateFlow lt User gt = userRepository userFlow stateIn viewModelScope Eagerly null ``` This case always show null state on UI started even if repository has initial value This problem may be solved by exposing `StateFlow` instead of regular `Flow` in repository is it bad practice

Nick Allen · Answer

gt even if repository has initial value Repository data usually comes from a web service or database in which case it takes time to get an initial value so it s not really initial at all What kind of repository do you have such that you think it has an initial value Do you for example want a list to show as empty while data is loading or some similar default value while loading If you changed the UI to show a spinner would you need to make a change in your repository If you need to change the repository in order to change the way data is presented then that is not a great design

gildor · Answer

gt This case always show null state on UI started even if repository has initial value It s implementation detail userFlow can expose StateFlow if it under it s contract to provide default value even if it null The reason why you get null at the first place is because subscription is asynchronous so first it will have null as default value in StateFlow after that it will subscribe on flow and start collecting

gildor · Answer

Nothing wrong with exposing StateFlow but you just should understand that it s a contract of this API now to provide default value you can think about it as about interface interface UserRepository val userFlow Flow lt User gt val currentUser User If it s fine just use StateFlow But not every API have meaningful default value or value at all many are asyncronous so instead exposing something like StateFlow lt T gt it s better to expose Flow lt T gt and let client to convert it to StateFlow is needed with any default value depending on case

Ryosuke Yamada · Answer

Yes in my case default value null is not meaningful value it only means data has not been already fetched But how to avoid meaningless transition in ViewModel and UI without exposing StateFlow With above example UI always shows default value to Flow value transition at opening screen even if data has been already fetched and cached by the repository It may cause bad UX

gildor · Answer

gt But how to avoid meaningless transition in ViewModel and UI without exposing StateFlow This transition is caused by asyncronous nature of subscription There are 2 points which are asyncronous stateIn and subscription on userState In theory if you use Dispatchers Main immediate and do subscription on main thread it should be almost immediate so not exactly because you will still get default value

gildor · Answer

you can mitigate it on UI level using loaders for example But it really depends on UI how to handle it Or you can expose StateFlow after all

Ryosuke Yamada · Answer

Thanks You mean this transition itself is unavoidable but doesn t matter in most case subscribing on default dispatcher is it right I ll watch UI behavior on this transition again and try to improve my UI logic first of all Thank you

gildor · Answer

gt You mean this transition itself is unavoidable When you create StateFlow from Flow and immediately subscribe on it so if eager subscription is slower than subscriber yes it s correct

Ryosuke Yamada · Answer

Thank you