how can I avoid the collect{} code execute again when navigate back to the fragment.
ViewModel class
private val _commitResult = MutableStateFlow<Map<String, Any>>(mapOf())
val commitResult: StateFlow<Map<String, Any>> = _commitResult
Fragment code like this:
viewLifecycleOwner.lifecycleScope.launch {
viewLifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED){
viewModel.commitResult.collect { data ->
Logger.i("commitResult $data")
//navigate to another fragment
}
}
}
when I change the _commitResult value in viewModel first, jump to another fragment works fine.
unfortunately, when I go back to the fragment. collect{ // navigate to another fragment} will
excute again.
I know when back to the fragment. onCreateView excute again and viewModel will emit the data store
before, so thecollect { // navigate to another fragment} excute. How can I avoid this?
same as LiveData, I use Event to fix this with LiveData.
open class Event<out T>(private val content: T) {
var hasBeenHandled = false
private set // Allow external read but not write
/**
* Returns the content and prevents its use again.
*/
fun getContentIfNotHandled(): T? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
/**
* Returns the content, even if it's already been handled.
*/
fun peekContent(): T = content
}
how can I handle this with stateflow? actually I don't like Event<.> to handle this,
am I use the stateflow in a wrong way? how I can fix this?
If anyone who can help, thanks in advance.
StateFlow keeps it's state, so I'd suggest either:
A) Use SharedFlow. https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-shared-flow/
B) Use a pattern where you handle the dismissal of events
class Vm: ViewModel() {
private val mEvent = MutableStateFlow<MyResult?>(null)
val event = mEvent.asStateFlow()
fun dismissEvent() {
mEvent.value = null
}
}
class Frag: Fragment() {
override fun onViewCreated() {
vm.event.collect {
navigate()
vm.dismissEvent()
}
}
}
Related
I am using a shared view model and it is shared across two fragments. Both the fragments are listening for one live data and also handling it in the following way to consume it only once
fun getContentIfNotHandled(): StateData<T>? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
return this
}
}
Now since I am observing one live data in two fragments, one of them is consuming the live data and the other one is getting a null value.
Not sure if this is your exact use case, but in my case, I had multiple fragments in a viewpager subscribing to a livedata Event. To ensure their are no conflicts, here is how I ensured that the fragment I wanted was consuming the correct event.
Using:
class ConditionalEventObserver<T>(
private val shouldConsumeEvent: (T) -> Boolean,
private val consumeEvent: (T) -> Unit
) : Observer<Event<T>> {
override fun onChanged(event: Event<T>) {
if (!shouldConsumeEvent.invoke(event.peekContent())) return
event.consumeContentIfAvailable()?.let { value ->
consumeEvent(value)
}
}
}
Event class:
open class Event<out T>(private val content: T) {
private var hasBeenHandled = false
fun consumeContentIfAvailable(): T? =
if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
fun peekContent(): T = content
}
Assuming the event wraps an object like this for example
data class EventPayload(
val pageIndex: Int,
//Other attributes
)
Inside viewmodel:
val sharedEvent = MutableLiveData<Event<EventPayload>>()
In onViewCreated in each fragment:
viewModel.sharedEvent.observe(viewLifecycleOwner, conditionallyConsumeEvent())
private fun conditionallyConsumeEvent(): ConditionalEventObserver<ViewModel.SharedEvent> =
ConditionalEventObserver({ eventPayload ->
//Add your condition here, you can embed data in the Event Payload
eventPayload.pageIndex == THIS_FRAGMENT_PAGE_INDEX // In my case, defined in my viewpager callback
}) { eventPayload ->
handleEventInThisFragment(eventPayload)
}
Not entirely sure if this is the best approach, but it is how I did it.
If on the other hand, you wish to simply read the value without consuming it from your fragments, then just peaking content (peekContent) on the event will work. But if you expect to consume the event more than once and trigger fragment callbacks from that, then you shouldn't be using an Event & EventObserver.
