I was testing android listAdapter.
I update a single item in the list, then feed it to the List adapter.
I observe that first the view gets drawn using the old list, then the delta gets applied on top of the old list.
I am confused why the old list gets drawn? Shouldnt only the delta be bound if the old list is already bound to the UI.
Why rebound the old list (i.e. the previous state) if it is already bound?
Fixed the issue..
error was as follows:
in my MainActivity I had:
override fun onCreate(savedInstanceState: Bundle?) {
.....
userViewModel.fetchAllUsers.observe(this, Observer { it ->
submitToAdapter(it)
})
...
}
private fun submitToAdapter(
usersList: List<UserEntity>
) {
userAdapter.submitList(usersList)
binding?.rvItemsList?.layoutManager = LinearLayoutManager(this)
binding?.rvItemsList?.adapter = userAdapter
binding?.rvItemsList?.visibility = View.VISIBLE
binding?.tvNoRecordsAvailable?.visibility = View.GONE
}
The problem was the :
binding?.rvItemsList?.layoutManager = LinearLayoutManager(this)
binding?.rvItemsList?.adapter = userAdapter
This should be called exactly once in the OnCreate function and not each time we get result from the observer..
Fixed code:
override fun onCreate(savedInstanceState: Bundle?) {
.....
binding?.rvItemsList?.layoutManager = LinearLayoutManager(this)
binding?.rvItemsList?.adapter = userAdapter
userViewModel.fetchAllUsers.observe(this, Observer { it ->
submitToAdapter(it)
})
...
}
private fun submitToAdapter(
usersList: List<UserEntity>
) {
userAdapter.submitList(usersList)
binding?.rvItemsList?.visibility = View.VISIBLE
binding?.tvNoRecordsAvailable?.visibility = View.GONE
}
Related
I have a function like:
#Composable
fun LazyElementList(data: Flow<PagingData<Element>>) {
val scrollState = rememberLazyListState()
val elements = data.collectAsLazyPagingItems()
LazyColumn(state = scrollState) {
items(elements) {
DisplayElement(it)
}
}
}
I would like when navigating to another screen and back to maintain the place in the list.
Unexpectedly, the value of scrollState is maintained when visiting child screens. If it wasn't, it should be hoisted, probably into the ViewModel.
In the code in the question scrollState will be reset to the beginning of the list because there are no items in the list on the first composition. You need to wait to display the list until the elements are loaded.
#Composable
fun LazyElementList(data: Flow<PagingData<Element>>) {
val scrollState = rememberLazyListState()
val elements = data.collectAsLazyPagingItems()
if (elements.isLoading) {
DisplayLoadingMessage()
} else {
LazyColumn(state = scrollState) {
items(elements) {
DisplayElement(it)
}
}
}
}
fun LazyPagingItems.isLoading(): Boolean
get() = loadState.refresh is LoadState.Loading
Folks I am stuck engineering a proper solution to access a viewModel scoped to a nav graph , from a button that exists in the TopAppBar in a compose application
Scaffold{
TopAppBar-> Contains the Save Button
Body->
BioDataGraph() -> Contains 5 screens to gather biodata information , and a viewmodel scoped to the graph
}
}
My BioDataViewModel looks like this
class BioDataViewModel{
fun gatherPersonalInformation()
fun gatherPhotos()
...
fun onSaveEverything()
}
The issue i came across is as i described above , how should i go about access the BioDataViewModel , such that i can invoke onSaveEverything when save is clicked in the TopAppBar.
What I have tried
private val performSave by mutableStateOf(false)
Scaffold(
topBar = {
TopAppBar(currentDestination){
//save is clicked.
performSave = true
}
})
{
NavHost(
navController = navController,
startDestination = homeNavigationRoute,
modifier = Modifier
.padding(padding)
.consumedWindowInsets(padding),
) {
composable(route = bioDataRoute) {
val viewModel = hiltViewModel<BioDataViewModel>()
if (performSave){
viewModel.onSaveEverything()
}
BioDataScreen(
viewModel
)
}
}
}
The problem with the approach above is that how and when should i reset the state of performSave ? . Because if i do not set it to false; on every recomposition onSaveEverything would get called.
What would be the best way to engineer a solution for this ? . I checked to see if a similar situation was tackled in jetpack samples , but i found nothing there .
