I have a Composable and a viewmodel (VM) for it. The VM gets some data from a kotlin flow which I would like to expose as a State
Usually I would have the VM expose a state like this:
var title by mutableStateOf("")
private set
And I could use it in the Composable like this
Text(text = viewModel.title)
But since the data comes from a flow, i have to expose it like this
#Composable
fun title() = flowOf("TITLE").collectAsState(initial = "")
And have to use it in the Composable like this
Text(text = viewModel.title().value)
I try to minimize boilerplate code, so the .value kind of bothers me. Is there any way to collect the flow as state, but still expose it as viewModel.title or viewModel.title() and get the actual String and not the state object?
You can use delegated property.If your program just read it.
class FlowDeletedProperty<T>(val flow: Flow<T>, var initialValue: T, val scope: CoroutineScope) :
ReadOnlyProperty<ViewModel, T> {
private var _value = mutableStateOf(initialValue)
init {
scope.launch {
flow.collect {
_value.value = it
}
}
}
override fun getValue(thisRef: ViewModel, property: KProperty<*>): T {
return _value.value
}
}
fun <T> ViewModel.flowDeletedProperty(flow: Flow<T>, initialValue: T) =
FlowDeletedProperty(flow, initialValue, viewModelScope)
in viewModel
val a = flow {
while (true) {
kotlinx.coroutines.delay(100L)
println("out ")
emit((100..999).random().toString())
}
}
val title by flowDeletedProperty(a,"")
in ui
Text(text = viewModel.title)
Related
I have a composable with viewmodel and I want to pass an id from the composable to the viewmodel.
My composable is:
#Composable
fun HttpRequestScreen(
viewModel: HttpRequestViewModel = hiltViewModel(),
id: String = EMPTYUUID,
onClick: (String, String, Int) -> Unit // respond, request Function: 1 - send request, 2 - edit request
) {
I have the id from a different screen and I want to pass it to my Hilt viewmodel.
Assuming that you have followed the documentation for compose navigation, you can find your parameter here:
#HiltViewModel
class RouteDetailsViewModel #Inject constructor(
private val getRouteByIdUC: GetRouteByIdUC,
private val savedStateHandle: SavedStateHandle
): ViewModel() {
private val routeId = savedStateHandle.get<String>("your-param-name") // for example String in my case
}
You will need to think in a Unidirectional Data Flow pattern, where events flow up and state flows down. For this, you need to expose some sort of state from your viewmodel that sends down the state of the request to the Composable as an observable state.
Your viewmodel could look like this.
class HttpRequestViewModel: ViewModel() {
private val _httpResponse = mutableStateOf("")
val httpResponse: State<String> = _httpResponse
fun onHttpRequest(requestUrl: String) {
//Execute your logic
val result = "result of your execution"
_httpResponse.value = result
}
}
Then in your Composable, you can send events up by calling the ViewModel function on the button click like so
#Composable
fun HttpRequestScreen(viewModel: HttpRequestViewModel) {
val state by viewModel.httpResponse
var userInput = remember { TextFieldValue("") }
Column {
Text(text = "Http Response = $state")
}
BasicTextField(value = userInput, onValueChange = {
userInput = it
})
Button(onClick = { viewModel.onHttpRequest(userInput.text) }) {
Text(text = "Make Request")
}
}
I hope that points you in the right direction. Good luck.
I am struggling to understand what is the best way to get this to work.
I have some input fields and I created a TextFieldState to keep all the state in one place.
But it is not triggering a re-composition of the composable so the state never updates.
