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How to implement loadstate with adapter, recyclerView?

I would like to implement a feature like loadStateFlow in Paging 3.
I do not use pagination in my implementation and it is not necessary in my case.
Could I make it another way?
I have found something like LoadingStateAdapter library
https://developer.android.com/reference/kotlin/androidx/paging/LoadStateAdapter
For now I get a list using method in the fragment:
private fun collectNotificationItems() {
vm.notificationData.collectWith(viewLifecycleOwner) {
notificationAdapter.items = it
}
}
This is implementation I would like to achieve, example is in paging3:
private fun collectItems() {
vm.items.collectWith(viewLifecycleOwner, adapter::submitData)
adapter.loadStateFlow.collectWith(viewLifecycleOwner) { loadState ->
vm.setLoadingState(loadState.refresh is LoadState.Loading)
val isEmpty =
loadState.source.refresh is LoadState.NotLoading && loadState.append.endOfPaginationReached && archiveAdapter.itemCount < 1
vm.setEmptyStateVisible(isEmpty)
}
}
Where methods are:
in ViewModel
fun setLoadingState(isLoading: Boolean) {
_areShimmersVisible.value = isLoading && !_isSwipingToRefresh.value
if (!isLoading) _isSwipingToRefresh.value = false
}
areShimmers and isSwiping are MutableStateFlow
Could you recommend any other options?
EDIT:
I have the whole implementation a little bit different.
I have use case to make it
class GetListItemDetailsUseCase #Inject constructor(private val dao: Dao): BaseFlowUseCase<Unit, List<ItemData>>() {
override fun create(params: Unit): Flow<List<ItemData>> {
return flow{
emit(dao.readAllData())
}
}
}
For now it looks like the code above.
How to use DateState in that case?
EDIT2:
class GetNotificationListItemDetailsUseCase #Inject constructor(private val notificationDao: NotificationDao): BaseFlowUseCase<Unit, DataState<List<NotificationItemsResponse.NotificationItemData>>>() {
override fun create(params: Unit): Flow<DataState<List<NotificationItemsResponse.NotificationItemData>>> {
return flow{
emit(DataState.Loading)
try {
emit(DataState.Success(notificationDao.readAllDataState()))
} catch(e: Exception) {
emit(DataState.Error(e)) // error, and send the exception
}
}
}
}
DAO
#Query("SELECT * FROM notification_list ORDER BY id ASC")
abstract suspend fun readAllDataState(): DataState<List<NotificationItemsResponse.NotificationItemData>>
/\ error beacause of it:
error: Not sure how to convert a Cursor to this method's return type
fragment
private suspend fun collectNotificationItems() {
vm.notificationData.collectLatest { dataState ->
when(dataState) {
is DataState.Error -> {
collectErrorState()
Log.d("collectNotificationItems", "Collect ErrorState")
}
DataState.Loading -> {
Log.d("collectNotificationItems", "Collect Loading")
}
is DataState.Success<*> -> {
vm.notificationData.collectWith(viewLifecycleOwner) {
notificationAdapter.items = it
notificationAdapter.notifyDataSetChanged()
Log.d("collectNotificationItems", "Collect Sucess")
}
}
}
}

You could use a utility class (usually called DataState or something like that).
sealed class DataState<out T> {
data class Success<out T>(val data: T) : DataState<T>()
data class Error(val exception: Exception) : DataState<Nothing>()
object Loading : DataState<Nothing>()
}
Then, you change your flow's return type from Flow<YourObject> to Flow<DataState<YourObject>> and emit the DataStates within a flow {} or channelFlow {} block.
val notificationsFlow: Flow<DataState<YourObject>> get() = flow {
emit(DataState.Loading) // when you collect, you will receive this DataState telling you that it's loading
try {
// networking/database stuff
emit(DataState.Success(yourResultObject))
} catch(e: Exception) {
emit(DataState.Error(e)) // error, and send the exception
}
}
Finally, just change your collect {} to be like:
notificationsFlow.collectLatest { dataState ->
when(dataState) {
is DataState.Error -> { } // error occurred, deal with it here
DataState.Loading -> { } // it's loading, show progress bar or something
is DataState.Success -> { } // data received from the flow, access it with dataState.data
}
}
For more information on this regard, check this out.

