Currently I'm using compose with navigation and viewmodels.
The code of my NavHost is the following
composable(MyRoute.name + "/{param}") { backStackEntry ->
val param = backStackEntry.arguments?.getString("id") ?: ""
val viewModel = hiltViewModel<MyViewModel>()
MyComposable(
viewModel = viewModel
)
}
The issue I'm facing is that viewModel.init is called an infinite number of times (I guess it is recomposition), but the viewModel is supposed to have only one instance that outlives the lifecycle of the composables.
Use a LaunchedEffect to run your network call.
See this for reference.
Related
I've created simple application to scan products barcode and retrieve information from API by this code. My whole application is routed by this composable:
#Composable
fun AppNavigationHost(navController: NavHostController, modifier: Modifier = Modifier) {
val sharableViewModel: SharableViewModel = hiltViewModel()
NavHost(navController = navController, startDestination = Main.route, modifier = modifier) {
composable(route = Main.route) {
MainScreen(viewModel = sharableViewModel, onSuccessfulProductScan = {
LaunchedEffect(sharableViewModel.product) {
navController.navigate(ProductDetail.route) {
popUpTo(Main.route)
}
}
})
}
composable(route = ProductDetail.route) {
ProductDetailsScreen(viewModel = sharableViewModel)
}
}
}
It works in a simple way:
From MainScreen i call viewModel.findProduct when button is clicked.
When product not exists, it has state NOT_FOUND and simply returns message, that product does not exist. My state has type mutableStateFlow<ProductState>. When It was LiveData, I couldn't navigate between my composables.
When product exists, I update my product in viewModel of type mutableLiveData<Product>. When I change it to StateFlow, my whole app crushes.
ProductDetailsScreen observe product from viewModel. When it's filled with data, composable updates it's view with product information.
Now, I don't understand few things, like:
I must initialize my SharedViewModel in shareable place like AppNavigationHost. When I add hiltViewModel() as a default parameter in those composables, my app instantly crashes. Why I can't inject viewModel with it's state as a separated parameter in composable?
Why I need to use StateFlow when I have to navigate between composables, why LiveData is not enought to handle it? Is there any possibility to use StateFlow instead of LiveData through navigated views without handling LaunchedEffect scope?
Why I need to use LiveData to persist object in ViewModel between composables? StateFlow is bounded to viewModel, not to composable. It should "emit" my product once and next view should retrieve that event.
I wish I could understand well basics of composable concept, but some of them are not clear enough for me, e.g. passing data in other way than shareable view model.
I am using Navigation Compose, and I am trying to pass an entire object to the next screen, So I am using a shared ViewModel through hilt, and created a mutablestate variable of that object and want to get its value in the next screen.
Like This
var campaign = mutableStateOf<Campaign?>(null)
private set
fun addCampaign(campaign: Campaign){
this.campaign.value = campaign
}
where Campaign is just a data-class.
In my Screen in Navigation
LazyColumn (
content =
{
items(viewModel.campaignListCurrent){ campaign ->
CampaignItem(
image = campaign.brand?.image ?: "",
title = campaign.name ,
id = campaign.id ,
description =campaign.description ,
date =campaign.createdAt ) {
viewModel.addCampaign(
CampaignsViewModel.Campaign(...) // the "campaign" object is used to fill this
)
Timber.tag("CampaignObject").v(viewModel.campaign.value.toString())
viewModel.changed = true
navController.navigate(Screen.CampaignDetailsScreen.route)
}
}
})
I can see when I log the data that it was stored successfully. yet in the next screen when I get the data from the same ViewModel, it's null. even though its a mutablestate and supposed to change its value and be observables, I don't think I get mutablestate behavior at all, and any link for a proper explanation for it will be appreciated.
val campaign by remember { viewModel.campaign }
Timber.v("CampaignDetailsScreen2: " + campaign.toString())
can someone explain to me why it doesn't work here, even if I used Launched effect??
why doesn't change its value here?
I managed to solve this by Creating a SharedViewModel instance in the NavGraph to make sure it is the same Instance used in both composable functions.
#Composable
fun CampaignsScreen(
navController : NavController,
viewModel: CampaignsViewModel
) {
....