I'm not sure if I understand you correctly, but you can define the BioDataViewModel in activity level, and you can access it in the TopAppBar like this
class MyActivity: ComponentActivity() {
// BioDataViewModel definition here
private val viewModel: BioDataViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
Scaffold(
topBar = {
TopAppBar(currentDestination) {
//save is clicked.
viewModel.onSaveEverything() // call onSaveEverything here
}
})
{
...
...
}
...
...
Edit:
If you want to have the same instance of ViewModel from activity and NavGraph level, you can consider this, a reference from my other answer.
You can define the ViewModelStoreOwner in the navigation graph level.
NavHost(
navController = navController,
startDestination = homeNavigationRoute,
modifier = Modifier
.padding(padding)
.consumedWindowInsets(padding),
) {
val viewModelStoreOwner = checkNotNull(LocalViewModelStoreOwner.current) {
"LocalViewModelStoreOwner not available"
}
composable(route = bioDataRoute) {
val viewModel = hiltViewModel<BioDataViewModel>(viewModelStoreOwner)
if (performSave){
viewModel.onSaveEverything()
}
BioDataScreen(
viewModel
)
}
}
I am quite new in Android Flow and JetPack compose,
I am trying to update my UI when mutable state is being changed , but this is not calling our composable , here is my code
#Composable
fun Grid() {
val mainViewModel by viewModels<DashBoardViewModel>()
mainViewModel.getData()
when (val result = mainViewModel.mutableState.value) {
is Resource.Success -> {
LazyVerticalGrid(
cells = GridCells.Adaptive(100.dp)
) {
items(result.device.items.first().devices.count()) {
EachItem(it)
}
}
}
is Resource.Error -> { Text(text = result.message) }
Resource.Loading -> { CircularProgressIndicator() }
Resource.Empty -> {}
else -> { CircularProgressIndicator() }
}
}
ViewModel:
#HiltViewModel
class DashBoardViewModel #Inject constructor(
private val dashBoardRepository: DashBoardRepository
) : ViewModel() {
val mutableState = MutableLiveData<Resource>()
fun getData() = viewModelScope.launch {
flow {
emit(Resource.Loading)
try {
val mResponse = dashBoardRepository.getDevice()
emit(Resource.Success(mResponse))
} catch (e: Exception) {
e.printStackTrace()
emit(Resource.Error("Error"))
}
}.flowOn(Dispatchers.IO).collect {
mutableState.value = it
}
}
}
There are two problems in your code:
mainViewModel.mutableState.value gets only the current value from your mutable state. Your composable will not be notified when this value changes and thus it cannot reflect the change. If you want to use LiveData in your viewmodel, you have to use observeAsState() extension function which converts LivaData to State that can be automatically observed by composable function. Other option is to have (Mutable)State directly in your viewmodel. See this state explanation.
Your mainViewModel.getData() function will be called every time your Grid() function recomposes, which will be every time your mainViewModel.mutableState changes (once you observe it correctly). You definitely don't want that. Better solution would be to call getData() from your viewModel's init block, or, if you really need to call it from your composable function, use LaunchedEffect.
And, as a side note, the way you are creating flow and then collecting it into LiveData is really odd and unnecessary. You can do something like this instead:
fun getData() = viewModelScope.launch {
mutableState.value = Resource.Loading
try {
val mResponse = dashBoardRepository.getDevice()
mutableState.value = Resource.Success(mResponse)
} catch (e: Exception) {
e.printStackTrace()
mutableState.value = Resource.Error("Error")
}
}
I need to pass a compose content parameter in data class. For example a button can render when added into this content.
data class ContentData {
val content: #Composable ()-> Unit
}
This is working but when I get the app background I am getting parcelable exception. How to solve this problem.
One possible explanation I think that will occur related with a parcelable error, happens if you try to pass such object between activities as extras through Intent. Consider not use Composable as parameters in objects. Instead, try to represent the parameters of your Composable with a model which contains the parameters.
// your compose function
#Composable
fun Item(content: String = "Default", padding: Dp){
// ...