Elegant way of handling error using Retrofit + Kotlin Flow

I have a favorite way of doing network request on Android (using Retrofit). It looks like this:
// NetworkApi.kt
interface NetworkApi {
#GET("users")
suspend fun getUsers(): List<User>
}
And in my ViewModel:
// MyViewModel.kt
class MyViewModel(private val networkApi: NetworkApi): ViewModel() {
val usersLiveData = flow {
emit(networkApi.getUsers())
}.asLiveData()
}
Finally, in my Activity/Fragment:
//MyActivity.kt
class MyActivity: AppCompatActivity() {
private viewModel: MyViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
viewModel.usersLiveData.observe(this) {
// Update the UI here
}
}
}
The reason I like this way is because it natively works with Kotlin flow, which is very easy to use, and has a lot of useful operations (flatMap, etc).
However, I am not sure how to elegantly handle network errors using this method. One approach that I can think of is to use Response<T> as the return type of the network API, like this:
// NetworkApi.kt
interface NetworkApi {
#GET("users")
suspend fun getUsers(): Response<List<User>>
}
Then in my view model, I can have an if-else to check the isSuccessful of the response, and get the real result using the .body() API if it is successful. But it will be problematic when I do some transformation in my view model. E.g.
// MyViewModel.kt
class MyViewModel(private val networkApi: NetworkApi): ViewModel() {
val usersLiveData = flow {
val response = networkApi.getUsers()
if (response.isSuccessful) {
emit(response.body()) // response.body() will be List<User>
} else {
// What should I do here?
}
}.map { // it: List<User>
// transform Users to some other class
it?.map { oneUser -> OtherClass(oneUser.userName) }
}.asLiveData()
Note the comment "What should I do here?". I don't know what to do in that case. I could wrap the responseBody (in this case, a list of Users) with some "status" (or simply just pass through the response itself). But that means that I pretty much have to use an if-else to check the status at every step through the flow transformation chain, all the way up to the UI. If the chain is really long (e.g. I have 10 map or flatMapConcat on the chain), it is really annoying to do it in every step.
What is the best way to handle network errors in this case, please?

You should have a sealed class to handle for different type of event. For example, Success, Error or Loading. Here is some of the example that fits your usecases.
enum class ApiStatus{
SUCCESS,
ERROR,
LOADING
} // for your case might be simplify to use only sealed class
sealed class ApiResult <out T> (val status: ApiStatus, val data: T?, val message:String?) {
data class Success<out R>(val _data: R?): ApiResult<R>(
status = ApiStatus.SUCCESS,
data = _data,
message = null
)
data class Error(val exception: String): ApiResult<Nothing>(
status = ApiStatus.ERROR,
data = null,
message = exception
)
data class Loading<out R>(val _data: R?, val isLoading: Boolean): ApiResult<R>(
status = ApiStatus.LOADING,
data = _data,
message = null
)
}
Then, in your ViewModel,
class MyViewModel(private val networkApi: NetworkApi): ViewModel() {
// this should be returned as a function, not a variable
val usersLiveData = flow {
emit(ApiResult.Loading(true)) // 1. Loading State
val response = networkApi.getUsers()
if (response.isSuccessful) {
emit(ApiResult.Success(response.body())) // 2. Success State
} else {
val errorMsg = response.errorBody()?.string()
response.errorBody()?.close() // remember to close it after getting the stream of error body
emit(ApiResult.Error(errorMsg)) // 3. Error State
}
}.map { // it: List<User>
// transform Users to some other class
it?.map { oneUser -> OtherClass(oneUser.userName) }
}.asLiveData()
In your view (Activity/Fragment), observe these state.
viewModel.usersLiveData.observe(this) { result ->
// Update the UI here
when(result.status) {
ApiResult.Success -> {
val data = result.data <-- return List<User>
}
ApiResult.Error -> {
val errorMsg = result.message <-- return errorBody().string()
}
ApiResult.Loading -> {
// here will actually set the state as Loading
// you may put your loading indicator here.
}
}
}

