Develop Reference

Return data from Repository to ViewModel without LiveData

I'm just trying to find an answer how to pass the data from Repository to ViewModel without extra dependencies like RxJava. The LiveData seems as a not good solution here because I don't need to proceed it in my Presentation, only in ViewModel and it's not a good practice to use observeForever.
The code is simple: I use Firebase example trying to pass data with Flow but can't use it within a listener (Suspension functions can be called only within coroutine body error):
Repository
fun fetchFirebaseFlow(): Flow<List<MyData>?> = flow {
var ret: List<MyData>? = null
firebaseDb.child("data").addListenerForSingleValueEvent(
object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
val data = dataSnapshot.getValue<List<MyData>>()
emit(data) // Error. How to return the data here?
}
override fun onCancelled(databaseError: DatabaseError) {
emit(databaseError) // Error. How to return the data here?
}
})
// emit(ret) // Useless here
}
ViewModel
private suspend fun fetchFirebase() {
repo.fetchFirebaseFlow().collect { data ->
if (!data.isNullOrEmpty()) {
// Add data to something
} else {
// Something else
}
}

You can use callbackFlow
#ExperimentalCoroutinesApi
fun fetchFirebaseFlow(): Flow<List<String>?> = callbackFlow {
val listener = object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
val data = dataSnapshot.getValue<List<MyData>>()
offer(data)
}
override fun onCancelled(databaseError: DatabaseError) {
}
}
val ref =firebaseDb.child("data")
reef.addListenerForSingleValueEvent(listener)
awaitClose{
//remove listener here
ref.removeEventListener(listener)
}
}

ObservableField is like LiveData but not lifecycle-aware and may be used instead of creating an Observable object.
{
val data = repo.getObservable()
val cb = object : Observable.OnPropertyChangedCallback() {
override fun onPropertyChanged(observable: Observable, i: Int) {
observable.removeOnPropertyChangedCallback(this)
val neededData = (observable as ObservableField<*>).get()
}
}
data.addOnPropertyChangedCallback(cb)
}
fun getObservable(): ObservableField<List<MyData>> {
val ret = ObservableField<List<MyData>>()
firebaseDb.child("events").addListenerForSingleValueEvent(
object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
ret.set(dataSnapshot.getValue<List<MyData>>())
}
override fun onCancelled(databaseError: DatabaseError) {
ret.set(null)
}
})
return ret
}
It is also possible to use suspendCancellableCoroutine for a single result. Thanks to Kotlin forum.