Hope that helps!
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 usecase:
Open app + disable network -> display error
Exit app, then enable network, then open app again
Expected: app load data
Actual: app display error that meaning state error cached, liveData is not emit
Repository class
class CategoryRepository(
private val api: ApiService,
private val dao: CategoryDao
) {
val categories: LiveData<Resource<List<Category>>> = liveData {
emit(Resource.loading(null))
try {
val data = api.getCategories().result
dao.insert(data)
emit(Resource.success(data))
} catch (e: Exception) {
val data = dao.getCategories().value
if (!data.isNullOrEmpty()) {
emit(Resource.success(data))
} else {
val ex = handleException(e)
emit(Resource.error(ex, null))
}
}
}
}
ViewModel class
class CategoryListViewModel(
private val repository: CategoryRepository
): ViewModel() {
val categories = repository.categories
}
Fragment class where LiveDate obsever
viewModel.apply {
categories.observe(viewLifecycleOwner, Observer {
// live data only trigger first time, when exit app then open again, live data not trigger
})
}
can you help me explain why live data not trigger in this usecase and how to fix? Thankyou so much
Update
I have resolved the above problem by replace val categories by func categories() at repository class. However, I don't understand and can't explain why it works properly with func but not val.
Why does this happen? This happens because your ViewModel has not been killed yet. The ViewModel on cleared() is called when the Fragment is destroyed. In your case your app is not killed and LiveData would just emit the latest event already set. I don't think this is a case to use liveData builder. Just execute the method in the ViewModel when your Fragment gets in onResume():
override fun onResume(){
viewModel.checkData()
super.onResume()
}
// in the viewmodel
fun checkData(){
_yourMutableLiveData.value = Resource.loading(null)
try {
val data = repository.getCategories()
repository.insert(data)
_yourMutableLiveData.value = Resource.success(data)
} catch (e: Exception) {
val data = repository.getCategories()
if (!data.isNullOrEmpty()) {
_yourMutableLiveData.value = Resource.success(data)
} else {
val ex = handleException(e)
_yourMutableLiveData.value = Resource.error(ex,null)
}
}
}
Not sure if that would work, but you can try to add the listener directly in onResume() but careful with the instantiation of the ViewModel.
Small advice, if you don't need a value like in Resource.loading(null) just use a sealed class with object
UPDATE
Regarding your question that you ask why it works with a function and not with a variable, if you call that method in onResume it will get executed again. That's the difference. Check the Fragment or Activity lifecycle before jumping to the ViewModel stuff.
I am using MVVM in my app. When you enter a query and click search button, the chain is as follows: Fragment -> ViewModel -> Repository -> API -> Client. The client is where HTTP requests are made. But there is one thing here, the client needs to make a call and get a key from the server at initialization. Therefore, to prevent any call before it this first call completes, I need to be able to observe it from Fragment so that I can disable search button. Since each component in the chain can communicate with adjacent components, all components should have a state.
I am thinking to implement a StatefulComponent class and make all components to extend it:
open class StatefulComponent protected constructor() {
enum class State {
CREATED, LOADING, LOADED, FAILED
}
private val currentState = MutableLiveData(State.CREATED)
fun setState(newState: State) {
currentState.value = newState
}
val state: LiveData<State> = currentState
val isLoaded: Boolean = currentState.value == State.LOADED
val isFailed: Boolean = currentState.value == State.FAILED
val isCompleted: Boolean = isLoaded || isFailed
}
The idea is that each component observers the next one and updates itself accordingly. However, this is not possible for ViewModel since it is already extending ViewModel super class.
How can I implement a solution for this problem?