//this class represent load statement management operation
/*
What is a sealed class
A sealed class is an abstract class with a restricted class hierarchy.
Classes that inherit from it have to be in the same file as the sealed class.
This provides more control over the inheritance. They are restricted but also allow freedom in state representation.
Sealed classes can nest data classes, classes, objects, and also other sealed classes.
The autocomplete feature shines when dealing with other sealed classes.
This is because the IDE can detect the branches within these classes.
*/
ٍٍٍٍٍ
sealed class APIResponse<out T>{
class Success<T>(response: Response<T>): APIResponse<T>() {
val data = response.body()
}
class Failure<T>(response: Response<T>): APIResponse<T>() {
val message:String = response.errorBody().toString()
}
class Exception<T>(throwable: Throwable): APIResponse<T>() {
val message:String? = throwable.localizedMessage
}
}
create extention file called APIResponsrEX.kt
and create extextion method
fun <T> APIResponse<T>.onSuccess(onResult :APIResponse.Success<T>.() -> Unit) : APIResponse<T>{
if (this is APIResponse.Success) onResult(this)
return this
}
fun <T> APIResponse<T>.onFailure(onResult: APIResponse.Failure<*>.() -> Unit) : APIResponse<T>{
if (this is APIResponse.Failure<*>)
onResult(this)
return this
}
fun <T> APIResponse<T>.onException(onResult: APIResponse.Exception<*>.() -> Unit) : APIResponse<T>{
if (this is APIResponse.Exception<*>) onResult(this)
return this
}
merge it with Retrofit
inline fun <T> Call<T>.request(crossinline onResult: (response: APIResponse<T>) -> Unit) {
enqueue(object : retrofit2.Callback<T> {
override fun onResponse(call: Call<T>, response: Response<T>) {
if (response.isSuccessful) {
// success
onResult(APIResponse.Success(response))
} else {
//failure
onResult(APIResponse.Failure(response))
}
}
override fun onFailure(call: Call<T>, throwable: Throwable) {
onResult(APIResponse.Exception(throwable))
}
})
}

Flow exception transparency is violated : Emission from another coroutine is detected

I'm tring to implement NetworkBoundResource class in my project and this is what i'm trying. Everything work correctly getting response, caching but when im emiting value inside flowBuilder then it crashes and showing this error.
error i'm getting:
Emission from another coroutine is detected.
Child of ProducerCoroutine{Active}#df26eb9, expected child of FlowCoroutine{Active}#a0bb2fe.
FlowCollector is not thread-safe and concurrent emissions are prohibited.
To mitigate this restriction please use 'channelFlow' builder instead of 'flow')' has been detected.
Emissions from 'catch' blocks are prohibited in order to avoid unspecified behaviour, 'Flow.catch' operator can be used instead.
For a more detailed explanation, please refer to Flow documentation.
NetworkBoundResource class:
abstract class NetworkBoundResource<ResultType, RequestType> {
fun invoke(): Flow<Resource<ResultType>> = flow {
val rawData = loadFromDb()
if (shouldFetch(rawData)) {
fetchDataFromServer()
.onStart { emit(Resource.loading(rawData)) } // emit() causing issue
.catch { emit(Resource.error(it, null)) } // emit() causing issue
.collectLatest { }
}
}
// Save API response result into the database
protected abstract suspend fun cacheInDb(items: RequestType)
// Need to fetch data from server or not.
protected abstract fun shouldFetch(data: ResultType?): Boolean
// Show cached data from the database.
protected abstract suspend fun loadFromDb(): ResultType
// Fetch the data from server.
protected abstract suspend fun fetchDataFromServer(): Flow<ApiResponse<List<Category>>>
// when the fetch fails.
protected open fun onFetchFailed() {}
}
Repository class:
fun getCategories(): Flow<Resource<List<Category>>> {
return object : NetworkBoundResource<List<Category>, List<Category>>() {
override suspend fun cacheInDb(items: List<Category>) {
withContext(Dispatchers.IO) { database.getCategories().insert(items) }
}
override fun shouldFetch(data: List<Category>?): Boolean {
return true
}
override suspend fun loadFromDb(): List<Category> {
return withContext(Dispatchers.IO) { database.getCategories().read() }
}
override suspend fun fetchDataFromServer(): Flow<ApiResponse<List<Category>>> {
return flow { emit(RetrofitModule.getCategories()) }
}
}.invoke()
}
myViewModelClass:
init {
viewModelScope.launch {
repository.getCategories().collectLatest {
if(it.data!=null){
_categories.value = it.data
Log.d("appDebug", " ViewModel : $it")
}
}
}
}

As the exception says, cold flows doesn't allow to emit() concurrently.
You have two options:
Replace flow { } with channelFlow { } and send values with send() (Probably easier in your case)
Make sure no emit() is called concurrently

