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Emitting ui state while collecting does not update ui

This init block is in my ViewModel:
init {
viewModelScope.launch {
userRepository.login()
userRepository.user.collect {
_uiState.value = UiState.Success(it)
}
}
}
This is very similar to what's actually written on the app, but even this simple example doesn't work. After userRepository.login(), user which is a SharedFlow emits a new user state. This latest value DOES get collected within this collect function shown above, but when emitting a new uiState containing the result, the view does not get such update.
val uiState by viewModel.uiState.collectAsStateWithLifecycle()
Doing this for some reason, does not work. I suspect the issue is related to the lifecycle of the viewmodel, because when I treat the viewmodel as a singleton, this doesn't happen. It happens only when the viewmodel gets destroyed and then created a 2nd (or more) time(s).
What I'm trying to achieve is that the screen containing the view model is aware of the user state. Meaning that when I navigate to the screen, I want it to collect the latest user state, and then decide which content to show.
I also realize this is not the best pattern, most likely. I'm currently looking into a solution that holds the User as part of the app state and collecting per screen (given that it basically changes all or many screens and functionalities) so if you have any resources on an example on such implementation I'd be thankful. But I can't get my head around why this current implementation doesn't work so any light shed on the situation is much appreciated.
EDIT
This is what I have in mind for the repository
private val _user = MutableSharedFlow<User>()
override val user: Flow<User> = _user
override suspend fun login() {
delay(2000)
_user.emit(LoggedUser.aLoggedUser())
}
override suspend fun logout() {
delay(2000)
_user.emit(GuestUser)
}

For your case better to use this pattern:
ViewModel class:
sealed interface UserUiState {
object NotLoggedIn : UserUiState
object Error : UserUiState
data class LoggedIn(val user: User) : UserUiState
}
class MyViewModel #Inject constructor(
userRepository: UserRepository
) : ViewModel() {
val userUiState = userRepository.login()
.map { user ->
if (user != null)
UserUiState.LoggedIn(user)
else
UserUiState.Error
}
.stateIn(
scope = viewModelScope,
started = SharingStarted.WhileSubscribed(5_000),
initialValue = UserUiState.NotLoggedIn
)
}
Repository class:
class UserRepository {
fun login(): Flow<User?> = flow {
val user = TODO("Your code to get user")
if (isSuccess) {
emit(user)
} else {
emit(null)
}
}
}
Your screen Composable:
#Composable
fun Screen() {
val userUiState by viewModel.userUiState.collectAsStateWithLifecycle()
when (userUiState) {
is UserUiState.LoggedIn -> { TODO("Success code") }
UserUiState.NotLoggedIn -> { TODO("Waiting for login code") }
UserUiState.Error -> { TODO("Error display code") }
}
}
How it works: login() in repository returns autorized user flow which can be used in ViewModel. I use UserUiState sealed class to handle possible user states. And then I convert User value in map {} to UserUiState to display it in the UI Layer. Then Flow of UserUiState needs to be converted to StateFlow to obtain it from the Composable function, so I made stateIn.
And of course, this will solve your problem
Tell me in the comments if I got something wrong or if the code does not meet your expectations
Note: SharedFlow and StateFlow are not used in the Data Layer like you do.
EDIT:
You can emiting flow like this if you are working with network:
val user = flow of {
while (true) {
// network call to get user
delay(2000)
}
}
If you use Room you can do this in your dao.
#Query(TODO("get actual user query"))
fun getUser(): Flow<User>
It is a better way and it recommended by android developers YouTube channel

one-shot operation with Flow in Android

I'm trying to show a user information in DetailActivity. So, I request a data and get a data for the user from server. but in this case, the return type is Flow<User>. Let me show you the following code.
ServiceApi.kt
#GET("endpoint")
suspend fun getUser(#Query("id") id: Int): Response<User>
Repository.kt
fun getUser(id: Int): Flow<User> = flow<User> {
val userResponse = api.getUser(id = id)
if (userResponse.isSuccessful) {
val user = userResponse.body()
emit(user)
}
}
.flowOn(Dispatchers.IO)
.catch { // send error }
DetailViewModel.kt
class DetailViewModel(
private val repository : Repository
) {
val uiState: StateFlow<User> = repository.getUser(id = 369).stateIn(
scope = viewModelScope,
started = SharingStarted.WhileSubscribed(5000),
initialValue = User() // empty user
)
}
DetailActivity.kt
class DetailActivity: AppCompatActivity() {
....
initObersevers() {
lifecycleScope.launch {
// i used the `flowWithLifecycle` because the data is just a single object.
viewModel.uiState.flowWithLifecycle(lifecycle, Lifecycle.State.STARTED).collect { state ->
// show data
}
}
}
...
}
But, all of sudden, I just realized that this process is just an one-shot operation and thought i can use suspend function and return User in Repository.kt.
So, i changed the Repository.kt.
Repository.kt(changed)
suspend fun getUser(id: Int): User {
val userResponse = api.getUser(id = id)
return if(userResponse.isSuccessful) {
response.body()
} else {
User() // empty user
}
}
And in DetailViewModel, i want to convert the User into StateFlow<User> because of observing from DetailActivity and I'm going to use it the same way as before by using flowWithLifecycle.
the concept is... i thought it's just one single data and i dind't need to use Flow in Repository. because it's not several items like List.
is this way correct or not??