#Composable
fun CampaignDetailsScreen(
navController : NavController,
viewModel: CampaignsViewModel
) {
and in my NavGraph I passed the ViewModel as HiltViewmodel in the constructor and from it to the two screens
#Composable
fun SetUpNavGraph (navController : NavHostController,
campaignViewModel: CampaignsViewModel = hiltViewModel()) {
...........
loginNavGraph(navController = navController)
homeNavGraph(navController = navController, campaignsViewModel = campaignViewModel)
}
}
and in the home NavGraph I passed the parametere and its the same Instance in both
fun NavGraphBuilder.homeNavGraph(
navController : NavHostController,
campaignsViewModel: CampaignsViewModel
){
navigation(startDestination = Screen.HomeScreen.route, route = HOME_GRAPH_ROUTE) {
................
composable(Screen.Campaigns.route){
CampaignsScreen(navController = navController, viewModel = campaignsViewModel)
}
composable(Screen.CampaignDetailsScreen.route){
CampaignDetailsScreen(navController, viewModel = campaignsViewModel)
}
}
}
You can use backstackEntry and PreviouseBackStack to solve this but it won't work if you have a JSON Object within your object as you will need to Serialize and Deserialize this object manually and it will take more time and code to achieve the same goal for a shared ViewModel to pass Parclized object. until Google Solve passing Parclized objects in Compose Navigation I recommend backStack Entry if you don't have a JSON object within your paralyzed object and if you do, use a shared ViewModel between the tow composable function.
Why would it work if you are just remembering a local value and not even reading from the proper model?
val campaign = viewModel.campaign
Can't just use remember willy-nilly without understanding the proper usage.
I'm trying to follow the official guidelines to migrate from LiveData to Flow/StateFlow with Compose, as per these articles:
A safer way to collect flows from Android UIs
Migrating from LiveData to Kotlin’s Flow
I am trying to follow what is recommended in the first article, in the Safe Flow collection in Jetpack Compose section near the end.
In Compose, side effects must be performed in a controlled
environment. For that, use LaunchedEffect to create a coroutine that
follows the composable’s lifecycle. In its block, you could call the
suspend Lifecycle.repeatOnLifecycle if you need it to re-launch a
block of code when the host lifecycle is in a certain State.
I have managed to use .flowWithLifecycle() in this way to make sure the flow is not emmiting when the app goes to the background:
#Composable
fun MyScreen() {
val lifecycleOwner = LocalLifecycleOwner.current
val someState = remember(viewModel.someFlow, lifecycleOwner) {
viewModel.someFlow
.flowWithLifecycle(lifecycleOwner.lifecycle, Lifecycle.State.STARTED)
.stateIn(
scope = viewModel.viewModelScope,
started = SharingStarted.WhileSubscribed(5000),
initialValue = null
)
}.collectAsState()
}
I find this very "boilerplatey" -there must be something better. I would like to have StateFlow in the ViewModel, instead of Flow that gets converted to StateFLow in the #Composable, and use .repeatOnLifeCycle(), so I can use multiple .collectAsState() with less boilerplate.
When I try to use .collectAsState() inside a coroutine (LaunchedEffect), I obviously get an error about .collectAsState() having to be called from the context of #Composable function.
How can I achieve similar functionality as with .collectAsState(), but inside .repeatOnLifecycle(). Do I have to use .collect() on the StateFlow and then wrap the value with State? Isn't there anything with less boilerplate than that?
From "androidx.lifecycle:lifecycle-runtime-compose:2.6.0-alpha01" you can use the collectAsStateWithLifecycle() extension function to collect from flow/stateflow and represents its latest value as Compose State in a lifecycle-aware manner.
import androidx.lifecycle.compose.collectAsStateWithLifecycle
#Composable
fun MyScreen() {
val state by viewModel.state.collectAsStateWithLifecycle()
}
Source: Android Lifecycle release
After reading a few more articles, including
Things to know about Flow’s shareIn and stateIn operators
repeatOnLifecycle API design story
and eventually realising that I wanted to have the StateFlow in the ViewModel instead of within the composable, I came up with these two solutions:
1. What I ended up using, which is better for multiple StateFlows residing in the ViewModel that need to be collected in the background while there is a subscriber from the UI (in this case, plus 5000ms delay to deal with configuration changes, like screen rotation, where the UI is still interested in the data, so we don't want to restart the StateFlow collecting routine). In my case, the original Flow is coming from Room, and been made a StateFlow in the VM so other parts of the app can have access to the latest data.
class MyViewModel: ViewModel() {
//...
val someStateFlow = someFlow.stateIn(
scope = viewModelScope,
started = SharingStarted.WhileSubscribed(5000),
initialValue = Result.Loading()
)
val anotherStateFlow = anotherFlow.stateIn(
scope = viewModelScope,
started = SharingStarted.WhileSubscribed(5000),
initialValue = Result.Loading()
)
//...