using countDownLatch.await() to make sure result is delivered

Full source code can be found here : https://github.com/alirezaeiii/SavingGoals-Cache
This is LocalDataSource class :
#Singleton
class QapitalLocalDataSource #Inject constructor(
private val goalsDao: GoalsDao
) : LocalDataSource {
override fun getSavingsGoals(): Single<List<SavingsGoal>> =
Single.create { singleSubscriber ->
goalsDao.getGoals()
.subscribe {
if (it.isEmpty()) {
singleSubscriber.onError(NoDataException())
} else {
singleSubscriber.onSuccess(it)
}
}
}
}
Above Method has been used in Repository class :
#Singleton
class GoalsRepository #Inject constructor(
private val remoteDataSource: QapitalService,
private val localDataSource: LocalDataSource,
private val schedulerProvider: BaseSchedulerProvider
) {
private var cacheIsDirty = false
fun getSavingsGoals(): Observable<List<SavingsGoal>> {
lateinit var goals: Observable<List<SavingsGoal>>
if (cacheIsDirty) {
goals = getGoalsFromRemoteDataSource()
} else {
val latch = CountDownLatch(1)
var disposable: Disposable? = null
disposable = localDataSource.getSavingsGoals()
.observeOn(schedulerProvider.io())
.doFinally {
latch.countDown()
disposable?.dispose()
}.subscribe({
goals = Observable.create { emitter -> emitter.onNext(it) }
}, { goals = getGoalsFromRemoteDataSource() })
latch.await()
}
return goals
}
}
As you see I am using countDownLatch.await() to make sure result is emmited in subscribe or error block. Is there any better solution than using CountDownLatch while using RxJava?

latch.await() blocks the thread which kinda defeats the whole point of using an async API like RxJava.
RxJava has APIs like onErrorResumeNext to handle exceptions and toObservable to convert a Single result to an Observable result.
Also, RxJava types like this are typically intended to be cold (they don't run or figure anything out until you subscribe) so I'd recommend not checking cacheIsDirty until the subscription happens.
I'd go with something like:
fun getSavingsGoals(): Observable<List<SavingsGoal>> {
return Observable
.fromCallable { cacheIsDirty }
.flatMap {
if (it) {
getGoalsFromRemoteDataSource()
} else {
localDataSource.getSavingsGoals()
.toObservable()
.onErrorResumeNext(getGoalsFromRemoteDataSource())
}
}
}
Btw, if you are already using Kotlin, I highly recommend coroutines. Then you async code ends up reading just like regular sequential code.

Sharing same MutableLiveData between Repository and ViewModel

I'm in the process of wrapping my head around Architecture Components / MVVM.
Let's say I have a repository, a ViewModel and a Fragment. I'm using a Resource class as a wrapper to expose network status, like suggested in the Guide to architecture components.
My repository currently looks something like this (simplified for brevity):
class MyRepository {
fun getLists(organizationId: String) {
var data = MutableLiveData<Resource<List<Something>>>()
data.value = Resource.loading()
ApolloClient().query(query)
.enqueue(object : ApolloCall.Callback<Data>() {
override fun onResponse(response: Response<Data>) {
response.data()?.let {
data.postValue(Resource.success(it))
}
}
override fun onFailure(exception: ApolloException) {
data.postValue(Resource.exception(exception))
}
})
}
Then in the ViewModel, I also declare a MutableLiveData:
var myLiveData = MutableLiveData<Resource<List<Something>>>()
fun getLists(organizationId: String, forceRefresh: Boolean = false) {
myLiveData = myRepository.getLists(organizationId)
}
Finally, the Fragment:
viewModel.getLists.observe(this, Observer {
it?.let {
if (it.status.isLoading()) showLoading() else hideLoading()
if (it.status == Status.SUCCESS) {
it.data?.let {
adapter.replaceData(it)
setupViews()
}
}
if (it.status == Status.ERROR) {
// Show error
}
}
})
As you see, there will be an issue with the observer not being triggered, since the LiveData variable will be reset in the process (the Repository creates a new instance).
I'm trying to figure out the best way to make sure that the same LiveData variable is used between the Repository and ViewModel.
I thought about passing the LiveData from the ViewModel to the getLists method, so that the Repository would be using the object from the ViewModel, but even if it works, it seems wrong to do that.
What I mean is something like that:
ViewModel
var myLiveData = MutableLiveData<Resource<List<Something>>>()
fun getLists(organizationId: String, forceRefresh: Boolean = false) {
myRepository.getLists(myLiveData, organizationId)
}
Repository
fun getLists(data: MutableLiveData<Resource<List<Something>>>, organizationId: String) {
...
}

I think I figured out how to do it, thanks to #NSimon for the cue.
My repository stayed the same, and my ViewModel looks like this:
class MyViewModel : ViewModel() {
private val myRepository = MyRepository()
private val organizationIdLiveData = MutableLiveData<String>()
private val lists = Transformations.switchMap(organizationIdLiveData) { organizationId -> myRepository.getLists(organizationId) }
fun getLists() : LiveData<Resource<MutableList<Something>>> {
return lists
}
fun fetchLists(organizationId: String, forceRefresh: Boolean = false) {
if (organizationIdLiveData.value == null || forceRefresh) {
organizationIdLiveData.value = organizationId
}
}
}
I observe getLists() in my fragment, and call viewModel.fetchLists(id) when I want the data. Seems legit?