Yeap, this one-time flow doesn't make any sense - it emits only once and that's it.
You've got two different ways. First - is to create a state flow in your repo and emit there any values each time you're doing your GET request. This flow will be exposed to the use case and VM levels. I would say that it leads to more difficult error handling (I'm not fond of this way, but these things are always arguable, haha), but it also has some pros like caching, you can always show/get the previous results.
Second way is to leave your request as a simple suspend function which sends a request, returns the result of it back to your VM (skipping error handling here to be simple):
val userFlow: Flow<User>
get() = _userFlow
private val _userFlow = MutableStateFlow(User())
fun getUser() = launch(viewModelScope) {
_userFlow.value = repository.getUser()
}
This kind of implementation doesn't provide any cache out of scope of this VM's lifecycle, but it's easy to test and use.
So it's not like there is only one "the-coolest-way-to-do-it", it's rather a question what suits you more for your needs.

Is livedata builder ok for one-shot operations?

For example, let's say that we have a product catalog view with an option to add product to a cart.
Each time when user clicks add to cart, a viewModel method addToCart is called, that could look like this:
//inside viewModel
fun addToCart(item:Item): LiveData<Result> = liveData {
val result = repository.addToCart(item) // loadUser is a suspend function.
emit(result)
}
//inside view
addButton.onClickListener = {
viewModel.addToCart(selectedItem).observe (viewLifecycleOwner, Observer () {
result -> //show result
}
}
What happens after adding for example, 5 items -> will there be 5 livedata objects in memory observed by the view?
If yes, when will they be cleanup? And if yes, should we avoid livedata builder for one-shot operations that can be called multiple times?

Your implementation seems wrong! You are constantly returning a new LiveData object for every addToCard function call. About your first question, it's a Yes.
If you want to do it correctly via liveData.
// In ViewModel
private val _result = MutableLiveData<Result>()
val result: LiveData<Result>
get() = _result;
fun addToCart(item: Item) {
viewModelScope.launch {
// Call suspend functions
result.value = ...
}
}
// Activity/Fragment
viewModel.result.observe(lifecycleOwner) { result ->
// Process the result
...
}
viewModel.addToCart(selectedItem)
All you have to do is call it from activity & process the result. You can also use StateFlow for this purpose. It also has an extension asLiveData which converts Flow -> LiveData as well.

According to LiveData implementation of:
public void observe(#NonNull LifecycleOwner owner, #NonNull Observer<? super T> observer) {
assertMainThread("observe");
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
// ignore
return;
}
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && !existing.isAttachedTo(owner)) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
owner.getLifecycle().addObserver(wrapper);
}
a new Observer (wrapper) is added every time you observe a LiveData. Looking at this I would be carefull creating new Observers from a view (click) event. At the moment I can not tell if a Garbage Collector can free this resources.
As #kaustubhpatange mentioned, you should have one LiveData with a state/value that can be changed by the viewModel, with every new result. That LiveData can be observed (once) in your Activity or Fragment onCreate() function:
fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewModel.result.observe(lifecycleOwner) { result ->
// handle the result
}
}
Using MutableLiveData in your ViewModel, you can mostly create LiveData only once, and populate it later with values from click events, responses etc.

TL;DR
If your operation is One-Shot use Coroutine and LiveData.
If your operation serving with Streams you can use Flow.
For one-shot operations, your approach it's OK.
I think with liveData builder there is no any Memory leak.
If you use for example private backing property for LiveData and observe an public LiveData it might occurs different behavior like get latest value before assign new value to that.

