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.
Related
Is this good to put the collect latest inside observe?
viewModel.fetchUserProfileLocal(PreferencesManager(requireContext()).userName!!)
.observe(viewLifecycleOwner) {
if (it != null) {
viewLifecycleOwner.lifecycleScope.launch {
viewLifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED) {
launch {
viewModel.referralDetailsResponse.collect { referralResponseState ->
when (referralResponseState) {
State.Empty -> {
}
is State.Failed -> {
Timber.e("${referralResponseState.message}")
}
State.Loading -> {
Timber.i("LOADING")
}
is State.Success<*> -> {
// ACCESS LIVEDATA RESULT HERE??
}}}}
I'm sure it isn't, my API is called thrice too as the local DB changes, what is the right way to do this?
My ViewModel looks like this where I'm getting user information from local Room DB and referral details response is the API response
private val _referralDetailsResponse = Channel<State>(Channel.BUFFERED)
val referralDetailsResponse = _referralDetailsResponse.receiveAsFlow()
init {
val inviteSlug: String? = savedStateHandle["inviteSlug"]
// Fire invite link
if (inviteSlug != null) {
referralDetail(inviteSlug)
}
}
fun referralDetail(referral: String?) = viewModelScope.launch {
_referralDetailsResponse.send(State.Loading)
when (
val response =
groupsRepositoryImpl.referralDetails(referral)
) {
is ResultWrapper.GenericError -> {
_referralDetailsResponse.send(State.Failed(response.error?.error))
}
ResultWrapper.NetworkError -> {
_referralDetailsResponse.send(State.Failed("Network Error"))
}
is ResultWrapper.Success<*> -> {
_referralDetailsResponse.send(State.Success(response.value))
}
}
}
fun fetchUserProfileLocal(username: String) =
userRepository.getUserLocal(username).asLiveData()
You can combine both streams of data into one stream and use their results. For example we can convert LiveData to Flow, using LiveData.asFlow() extension function, and combine both flows:
combine(
viewModel.fetchUserProfileLocal(PreferencesManager(requireContext()).userName!!).asFlow(),
viewModel.referralDetailsResponse
) { userProfile, referralResponseState ->
...
}.launchIn(viewLifecycleOwner.lifecycleScope)
But it is better to move combining logic to ViewModel class and observe the overall result.
Dependency to use LiveData.asFlow() extension function:
implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.4.0"
it certainly is not a good practice to put a collect inside the observe.
I think what you should do is collect your livedata/flows inside your viewmodel and expose the 'state' of your UI from it with different values or a combined state object using either Flows or Livedata
for example in your first code block I would change it like this
get rid of "userProfile" from your viewmodel
create and expose from your viewmodel to your activity three LiveData/StateFlow objects for your communityFeedPageData, errorMessage, refreshingState
then in your viewmodel, where you would update the "userProfile" update the three new state objects instead
this way you will take the business logic of "what to do in each state" outside from your activity and inside your viewmodel, and your Activity's job will become to only update your UI based on values from your viewmodel
For the specific case of your errorMessage and because you want to show it only once and not re-show it on Activity rotation, consider exposing a hot flow like this:
private val errorMessageChannel = Channel<CharSequence>()
val errorMessageFlow = errorMessageChannel.receiveAsFlow()
What "receiveAsFlow()" does really nicely, is that something emitted to the channel will be collected by one collector only, so a new collector (eg if your activity recreates on a rotation) will not receive the message and your user will not see it again
I'm using Flow to get data from Room then I call Flow.collect in my FragmentLogic class. Inside collect{} I do some work and update the View through an Interface( example: view.updateAdapter(collectData)). This works fine until an onConfigurationChanged is called and the screen rotates, the code inside collect executes and works in Logcat but anything that changes the UI does not work, the function updateAdapter() is called however nothing happens. One solution I found is to call the function beginObservingProductList() again in onStart() if savedinstance is not null but that creates two instances of the collect{} and they both show in logcat.
I want the UI changes to work even after onConfigurationChanged is called.
Room Dao class:
#Query("SELECT * FROM product")
fun observableList(): Flow<List<ProductEntity>>
then in the implementation:
productDao.observableList().map { collection ->
collection.map { entity ->
entity.toProduct
}
}.flowOn(DispatchThread.io())
And finally I collect the data and change the view:
private fun beginObservingProductList() = this.launch {
vModel.liveProductList.map {
mapToSelectionProduct(it)
}.collect {
ui { view.updateAdapter(it) }
if (it.isNotEmpty()) {
filledListState()
} else {
emptyListState()
}
updateCosts()
vModel.firstTime = false
}
}
Flow is not lifecycle aware, you should use LiveData to handle configuration changes.
To use LiveData with Flow,
implement androidx.lifecycle:lifecycle-livedata-ktx:2.2.0, then you can use .asLiveData() extension func.
Repository
fun getList()= productDao.observableList().map { collection ->
collection.map { entity ->
entity.toProduct
}
}.flowOn(DispatchThread.io())
ViewModel
val liveData = repository.getList().asLiveData()
Activity/Fragment
viewModel.liveData.observe(this,Observer{ list->
//do your thing
})
Scenario
Hi,
I have an Activity with a ViewPager. In the ViewPagerAdapter, I create instances of a same fragment with different data.
And in each instance I initialize a ViewModel
val dataViewModelFactory = this.activity?.let { DataViewModelFactory(it) }
mainViewModel = ViewModelProviders.of(this, dataViewModelFactory).get(MainViewModel::class.java)
In my fragment, I observe two MutableLiveData when I call APIs
mainViewModel.isResponseSuccessful.observe(this, Observer { it ->
if(it) {
//do Something
}else{
Toast.makeText(activity, "Error in Sending Request", Toast.LENGTH_SHORT).show()
}
})
mainViewModel.isLoading.observe(this, Observer {
if (it) {
println("show progress")
} else {
println("dismiss progress")
}
})
In each fragment, on a button click I load another fragment. And if required call and API to fetch data.
PROBLEM
The code comes to the observe block multiple times in my fragment. When I comeback from one fragment to previous fragment, even though no API is called, the code on observe block is executed.
What I tried
I tried using an activity instance in the ViewModel initialization
mainViewModel = ViewModelProviders.of(activity,dataViewModelFactory).get(MainViewModel::class.java)
But it did not work.
Please help,
If you want to prevent multiple calls of your observer u can just change MutableLiveData to SingleLiveEvent. Read this
It might help you:
import java.util.concurrent.atomic.AtomicBoolean
class OneTimeEvent<T>(
private val value: T
) {
private val isConsumed = AtomicBoolean(false)
private fun getValue(): T? =
if (isConsumed.compareAndSet(false, true)) value
else null
fun consume(block: (T) -> Unit): T? =
getValue()?.also(block)
}
fun <T> T.toOneTimeEvent() =
OneTimeEvent(this)
First, when you want to post a value on LiveData, use toOneTimeEvent() extension function to wrap it in a OneTimeEvent:
liveData.postValue(yourObject.toOneTimeEvent())
Second, when you are observing on the LiveData, use consume { } function on the delivered value to gain the feature of OneTimeEvent. You'll be sure that the block of consume { } will be executed only once.
viewModel.liveData.observe(this, Observer {
it.consume { yourObject ->
// TODO: do whatever with 'yourObject'
}
})
In this case, when the fragment resumes, your block of code does not execute again.
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. :)
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.