MVVM Add to favourites functionality

I am implementing an "Add to favourites" functionality in a detail screen. If the user taps the FAB, I want to set the fab as selected and update my database. How could I use the same value that I am sending to the database to be used in my fragment (to be consistent, in case there is some issue while updating the DB)
Fragment
class BeerDetailsFragment : Fragment(R.layout.fragment_beer_details) {
private val viewModel by viewModels<BeerDetailsViewModel>()
private val args by navArgs<BeerDetailsFragmentArgs>()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
subscribeToObservers()
viewModel.getBeer(args.beerId)
}
private fun subscribeToObservers() {
viewModel.beer.observe(viewLifecycleOwner, { resource ->
when(resource.status) {
Status.SUCCESS -> {
loadData(resource.data)
}
Status.ERROR -> {
showError(resource.message)
}
Status.LOADING -> {}
}
})
}
private fun loadData(beerDetails: BeerDomainModel?) {
if (beerDetails != null) {
Glide.with(requireContext())
.load(beerDetails.imageMedium)
.placeholder(R.drawable.ic_beer)
.error(R.drawable.ic_beer)
.fallback(R.drawable.ic_beer)
.into(beerDetailsImage)
beerDetailsName.text = beerDetails.name
beerDetailsDescription.text = beerDetails.description
fab.isSelected = beerDetails.isFavourite
fab.setOnClickListener {
viewModel.updateBeer(beerDetails)
// I shouldn't do it like this in case there is an issue while updating the DB
fab.isSelected = !beerDetails.isFavourite
}
}
}
...
View Model class
class BeerDetailsViewModel #ViewModelInject constructor(private val repository: BreweryRepository) : ViewModel() {
private val beerId = MutableLiveData<String>()
fun getBeer(id: String) {
beerId.value = id
}
var beer = beerId.switchMap { id ->
liveData {
emit(Resource.loading(null))
emit(repository.getBeer(id))
}
}
fun updateBeer(beer: BeerDomainModel) {
viewModelScope.launch {
repository.updateBeer(beer)
}
}
}
Repository
class BreweryRepository #Inject constructor(private val breweryApi: BreweryApi, private val beerDao: BeerDao, private val responseHandler: ResponseHandler) {
...
suspend fun getBeer(id: String): Resource<BeerDomainModel> {
return try {
withContext(IO) {
val isInDB = beerDao.isInDB(id)
if (!isInDB) {
val response = breweryApi.getBeer(id).beer.toDomainModel()
beerDao.insert(response.toBeerEntity())
responseHandler.handleSuccess(response)
} else {
val beer = beerDao.get(id).toDomainModel()
responseHandler.handleSuccess(beer)
}
}
} catch (e: Exception) {
responseHandler.handleException(e)
}
}
suspend fun updateBeer(beer: BeerDomainModel) {
withContext(IO) {
val dbBeer = with(beer) {
copy(isFavourite = !isFavourite)
toBeerEntity()
}
beerDao.update(dbBeer)
}
}
}

I would prefer to use a unidirectional flow with the following implementation:
Not sure how is your DAO implemented, but if you are using Room you could update your get method to return a Flow instead. That way whenever your data is updated, you will get back the updated data.
Then in your VM you just get that Flow or stream of data and subscribe to the updates. Flow has a very convenient method: asLiveData() so your code will look much cleaner.
If you are not using Room, then what I'd do is either construct a Flow or some type of stream and on updates successful updates send out the new data.

How to fix android architecture components paging onItemAtEndLoaded get in loop?

I am trying to practice the android architecture components Paging
Local + Remote Datasource with Room, MVVM and LiveData
When i first time scroll the list(get remote data), it get into loop by onItemAtEndLoaded in PagedList.BoundaryCallback, but it scroll smooth when open the activity next time (get local data)
Here is my github link here!
Can anyone take a look and help me how to fix it, Thanks!
Activity
class PagingActivity : AppCompatActivity() {
lateinit var viewModel: PagingViewModel
lateinit var adapter: PagingAdapter
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_paging)
val factory = PagingViewModelFactory(PagingRepository(), application)
viewModel = ViewModelProviders.of(this,factory).get(PagingViewModel::class.java)
adapter = PagingAdapter()
recyclerView.adapter = adapter
viewModel.pagedListLiveData.observe(this, Observer {
adapter.submitList(it)
})
}
}
ViewModel
class PagingViewModel(repository: PagingRepository, application: Application) :
AndroidViewModel(application) {
val pagedListLiveData = repository.getDataItem(application)
}
Repository
class PagingRepository : PagingRepositoryCallback {
private lateinit var localDataSource: DataSource.Factory<Int, DataItem>
override fun getDataItem(application: Application): LiveData<PagedList<DataItem>> {
val pagedListLiveData: LiveData<PagedList<DataItem>> by lazy {
localDataSource = DataItemDbHelper(application).getRoomDataItemDao().getAllDataItem()
val config = PagedList.Config.Builder()
.setPageSize(25)
.setEnablePlaceholders(false)
.build()
LivePagedListBuilder(localDataSource, config)
.setBoundaryCallback(PagingBoundaryCallback(application))
.build()
}
return pagedListLiveData
}
}
interface PagingRepositoryCallback {
fun getDataItem(application: Application): LiveData<PagedList<DataItem>>
}
BoundaryCallback
class PagingBoundaryCallback(context: Context) :
PagedList.BoundaryCallback<DataItem>() {
private var page = 2
private val api = AllPlayerApi.api
private val dao = DataItemDbHelper(context).getRoomDataItemDao()
override fun onZeroItemsLoaded() {
super.onZeroItemsLoaded()
api.getAllPlayer().enqueue(createWebserviceCallback())
}
override fun onItemAtEndLoaded(itemAtEnd: DataItem) {
super.onItemAtEndLoaded(itemAtEnd)
api.getAllPlayer(page).clone().enqueue(createWebserviceCallback())
}
private fun createWebserviceCallback(): Callback<AllPlayerData> {
return object : Callback<AllPlayerData> {
override fun onFailure(call: Call<AllPlayerData>?, t: Throwable?) {
Log.d("Huang", " get player fail ")
}
override fun onResponse(call: Call<AllPlayerData>?, response: Response<AllPlayerData>) {
Log.d("Huang", " onResponse " + page)
response.body()!!.data!!.forEach {
it.imageUrl = "https://pdc.princeton.edu/sites/pdc/files/events/new-nba-logo-1.png"
}
insertItemsIntoDb(response)
page++
}
}
}
private fun insertItemsIntoDb(response: Response<AllPlayerData>) {
GlobalScope.launch {
response.body()!!.data!!.forEach {
dao.insert(it)
}
}
}
}