The most common approach is to use sealed class as your state, so you have any paramaters as you want on each state case.
sealed class MyState {
object Loading : MyState()
data class Loaded(data: Data) : MyState()
data class Failed(message: String) : MyState()
}
On your viewmodel you will have only 1 livedata
class MyViewModel : ViewModel() {
private val _state = MutableLiveData<MyState>()
val state: LiveData<MyState> = _state
fun load() {
_state.postCall(Loading)
repo.loadSomeData(onData = { data ->
_state.postCall(Loaded(data))
}, onError = { error -> _state.postCall(Failed(error.message)) })
}
// coroutines approach
suspend fun loadSuspend() {
_state.postCall(Loading)
try {
_state.postCall(Loaded(repo.loadSomeDataSupend()))
} catch(e: Exception) {
_state.postCall(Failed(e.message))
}
}
}
And on the fragment, just observe the state
class MyFragment : Fragment() {
...
onViewCreated() {
viewModel.state.observer(Observer {
when (state) {
// auto casts to each state
Loading -> { button.isEnabled = false }
is Loaded -> { ... }
is Failed -> { ... }
}
}
)
}
}
As João Gouveia mentioned, we can make stateful components quite easily using kotlin's sealed classes.
But to make it further more useful, we can introduce Generics! So, our state class becomes StatefulData<T> which you can use pretty much anywhere (LiveData, Flows, or even in Callbacks).
sealed class StatefulData<out T : Any> {
data class Success<T : Any>(val result : T) : StatefulData<T>()
data class Error(val msg : String) : StatefulData<Nothing>()
object Loading : StatefulData<Nothing>()
}
I've wrote an article fully explaining this particular implementation here
https://naingaungluu.medium.com/stateful-data-on-android-with-sealed-classes-and-kotlin-flow-33e2537ccf55
If you are using the composable ... You can use produce state
#Composable
fun PokemonDetailScreen(
viewModel: PokemonDetailVm = hiltViewModel()
) {
/**
* This takes a initial state and with that we get a coroutine scope where we can call a API and assign the data into the value
*/
val pokemonInfo = produceState<Resource<Pokemon>>(initialValue = Resource.Loading()) {
value = viewModel.getPokemonInfo(pokemonName)
}.value
}
What is a proper way to communicate between the ViewModel and the View, Google architecture components give use LiveData in which the view subscribes to the changes and update itself accordingly, but this communication not suitable for single events, for example show message, show progress, hide progress etc.
There are some hacks like SingleLiveEvent in Googles example but it work only for 1 observer.
Some developers using EventBus but i think it can quickly get out of control when the project grows.
Is there a convenience and correct way to implement it, how do you implement it?
(Java examples welcome too)
Yeah I agree, SingleLiveEvent is a hacky solution and EventBus (in my experience) always lead to trouble.
I found a class called ConsumableValue a while back when reading the Google CodeLabs for Kotlin Coroutines, and I found it to be a good, clean solution that has served me well (ConsumableValue.kt):
class ConsumableValue<T>(private val data: T) {
private var consumed = false
/**
* Process this event, will only be called once
*/
#UiThread
fun handle(block: ConsumableValue<T>.(T) -> Unit) {
val wasConsumed = consumed
consumed = true
if (!wasConsumed) {
this.block(data)
}
}
/**
* Inside a handle lambda, you may call this if you discover that you cannot handle
* the event right now. It will mark the event as available to be handled by another handler.
*/
#UiThread
fun ConsumableValue<T>.markUnhandled() {
consumed = false
}
}
class MyViewModel : ViewModel {
private val _oneShotEvent = MutableLiveData<ConsumableValue<String>>()
val oneShotEvent: LiveData<ConsumableValue<String>>() = _oneShotData
fun fireEvent(msg: String) {
_oneShotEvent.value = ConsumableValue(msg)
}
}
// In Fragment or Activity
viewModel.oneShotEvent.observe(this, Observer { value ->
value?.handle { Log("TAG", "Message:$it")}
})
In short, the handle {...} block will only be called once, so there's no need for clearing the value if you return to a screen.
What about using Kotlin Flow?
I do not believe they have the same behavior that LiveData has where it would alway give you the latest value. Its just a subscription similar to the workaround SingleLiveEvent for LiveData.