}
// Ui Model which contains your data (instead of have a weird composable reference) as a parcelable.
data class ContentData(
val content: String = "Default",
val paddingRaw: Int = 0
) : Parcelable {
constructor(parcel: Parcel) : this(
parcel.readString().orEmpty(),
parcel.readInt()
) {
}
override fun writeToParcel(parcel: Parcel, flags: Int) {
parcel.writeString(content)
parcel.writeInt(paddingRaw)
}
override fun describeContents(): Int {
return 0
}
companion object CREATOR : Parcelable.Creator<ContentData> {
override fun createFromParcel(parcel: Parcel): ContentData {
return ContentData(parcel)
}
override fun newArray(size: Int): Array<ContentData?> {
return arrayOfNulls(size)
}
}
}
// Example if you need the model between activities through the intent as an extra.
val data = ContentData("your content", 11)
val intent = Intent().apply {
putExtra("keyContentData", data)
}
//The way of get and use your model.
val contentData = intent.extras?.get("keyContentData") as ContentData
#Composable
fun ParentComponent(){
// ...
Item(
contentData?.content.orEmpty(),
contentData?.paddingRaw?.dp ?: 0.dp
)
// ...
}
I know this breaks a lot of Rx rules, but I really like RxJava-JDBC and so do my teammates. Relational databases are very core to what we do and so is Rx.
However there are some occasions where we do not want to emit as an Observable<ResultSet> but would rather just have a pull-based Java 8 Stream<ResultSet> or Kotlin Sequence<ResultSet>. But we are very accustomed to the RxJava-JDBC library which only returns an Observable<ResultSet>.
Therefore, I am wondering if there is a way I can turn an Observable<ResultSet> into a Sequence<ResultSet> using an extension function, and not do any intermediary collection or toBlocking() calls. Below is all I have so far but my head is spinning now trying to connect push and pull based systems, and I cannot buffer either as the ResultSet is stateful with each onNext() call. Is this an impossible task?
import rx.Observable
import rx.Subscriber
import java.sql.ResultSet
fun Observable<ResultSet>.asSequence() = object: Iterator<ResultSet>, Subscriber<ResultSet>() {
private var isComplete = false
override fun onCompleted() {
isComplete = true
}
override fun onError(e: Throwable?) {
throw UnsupportedOperationException()
}
override fun onNext(rs: ResultSet?) {
throw UnsupportedOperationException()
}
override fun hasNext(): Boolean {
throw UnsupportedOperationException()
}
override fun next(): ResultSet {
throw UnsupportedOperationException()
}
}.asSequence()
I'm not sure that's the easiest way to achieve what you want but you can try this code. It converts an Observable to an Iterator by creating a blocking queue and publishing all events from the Observable to this queue. The Iterable pulls events from the queue and blocks if there're none. Then it modify its own state depending on received current event.
class ObservableIterator<T>(
observable: Observable<T>,
scheduler: Scheduler
) : Iterator<T>, Closeable {
private val queue = LinkedBlockingQueue<Notification<T>>()
private var cached: Notification<T>? = null
private var completed: Boolean = false
private val subscription =
observable
.materialize()
.subscribeOn(scheduler)
.subscribe({ queue.put(it) })
override fun hasNext(): Boolean {
cacheNext()
return !completed
}
override fun next(): T {
cacheNext()
val notification = cached ?: throw NoSuchElementException()
check(notification.isOnNext)
cached = null
return notification.value
}
private fun cacheNext() {
if (completed) {
return
}
if (cached == null) {
queue.take().let { notification ->
if (notification.isOnError) {
completed = true
throw RuntimeException(notification.throwable)
} else if (notification.isOnCompleted) {
completed = true
} else {
cached = notification
}
}
}
}
override fun close() {
subscription.unsubscribe()
completed = true
cached = null
}
}
You can use the following helper function:
fun <T> Observable<T>.asSequence() = Sequence { toBlocking().getIterator() }
The observable will be subscribed to when the sequence returned is called for iterator.
If an observable emits elements on the same thread it was subscribed to (like Observable.just for example), it will populate the buffer of the iterator before it gets a chance to be returned.
In this case you might need to direct subscription to the different thread with a call to subscribeOn:
observable.subscribeOn(scheduler).asSequence()
However, while toBlocking().getIterator() doesn't buffer all results it could buffer some of them if they aren't consumed timely by the iterator. That might be a problem if a ResultSet gets somehow expired when the next ResultSet arrives.