Logic for, If onItemAtEndLoaded get the same itemAtEnd , then do nothing.
var lastItemAtEnd:DataItem? = null
override fun onItemAtEndLoaded(itemAtEnd: DataItem) {
lastItemAtEnd?.timestamp?.apply{
if(itemAtEnd.timestamp==this){
return;
}
}
super.onItemAtEndLoaded(itemAtEnd)
api.getAllPlayer(page).clone().enqueue(createWebserviceCallback())
}

As your page size is 25 so Pagelist config should have setInitialLoadSizeHint as 25 for avoiding looping/unnecessary call of onItemAtEndLoaded method
val config = PagedList.Config.Builder()
.setPageSize(25)
.setInitialLoadSizeHint(25) //same as your page size
.setEnablePlaceholders(false)
.build()

I know it's been long but i just post the solution in case someone need.
you should register an observer for your adapter and listen for onItemRangeInserted event and if the start position of item range is zero just simply scroll adapter to zero position, this make your RecyclerView on first load stay in zero position and by the way you should set setPrefetchDistance value smaller than the setInitialLoadSizeHint.
This is the Java code for adapters observer
adapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
#Override
public void onItemRangeInserted(int positionStart, int itemCount) {
super.onItemRangeInserted(positionStart, itemCount);
if(positionStart == 0)
recyclerView.scrollToPosition(positionStart);
}
});

How to observe PagedList data?