}
Then collected in the UI:
#Composable
fun SomeScreen() {
var someUIState: Any? by remember { mutableStateOf(null)}
var anotherUIState: Any? by remember { mutableStateOf(null)}
LaunchedEffect(true) {
lifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED) {
launch {
viewModel.someStateFlow.collectLatest {
someUIState = it
}
}
launch {
viewModel.anotherStateFlow.collectLatest {
anotherUIState = it
}
}
}
}
}
2. An extension function to alleviate the boilerplate when collecting a single StateFlow as State within a #Composable. This is useful only when we have an individual HOT flow that won't be shared with other Screens/parts of the UI, but still needs the latest data at any given time (hot flows like this one created with the .stateIn operator will keep collecting in the background, with some differences in behaviour depending on the started parameter). If a cold flow is enough for our needs, we can drop the .stateIn operator together with the initial and scope parameters, but in that case there's not so much boilerplate and we probably don't need this extension function.
#Composable
fun <T> Flow<T>.flowWithLifecycleStateInAndCollectAsState(
scope: CoroutineScope,
initial: T? = null,
context: CoroutineContext = EmptyCoroutineContext,
): State<T?> {
val lifecycleOwner = LocalLifecycleOwner.current
return remember(this, lifecycleOwner) {
this
.flowWithLifecycle(
lifecycleOwner.lifecycle,
Lifecycle.State.STARTED
).stateIn(
scope = scope,
started = SharingStarted.WhileSubscribed(5000),
initialValue = initial
)
}.collectAsState(context)
}
This would then be used like this in a #Composable:
#Composable
fun SomeScreen() {
//...
val someState = viewModel.someFlow
.flowWithLifecycleStateInAndCollectAsState(
scope = viewModel.viewModelScope //or the composable's scope
)
//...
}
Building upon OP's answer, it can be a bit more light-weight by not going through StateFlow internally, if you don't care about the WhileSubscribed(5000) behavior.
#Composable
fun <T> Flow<T>.toStateWhenStarted(initialValue: T): State<T> {
val lifecycleOwner = LocalLifecycleOwner.current
return produceState(initialValue = initialValue, this, lifecycleOwner) {
lifecycleOwner.lifecycle.repeatOnLifecycle(Lifecycle.State.STARTED) {
collect { value = it }
}
}
}
I'm converting my project from view system to compose. In one of app page's in old view system I have a fragment with one viewPager which just make some pages of same fragment with different data to show. While each fragment has it's own lifecycle I can have multiple isolated viewModel per each fragment. In Compose as far as I know viewModel life cycle is attached to navigation graph, therefor each time I try to access viewModel it just return same viewModel object that's created in first call. how can I achieve same view system behavior in compose?
this is simplified version of what my app is doing now
#Composable
fun MainScreen(mainViewModel: MainViewModel = hiltViewModel()) {
val pages = mainViewModel.pages.collectAsState()
val pagerState = rememberPagerState(pageCount = pages.size)
HorizontalPager(state = pagerState) {
ChildScreen()
}
}
#Composable
fun ChildScreen(childViewModel:ChildViewModel = hiltViewModel()){
}
here childViewModel is always one object for all pages
I'm working on a tutorial for working with LiveData in Kotlin here and I've come to a point in the instructions where one of my fragments says to use the NavHostFragment like so:
/**
* Called when the game is finished
*/
private fun gameFinished() {
Toast.makeText(activity, "Game has just finished.", Toast.LENGTH_LONG).show()
val action = GameFragmentDirections.actionGameToScore()
action.score = viewModel.score.value?:0
NavHostFragment.findNavController(this).navigate(action)
viewModel.onGameFinishComplete()
}
But then, I ended up setting up an Observer with a lambda function in it in another fragment, and they say to use findNavController by itself with no arguments in this section:
viewModelFactory = ScoreViewModelFactory(ScoreFragmentArgs.fromBundle(requireArguments()).score)
viewModel = ViewModelProvider(this, viewModelFactory).get(ScoreViewModel::class.java)
// sets observer
viewModel.score.observe(viewLifecycleOwner, Observer {newScore ->
binding.scoreText.text = newScore.toString()
})
viewModel.eventPlayAgain.observe(viewLifecycleOwner, Observer {playAgain ->
if(playAgain) {
findNavController().navigate(ScoreFragmentDirections.actionRestart())
viewModel.onPlayAgainComplete()
}
})
I'm not entirely sure why one would require the call to the static method of the NavHostFragment class with 'this' sent in as a context in the first piece of code, but why one is able to call the method alone in the second one. Does the Observer or the viewLifecycleOwner or the viewModel generate an implied context in this wrapped block or something?
They are both same methods. Fragment's findNavController source code is
fun Fragment.findNavController(): NavController =
NavHostFragment.findNavController(this)