ViewModel refetching data again with distinctUntilChanged()

I have a Fragment that I want to do a fetch once on its data, I have used distinctUntilChanged() to fetch just once because my location is not changing during this fragment.
Fragment
private val viewModel by viewModels<LandingViewModel> {
VMLandingFactory(
LandingRepoImpl(
LandingDataSource()
)
)
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val sharedPref = requireContext().getSharedPreferences("LOCATION", Context.MODE_PRIVATE)
val nombre = sharedPref.getString("name", null)
location = name!!
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
setupRecyclerView()
fetchShops(location)
}
private fun fetchShops(localidad: String) {
viewModel.setLocation(location.toLowerCase(Locale.ROOT).trim())
viewModel.fetchShopList
.observe(viewLifecycleOwner, Observer {
when (it) {
is Resource.Loading -> {
showProgress()
}
is Resource.Success -> {
hideProgress()
myAdapter.setItems(it.data)
}
is Resource.Failure -> {
hideProgress()
Toast.makeText(
requireContext(),
"There was an error loading the shops.",
Toast.LENGTH_SHORT
).show()
}
}
})
}
Viewmodel
private val locationQuery = MutableLiveData<String>()
fun setLocation(location: String) {
locationQuery.value = location
}
val fetchShopList = locationQuery.distinctUntilChanged().switchMap { location ->
liveData(viewModelScope.coroutineContext + Dispatchers.IO) {
emit(Resource.Loading())
try{
emit(repo.getShopList(location))
}catch (e:Exception){
emit(Resource.Failure(e))
}
}
}
Now, if I go to the next fragment and press back, this fires again, I know that maybe this is because the fragment is recreating and then passing a new instance of viewmodel and thats why the location is not retained, but if I put activityViewModels as the instance of the viewmodel, it also happends the same, the data is loaded again on backpress, this is not acceptable since going back will get the data each time and this is not server efficient for me, I need to just fetch this data when the user is in this fragment and if they press back to not fetch it again.
Any clues ?
I'm using navigation components, so I cant use .add or do fragment transactions, I want to just fetch once on this fragment when creating it first time and not refetching on backpress of the next fragment

TL;DR
You need to use a LiveData that emits its event only once, even if the ui re-subscribe to it. for more info and explanation and ways to fix, continue reading.
When you go from Fragment 1 -> Fragment 2, Fragment 1 is not actually destroyed right away, it just un-subscribe from your ViewModel LiveData.
Now when you go back from F2 to F1, the fragment will re-subscribe back to ViewModel LiveData, and since the LiveData is - by nature - state holder, then it will re-emit its latest value right away, causing the ui to rebind.
What you need is some sort of LiveData that won't emit an event that has been emitted before.
This is common use case with LiveData, there's a pretty nice article talking about this need for a similar LiveData for different types of use cases, you can read it here.
Although the article proposed a couple of solutions but those can be a bit of an overkill sometimes, so a simpler solution would be using the following ActionLiveView
// First extend the MutableLiveData class
class ActionLiveData<T> : MutableLiveData<T>() {
#MainThread
override fun observe(owner: LifecycleOwner, observer: Observer<T?>) {
// Being strict about the observer numbers is up to you
// I thought it made sense to only allow one to handle the event
if (hasObservers()) {
throw Throwable("Only one observer at a time may subscribe to a ActionLiveData")
}
super.observe(owner, Observer { data ->
// We ignore any null values and early return
if (data == null) return
observer.onChanged(data)
// We set the value to null straight after emitting the change to the observer
value = null
// This means that the state of the data will always be null / non existent
// It will only be available to the observer in its callback and since we do not emit null values
// the observer never receives a null value and any observers resuming do not receive the last event.
// Therefore it only emits to the observer the single action so you are free to show messages over and over again
// Or launch an activity/dialog or anything that should only happen once per action / click :).
})
}
// Just a nicely named method that wraps setting the value
#MainThread
fun sendAction(data: T) {
value = data
}
}
You can find more explainiation for ActionLiveData in this link if you want.
I would advise using the ActionLiveData class, I've been using it for small to medium project size and it's working alright so far, but again, you know your use cases better than me. :)

How to call again LiveData Coroutine Block

I'm using LiveData's version "androidx.lifecycle:lifecycle-livedata-ktx:2.2.0-alpha05". Once my LiveData block executes successfully I want to explicitly trigger it to execute again, e.g.
I navigate to a fragment
User's data loads
I click delete btn while being in the same fragment
User's data should refresh
I have a fragment where I observe my LiveData, a ViewModel with LiveData and Repository:
ViewModel:
fun getUserLiveData() = liveData(Dispatchers.IO) {
val userData = usersRepo.getUser(userId)
emit(userData)
}
Fragment:
viewModel.getUserLiveData.observe(viewLifecycleOwner,
androidx.lifecycle.Observer {..
Then I'm trying to achieve desired behaviour like this:
viewModel.deleteUser()
viewModel.getUserLiveData()
According to the documentation below LiveData block won't execute if it has completed successfully and if I put a while(true) inside the LiveData block, then my data refreshes, however I don't want this to do since I need to update my view reactively.
If the [block] completes successfully or is cancelled due to reasons other than [LiveData]
becoming inactive, it will not be re-executed even after [LiveData] goes through active
inactive cycle.
Perhaps I'm missing something how I can reuse the same LiveDataScope to achieve this? Any help would be appreciated.