I'm using Paging Library and Android Architecture Components. I simply want to observe pagedlist livedata and update my RecyclerView when there is a change.
I'm observing isLoadingLiveData, isEmptyLiveData and errorLiveData objects which are MediatorLiveData objects created in my ViewModel and observed in my fragment. And also observing resultLiveData which returns the fetched Gist list from remote.
In my ViewModel, I created a PagedList LiveData and whenever it's data changed, I wanted to update isLoadingLiveData, isEmptyLiveData, errorLiveData and PagedListAdapter. Therefore, I defined isLoadingLiveData, isEmptyLiveData, errorLiveData and resultLiveData as MediatorLiveData objects. I added resultLiveData as a source of these objects. So when resultLiveData has changed, these objects' onChanged methods will be called. And resultLiveData is depend on userNameLiveData, so when userNameLiveData has changed, allGistsLiveData will be called and it will fetch the data. For example when the user swipe the list, I'm setting userNameLiveData and doing network call again.
My ViewModel:
private val userNameLiveData = MutableLiveData<String>()
private var gists: LiveData<PagedList<Gist>>? = null
val allGistsLiveData: LiveData<PagedList<Gist>>
get() {
if (null == gists) {
gists = GistModel(NetManager(getApplication()), getApplication()).getYourGists(userNameLiveData.value!!).create(0,
PagedList.Config.Builder()
.setPageSize(PAGED_LIST_PAGE_SIZE)
.setInitialLoadSizeHint(PAGED_LIST_PAGE_SIZE)
.setEnablePlaceholders(PAGED_LIST_ENABLE_PLACEHOLDERS)
.build())
}
return gists!!
}
val resultLiveData = MediatorLiveData<LiveData<PagedList<Gist>>>().apply {
this.addSource(userNameLiveData) {
gists = null
this.value = allGistsLiveData
}
}
val isLoadingLiveData = MediatorLiveData<Boolean>().apply {
this.addSource(resultLiveData) { this.value = false }
}
val isEmptyLiveData = MediatorLiveData<Boolean>().apply {
this.addSource(resultLiveData) { this.value = false }
}
val errorLiveData = MediatorLiveData<Boolean>().apply {
this.addSource(resultLiveData) {
if (it == null) {
this.value = true
}
}
}
fun setUserName(userName: String) {
userNameLiveData.value = userName
}
and my fragment:
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel.isEmptyLiveData.observe(this#YourGistsFragment, Observer<Boolean> { isVisible ->
emptyView.visibility = if (isVisible!!) View.VISIBLE else View.GONE
})
viewModel.isLoadingLiveData.observe(this#YourGistsFragment, Observer<Boolean> {
it?.let {
swipeRefreshLayout.isRefreshing = it
}
})
viewModel.errorLiveData.observe(this#YourGistsFragment, Observer<Boolean> {
it?.let {
showSnackBar(context.getString(R.string.unknown_error))
}
})
viewModel.setUserName("melomg")
viewModel.resultLiveData.observe(this#YourGistsFragment, Observer { it -> gistAdapter.setList(it?.value) })
}
override fun onRefresh() {
viewModel.setUserName("melomg")
}
my repository:
fun getYourGists(userName: String): LivePagedListProvider<Int, Gist> {
return remoteDataSource.getYourGists(userName, GitHubApi.getGitHubService(context))
}
my remoteDataSource:
fun getYourGists(username: String, dataProvider: GitHubService): LivePagedListProvider<Int, Gist> {
return object : LivePagedListProvider<Int, Gist>() {
override fun createDataSource(): GistTiledRemoteDataSource<Gist> = object : GistTiledRemoteDataSource<Gist>(username, dataProvider) {
override fun convertToItems(items: ArrayList<Gist>?): ArrayList<Gist>? {
return items
}
}
}
}
I tried to create this solution from this project. But my problem is
resultLiveData has been changing without waiting the network call response and therefore the result of response ignored and my ui is being updated before the data has arrived. Since resultLiveData is changing before request and therefore there is no data yet. Simply how can I observe pagedlist livedata?

Finally, I found a working solution but I don't think it is a best solution. So if you have any ideas please don't hesitate to answer. Here is my solution:
I gave up from wrapping my PagedList data with MediatorLiveData. I still leave my all other live data(isLoadingLiveData, isEmptyLiveData, errorLiveData) as MediatorLiveData but added their source when my gists PagedList LiveData initialized. Here is the code:
private var gists: LiveData<PagedList<Gist>>? = null
private val userNameLiveData = MutableLiveData<String>()
val isLoadingLiveData = MediatorLiveData<Boolean>()
val isEmptyLiveData = MediatorLiveData<Boolean>()
val errorLiveData = MediatorLiveData<ErrorMessage>()
val allGistsLiveData: LiveData<PagedList<Gist>>
get() {
if (gists == null) {
gists = GistModel(NetManager(getApplication()), getApplication()).getYourGists(userNameLiveData.value!!).create(0,
PagedList.Config.Builder()
.setPageSize(Constants.PAGED_LIST_PAGE_SIZE)
.setInitialLoadSizeHint(Constants.PAGED_LIST_PAGE_SIZE)
.setEnablePlaceholders(Constants.PAGED_LIST_ENABLE_PLACEHOLDERS)
.build())
isLoadingLiveData.addSource(gists) { isLoadingLiveData.value = false }
isEmptyLiveData.addSource(gists) {
if (it?.size == 0) {
isEmptyLiveData.value = true
}
}
errorLiveData.addSource(gists) {
if (it == null) {
errorLiveData.value = ErrorMessage(errorCode = ErrorCode.GENERAL_ERROR)
}
}
}
}
return gists!!
}
fun setUserName(userName: String) {
isLoadingLiveData.value = true
userNameLiveData.value = userName
gists = null
allGistsLiveData
}