I saw this stack overflow answer on a similar question, but I just find it confusing and it doesn't make sense to me
Here is the code:
The Composable:
#Composable
fun AddTrip (
addTripVm: AddTripVm = hiltViewModel()
) {
var name = addTripVm.getNameState()
var stateTest = addTripVm.getStateTest()
Column(
//verticalArrangement = Arrangement.Center,
modifier = Modifier
.fillMaxSize()
) {
Text(text = "Add Trip")
Column(
){
println("From Composable: ${name.value.value}") //No Recomposition
meTextField(
value = name.value.value,
onChange = {
addTripVm.updateName(it)
},
placeholder = "Name",
)
}
View Model code:
#HiltViewModel
class AddTripVm #Inject constructor(
private val tripRepository: TripRepositoryContract,
private val tripValidator: TripValidatorContract
): TripValidatorContract by tripValidator, ViewModel() {
/**
* Name of the trip, this is required
*/
private val nameState: MutableState<TextFieldState> = mutableStateOf(TextFieldState())
private var stateTest = mutableStateOf("");
fun updateStateTest(newValue: String) {
stateTest.value = newValue
}
fun getStateTest(): MutableState<String> {
return stateTest
}
fun getNameState(): MutableState<TextFieldState> {
return nameState;
}
fun updateName(name: String) {
println("From ViewModel? $name")
nameState.value.value = name
println("From ViewModel after update: ${nameState.value.value}") //Updates perfectly
}
}
Text field state:
data class TextFieldState(
var value: String = "",
var isValid: Boolean? = null,
var errorMessage: String? = null
)
Is this possible? Or do I need to separate the value as a string and keep the state separate for if its valid or not?
You don't change instance of nameState's value with
nameState.value.value = name
It's the same object which State checks by default with
fun <T> structuralEqualityPolicy(): SnapshotMutationPolicy<T> =
StructuralEqualityPolicy as SnapshotMutationPolicy<T>
private object StructuralEqualityPolicy : SnapshotMutationPolicy<Any?> {
override fun equivalent(a: Any?, b: Any?) = a == b
override fun toString() = "StructuralEqualityPolicy"
}
MutableState use this as
fun <T> mutableStateOf(
value: T,
policy: SnapshotMutationPolicy<T> = structuralEqualityPolicy()
): MutableState<T> = createSnapshotMutableState(value, policy)
Easiest way is to set
nameState.value = nameState.value.copy(value= name)
other option is to write your own SnapshotMutationPolicy
Im trying to use datastore inside Composable to read user data but cant read the value as string to put inside Text.
That's the datastore
private val Context.userPreferencesDataStore: DataStore<Preferences> by preferencesDataStore(
name = "user"
)
private val USER_FIRST_NAME = stringPreferencesKey("user_first_name")
suspend fun saveUserToPreferencesStore(context: Context) {
context.userPreferencesDataStore.edit { preferences ->
preferences[USER_FIRST_NAME] = "user1"
}
}
fun getUserFromPreferencesStore(context: Context): Flow<String> = context.userPreferencesDataStore.data
.map { preferences ->
preferences[USER_FIRST_NAME] ?: ""
}
and inside Composable:
#Composable
fun myComposable() {
var context = LocalContext.current
LaunchedEffect( true){
saveUserToPreferencesStore(context )
}
Text(getUserFromPreferencesStore(context ))
}
so in your code, getUserFromPreferencesStore() is returning a Flow. so you should collect that as flow, and then compose will auto update once the data is being changed. For example (something similar to this):
val user by getUserFromPreferencesStore(context).collectAsStateWithLifecycleAware(initValue)
We can create LiveData or StateFlow in a similar way as below
val _liveData = MutableLiveData(0)
val _stateFlow = MutableStateFlow(0)
But In LiveData, we can ensure the data is saved and restored by using
val _liveData: MutableLiveData<Int> = savedStateHandle.getLiveData("Key", 0)
For StateFlow, is there a way (or API) to keep the last value and restore it, like what we have in LiveData?
I use the conventional way in ViewModel to store and restore the value
private val _stateFlow = MutableStateFlow(
savedStateHandle.get("key") ?: InitialValue
)
And when setting the value
_stateFlow.value = ChangeValue
savedStateHandle.set("key", ChangeValue)
Not ideal, but at least we have a way to save the latest value.