To do this with liveData { .. } block you need to define some source of commands and then subscribe to them in a block. Example:
MyViewModel() : ViewModel() {
val commandsChannel = Channel<Command>()
val liveData = livedata {
commandsChannel.consumeEach { command ->
// you could have different kind of commands
//or emit just Unit to notify, that refresh is needed
val newData = getSomeNewData()
emit(newData)
}
}
fun deleteUser() {
.... // delete user
commandsChannel.send(RefreshUsersListCommand)
}
}
Question you should ask yourself: Maybe it would be easier to use ordinary MutableLiveData instead, and mutate its value by yourself?
livedata { ... } builder works well, when you can collect some stream of data (like a Flow / Flowable from Room DB) and not so well for plain, non stream sources, which you need to ask for data by yourself.

I found a solution for this. We can use switchMap to call the LiveDataScope manually.
First, let see the official example for switchMap:
/**
* Here is an example class that holds a typed-in name of a user
* `String` (such as from an `EditText`) in a [MutableLiveData] and
* returns a `LiveData` containing a List of `User` objects for users that have
* that name. It populates that `LiveData` by requerying a repository-pattern object
* each time the typed name changes.
* <p>
* This `ViewModel` would permit the observing UI to update "live" as the user ID text
* changes.
**/
class UserViewModel: AndroidViewModel {
val nameQueryLiveData : MutableLiveData<String> = ...
fun usersWithNameLiveData(): LiveData<List<String>> = nameQueryLiveData.switchMap {
name -> myDataSource.usersWithNameLiveData(name)
}
fun setNameQuery(val name: String) {
this.nameQueryLiveData.value = name;
}
}
The example was very clear. We just need to change nameQueryLiveData to your own type and then combine it with LiveDataScope. Such as:
class UserViewModel: AndroidViewModel {
val _action : MutableLiveData<NetworkAction> = ...
fun usersWithNameLiveData(): LiveData<List<String>> = _action.switchMap {
action -> liveData(Dispatchers.IO){
when (action) {
Init -> {
// first network request or fragment reusing
// check cache or something you saved.
val cache = getCache()
if (cache == null) {
// real fecth data from network
cache = repo.loadData()
}
saveCache(cache)
emit(cache)
}
Reload -> {
val ret = repo.loadData()
saveCache(ret)
emit(ret)
}
}
}
}
// call this in activity, fragment or any view
fun fetchData(ac: NetworkAction) {
this._action.value = ac;
}
sealed class NetworkAction{
object Init:NetworkAction()
object Reload:NetworkAction()
}
}

First add implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0" to your gradle file. Make your ViewModel as follows:
MyViewModel() : ViewModel() {
val userList = MutableLiveData<MutableList<User>>()
fun getUserList() {
viewModelScope.launch {
userList.postValue(usersRepo.getUser(userId))
}
}
}
Then onserve the userList:
viewModel.sessionChartData.observe(viewLifecycleOwner, Observer { users ->
// Do whatever you want with "users" data
})
Make an extension to delete single user from userList and get notified:
fun <T> MutableLiveData<MutableList<T>>.removeItemAt(index: Int) {
if (!this.value.isNullOrEmpty()) {
val oldValue = this.value
oldValue?.removeAt(index)
this.value = oldValue
} else {
this.value = mutableListOf()
}
}
Call that extension function to delete any user and you will be notified in your Observer block after one user get deleted.
viewModel.userList.removeItemAt(5) // Index 5
When you want to get userList from data source just call viewModel.getUserList() You will get data to the observer block.

private val usersLiveData = liveData(Dispatchers.IO) {
val retrievedUsers = MyApplication.moodle.getEnrolledUsersCoroutine(course)
repo.users = retrievedUsers
roles.postValue(repo.findRolesByAll())
emit(retrievedUsers)
}
init {
usersMediator.addSource(usersLiveData){ usersMediator.value = it }
}
fun refreshUsers() {
usersMediator.removeSource(usersLiveData)
usersMediator.addSource(usersLiveData) { usersMediator.value = it }
The commands in liveData block {} doesn't get executed again.
Okay yes, the observer in the viewmodel holding activity get's triggered, but with old data.
No further network call.
Sad. Very sad. "Solution" seemed promisingly and less boilerplaty compared to the other suggestions with Channel and SwitchMap mechanisms.

You can use MediatorLiveData for this.
The following is a gist of how you may be able to achieve this.
class YourViewModel : ViewModel() {
val mediatorLiveData = MediatorLiveData<String>()
private val liveData = liveData<String> { }
init {
mediatorLiveData.addSource(liveData){mediatorLiveData.value = it}
}
fun refresh() {
mediatorLiveData.removeSource(liveData)
mediatorLiveData.addSource(liveData) {mediatorLiveData.value = it}
}
}
Expose mediatorLiveData to your View and observe() the same, call refresh() when your user is deleted and the rest should work as is.