There is a better way to do this. Look this example:
https://github.com/android/architecture-components-samples/tree/master/PagingWithNetworkSample
The trick is putting inside your DataSource a mutable live data like this:
enum class NetworkState { LOADING, LOADED, FAILURE }
val networkState = MutableLiveData<NetworkState>()
override fun loadInitial(params: LoadInitialParams<String>, callback: LoadInitialCallback<String, OrderService.Resource>) {
networkState.post(NetworkState.LOADING)
ApiClient.listMyObjects(
onSuccess = { list ->
networkState.post(NetworkState.LOADED)
callback.onResult(list)
}
onFailure = {
networkState.post(NetworkState.FAILURE)
}
)
}
Then you can listen to your paged list and network state with:
val myDataSource = MyDataSource.Factory().create()
myDataSource.toLiveData().observe(viewLifeCycle, Observer { pagedList ->
// Update your recycler view
myRecyclerViewAdapter.submitList(pagedList)
})
myDataSource.networkState.observe(viewLifeCycle, Observer { state ->
// Show errors, retry button, progress view etc. according to state
})

Firebase database query design

I have basically a list of entries that many users can read (but can't write). These items show up sorted in the app based on a unique integer that each entry has. I'd like to add a way to allow each individual user to favorite some x number of these items, making those x items appear first in the list. Is there a way I can achieve this using firebase's querying without having to duplicate the list for each user?

Rather than using FirebaseRecyclerAdapter and just passing in the query (which is what I was previously doing), I had to manually manage the results and use a RecyclerView.Adapter instead. This is the code that I ended up with. I'll clean it up a bit so it actually replaces items if they're already there and update and whatnot, but this is a quick summary of the necessary approach:
val data = ArrayList<SomeData>
val favorites = ArrayList<Favorite>
val dataWithFavorites = ArrayList<SomeData>
private val dbRef : DatabaseReference by lazy { FirebaseDatabase.getInstance().reference }
private val dataQuery : Query by lazy { dbRef.child("data").orderByChild("rank") }
private val favoritesQuery : Query by lazy { dbRef.child("users").child("$someUserId").child("favorites").orderByChild("name") }
init {
dataQuery.addValueEventListener(object : ValueEventListener {
override fun onCancelled(p0: DatabaseError?) { }
override fun onDataChange(p0: DataSnapshot?) {
data.clear()
p0!!.children.mapTo(dataList) { it.getValue(SomeData::class.java)!! }
updateEntries()
}
})
favoritesQuery.addValueEventListener(object : ValueEventListener {
override fun onCancelled(p0: DatabaseError?) { }
override fun onDataChange(p0: DataSnapshot?) {
favorites.clear()
p0!!.children.mapTo(dataList) { it.getValue(Favorite::class.java)!! }
updateEntries()
}
})
}
private fun updateEntries() {
if (data.isEmpty()) {
return
}
val favoritesStrings = favorites.map { (id) -> id }
val favoriteData = data
.filter { favoritesStrings.contains(it.id) }
.sortedBy { it.name }
.onEach { it.isFavorite = true }
dataWithFavorites.clear()
dataWithFavorites.addAll(data)
dataWithFavorites.removeAll(favoriteData)
dataWithFavorites.forEach { it.isFavorite = false }
dataWithFavorites.addAll(0, favoriteData)
recyclerView.adapter?.notifyDataSetChanged()
}