And thanks to https://www.reddit.com/user/coffeemongrul/, who provided a proposal to wrap it around with a class as per https://www.reddit.com/r/androiddev/comments/rlxrsr/in_stateflow_how_can_we_save_and_restore_android/
class MutableSaveStateFlow<T>(
private val savedStateHandle: SavedStateHandle,
private val key: String,
defaultValue: T
) {
private val _state: MutableStateFlow<T> =
MutableStateFlow(savedStateHandle.get<T>(key) ?: defaultValue)
var value: T
get() = _state.value
set(value) {
_state.value = value
savedStateHandle.set(key, value)
}
fun asStateFlow(): StateFlow<T> = _state
}
Can you try this? I haven't tried it yet but I think it might work.
private val _stateFlow: MutableStateFlow<Int>?
get()= savedStateHandle.get<MutableStateFlow<Int>>("KEY")
I can propose one more solution. A simple extension that will update SavedStateHandle once MutableStateFlow will be updated.
fun <T> SavedStateHandle.getMutableStateFlow(
scope: CoroutineScope,
key: String,
initialValue: T
): MutableStateFlow<T> {
return MutableStateFlow(get(key) ?: initialValue).apply {
scope.launch(Dispatchers.Main.immediate) {
this#apply.collect { value ->
set(key, value)
}
}
}
}
I'm migrating from Shared preference to data store using jetpack compose. everything works fine (data is saved and can be retreated successfully). However, whenever a Data is retrieved, the composable keeps on recomposing endlessly. I'm using MVVM architecture and below is how I have implemented data store.
Below is declared in my AppModule.kt
App module in SingletonComponent
#Provides
#Singleton
fun provideUserPreferenceRepository(#ApplicationContext context: Context):
UserPreferencesRepository = UserPreferencesRepositoryImpl(context)
Then here's my ViewModel:
#HiltViewModel
class StoredUserViewModel #Inject constructor(
private val _getUserDataUseCase: GetUserDataUseCase
): ViewModel() {
private val _state = mutableStateOf(UserState())
val state: State<UserState> = _state
fun getUser(){
_getUserDataUseCase().onEach { result ->
val name = result.name
val token = result.api_token
_state.value = UserState(user = UserPreferences(name, agentCode, token, balance))
}.launchIn(viewModelScope)
}}
Finally, Here's my Repository Implementation:
class UserPreferencesRepositoryImpl #Inject constructor(
private val context: Context
): UserPreferencesRepository {
private val Context.dataStore by preferencesDataStore(name = "user_preferences")
}
private object Keys {
val fullName = stringPreferencesKey("full_name")
val api_token = stringPreferencesKey("api_token")
}
private inline val Preferences.fullName get() = this[Keys.fullName] ?: ""
private inline val Preferences.apiToken get() = this[Keys.api_token] ?: ""
override val userPreferences: Flow<UserPreferences> = context.dataStore.data.catch{
// throws an IOException when an error is encountered when reading data
if (it is IOException) {
emit(emptyPreferences())
} else {
throw it
}
}.map { preferences ->
UserPreferences(name = preferences.fullName, api_token = preferences.apiToken)
}.distinctUntilChanged()
I don't know what causes the composable to recompose. Below Is the composable:
#Composable
fun LoginScreen(
navController: NavController,
userViewModel: StoredUserViewModel = hiltViewModel()
) {
Log.v("LOGIN_SCREEN", "CALLED!")
userViewModel.getUser()
}
If anyone can tell me where I've done wrong please enlighten me. I have tried to change the implementation in AppModule for UserPreferencesRepository but no luck.
Below is UseState.kt which is just a data class
data class UserState(
val user: UserPreferences? = null
)
Below is UserPreferences.kt
data class UserPreferences(val name: String, val api_token: String)
I also faced such problem. The solution was became to navigate with LauchedEffect in composable.
before:
if (hasFlight) {
navController.navigate(Screen.StartMovingScreen.route)
}
after:
if (hasFlight) {
LaunchedEffect(Unit) {
navController.navigate(Screen.StartMovingScreen.route)
}
}
This is expected behaviour: you're calling getUser on each recomposition.
#Composable function is a view builder, and should be side-effects free.
Instead you can use special side effect function, like LaunchedEffect, which will launch job only once, until it's removed from view tree or key argument is changed:
LaunchedEffect(Unit) {
userViewModel.getUser()
}
But this also will be re-called in case of configuration change, e.g. screen rotation. To prevent this case, you have two options:
Call getUser inside view model init: in this case it's guarantied that it's called only once.
Create some flag inside view model to prevent redundant request.
More info about Compose side effects in documentation.