Here is a video explaining the difference that I think you will find interesting and answer your questions
https://youtu.be/B8ppnjGPAGE?t=535
try this:
/**
* Used as a wrapper for data that is exposed via a LiveData that represents an event.
*/
open class Event<out T>(private val content: T) {
var hasBeenHandled = false
private set // Allow external read but not write
/**
* Returns the content and prevents its use again.
*/
fun getContentIfNotHandled(): T? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
/**
* Returns the content, even if it's already been handled.
*/
fun peekContent(): T = content
}
And wrapper it into LiveData
class ListViewModel : ViewModel {
private val _navigateToDetails = MutableLiveData<Event<String>>()
val navigateToDetails : LiveData<Event<String>>
get() = _navigateToDetails
fun userClicksOnButton(itemId: String) {
_navigateToDetails.value = Event(itemId) // Trigger the event by setting a new Event as a new value
}
}
And observe
myViewModel.navigateToDetails.observe(this, Observer {
it.getContentIfNotHandled()?.let { // Only proceed if the event has never been handled
startActivity(DetailsActivity...)
}
})
link reference: Use an Event wrapper
For showing/hiding progress dialogs and showing error messages from a failed network call on loading of the screen, you can use a wrapper that encapsulates the LiveData that the View is observing.
Details about this method are in the addendum to app architecture:
https://developer.android.com/jetpack/docs/guide#addendum
Define a Resource:
data class Resource<out T> constructor(
val state: ResourceState,
val data: T? = null,
val message: String? = null
)
And a ResourceState:
sealed class ResourceState {
object LOADING : ResourceState()
object SUCCESS : ResourceState()
object ERROR : ResourceState()
}
In the ViewModel, define your LiveData with the model wrapped in a Resource:
val exampleLiveData = MutableLiveData<Resource<ExampleModel>>()
Also in the ViewModel, define the method that makes the API call to load the data for the current screen:
fun loadDataForView() = compositeDisposable.add(
exampleUseCase.exampleApiCall()
.doOnSubscribe {
exampleLiveData.setLoading()
}
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{
exampleLiveData.setSuccess(it)
},
{
exampleLiveData.setError(it.message)
}
)
)
In the View, set up the Observer on creation:
viewModel.exampleLiveData.observe(this, Observer {
updateResponse(it)
})
Here is the example updateResponse() method, showing/hiding progress, and showing an error if appropriate:
private fun updateResponse(resource: Resource<ExampleModel>?) {
resource?.let {
when (it.state) {
ResourceState.LOADING -> {
showProgress()
}
ResourceState.SUCCESS -> {
hideProgress()
// Use data to populate data on screen
// it.data will have the data of type ExampleModel
}
ResourceState.ERROR -> {
hideProgress()
// Show error message
// it.message will have the error message
}
}
}
}
You can easily achieve this by not using LiveData, and instead using Event-Emitter library that I wrote specifically to solve this problem without relying on LiveData (which is an anti-pattern outlined by Google, and I am not aware of any other relevant alternatives).
allprojects {
repositories {
maven { url "https://jitpack.io" }
}
}
implementation 'com.github.Zhuinden:event-emitter:1.0.0'
If you also copy the LiveEvent class , then now you can do
private val emitter: EventEmitter<String> = EventEmitter()
val events: EventSource<String> get() = emitter
fun doSomething() {
emitter.emit("hello")
}
And
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel = getViewModel<MyViewModel>()
viewModel.events.observe(viewLifecycleOwner) { event ->
// ...
}
}
// inline fun <reified T: ViewModel> Fragment.getViewModel(): T = ViewModelProviders.of(this).get(T::class.java)
For rationale, you can check out my article I wrote to explain why the alternatives aren't as valid approaches.
You can however nowadays also use a Channel(UNLIMITED) and expose it as a flow using asFlow(). That wasn't really applicable back in 2019.