Why isn t there a coroutine Flow livedata builder which retu

Question

Why isn t there a coroutine Flow livedata builder which returns mutableLivedata

Remy Benza · Answer

This would be the old patern ```class MyViewModel ViewModel private val items = MutableLiveData lt List lt Item gt gt val items LiveData lt List lt Item gt gt get = items init viewModelScope launch items value = fetchDataFromNetwork ```

Remy Benza · Answer

With the new coroutines Flow livedata builder you can reduce this to ```val items LiveData lt List lt Item gt gt = liveData emit fetchDataFromNetwork ```

Remy Benza · Answer

But what if I want to add logic inside my viewmodel that alters the value inside the items value

Vishal Choudhary · Answer

I believe you need to use MediatorLiveData for that

Remy Benza · Answer

Can you provide a code snippet that demonstrates this

Faraz · Answer

Just for Clarity What is it you re trying to do do you want to change the livedata value from viewmodel If yes try changing the source of flow using `emitSource` this acts as a `MediatorLiveData addSource` as Vishal said Then something like this should work ```fun getItems changeSource gt LiveData lt List lt Item gt gt = liveData emit fetchDatafromNetwork change source emitSource changeSource ``` But then this changing of source will be invoked from within livedata scope So if you want to change it from outside and want full control you can extend MediatorLiveData and run your coroutine scope inside it

Faraz · Answer

and the last sentence is the reason why you don t have builder which returns mutablelivedata because you can always build one using `MediatorLiveData` which is also used in building the current flow livedata

Remy Benza · Answer

Below my full code I have logic for animations I am fetching tags from a network API I then alter the result inside the viewModel

Remy Benza · Answer

```class OnboardingGetToKnowViewModel ViewModel private val onboardingTags = MutableLiveData lt List lt OnboardingTag gt gt val onboardingTags LiveData lt List lt OnboardingTag gt gt get = onboardingTags private var selectedTags = MutableLiveData lt List lt OnboardingTag gt gt val selectedTags LiveData lt List lt OnboardingTag gt gt get = selectedTags fun addTag tag OnboardingTag val existingList = selectedTags value emptyList val newList = existingList + listOf tag selectedTags value = newList for grid animation purpose change photo index so 1 becomes 3 3 becomes 5 etc val index = onboardingTags value indexOf tag val list = onboardingTags value toMutableList for i in index until onboardingTags value size step 2 val nextItem = list getOrNull i + 2 if nextItem = null list i = nextItem else list removeAt i onboardingTags value = list fun removeTag tag OnboardingTag val existingList = selectedTags value emptyList val newList = existingList listOf tag selectedTags value = newList for grid animation purpose change photo index so it gets added at the top val oldPhotos = onboardingTags value val index = if oldPhotos size 2 == 0 2 else 3 val list = oldPhotos toMutableList for i in index until oldPhotos size step 2 list i = oldPhotos i 2 if oldPhotos size gt 1 list add oldPhotos oldPhotos size 2 list index 2 = tag else list add tag onboardingTags value = list val lastItemTagged = Transformations map selectedTags list gt list lastOrNull val secondLastItemTagged = Transformations map selectedTags list gt list getOrNull list lastIndex 1 val extraItems = Transformations map selectedTags list gt list size 2 init viewModelScope launch onboardingTags value = Services followApi fetchOnboardingTags ```

Zach Klippenstein (he/him) [MOD] · Answer

Seems like your code has a race what if someone calls addTag before the service call returns and then the call returns in middle of addTag executing in between some of the accesses of onboardingTags Or if the call returns after addTag updates the value of onboardingTags and overwrites it

Zach Klippenstein (he/him) [MOD] · Answer

To your original question that would mean the LiveData would have two+ sources of truth which can make code tricky to reason about and result in races like this for example