Develop Reference

Firebase Realtime Database wait until the data is retrieved

I have a map containing the keys of my Firebase Realtime Database and want to retrieve the corresponding key data and put it in the result data list. How can I execute the loop sequentially? Basically, block the Firebase listener until it gets the result and only then iterate to the next key in the loop.
fun functionA() {
val resultFileDataList = List<DataSnapshot>()
for ((key, value) in filesMap) {
val dbRef = database.child("files").child(key)
dbRef.addListenerForSingleValueEvent(object : ValueEventListener {
override fun onCancelled(p0: DatabaseError) {}
override fun onDataChange(dataSnapshot: DataSnapshot) {
resultFileDataList.add(dataSnapshot)
}
})
}
callFunctionB() // call this function only after all the data in the loop above is retrieved
}
I tried runBlocking {} but no luck.

You can achieve it using this way by utilizing the Task. Tasks.whenall() will wait until all task are done.
fun functionA() {
val taskList = mutableListOf<Task<DataSnapshot>>()
val resultFileDataList = List<DataSnapshot>()
for ((key, value) in filesMap) {
val databaseReferenceTask: Task<DataSnapshot> = database.child("files").child(key).get()
taskList.add(databaseReferenceTask)
val resultTask = Tasks.whenAll(taskList)
resultTask.addOnCompleteListener {
for (task in taskList) {
val snapshotKey: String? = task.result.key
val snapShotValue = task.result
}
callFunctionB()
}
}
}

Since you are using Kotlin, then the simplest solution would be to use Kotlin Coroutines. In this way, you can use suspend functions and call await for each read operation. To achieve that, please check the following article:
How to read data from Firebase Realtime Database using get()?
If you need however to pipeline the requests over its existing connection, then you should consider using kotlinx-coroutines-play-services, case in which you can use awaitAll() function.

This is one way to do it:
suspend fun functionA() = suspendCoroutine<List<DataSnapshot>>{ continuation ->
val resultFileDataList = mutableListOf<DataSnapshot>()
for ((key, value) in filesMap) {
val dbRef = database.child("files").child(key)
dbRef.addListenerForSingleValueEvent(object : ValueEventListener {
override fun onCancelled(p0: DatabaseError) {}
override fun onDataChange(dataSnapshot: DataSnapshot) {
resultFileDataList.add(dataSnapshot)
if(resultFileDataList.size == fileMaps.size){
continuation.resume(resultFileDataList)
}
}
})
}
}
And then you can call the functions wherever you want like so:
CoroutineScope(Dispatchers.IO).launch {
val dataSnapshotList = functionA()
functionB(dataSnapshotList)
}
Bear in mind that it is better to use the following to bind the coroutine to the lifecycle of the activity:
lifecycleScope.launch(Dispatchers.IO) {
val dataSnapshotList = functionA()
functionB(dataSnapshotList)
}
Note:
This will basically wait for all the data to change so that the onDataChanged() is triggered and when the last file is added, continues with the coroutine and returns the value. Depending on your user's behaviour, this could take a long time to complete since even if one of the files is not changed, the coroutine will not resume.
Also, if onCancelled() is triggered for one file, this will never complete. So if you are absolutely sure that onDataChanged() will be triggered for all files, use this. Otherwise, implement some sort of timeout functionality to resume with the incomplete data.

How to use Firestore databse : addSnapshotListener using await() in Kotlin?

I have a DAO class where I have fetchHubList method which fetches a collection of documents from cloud Firestore asynchronously using await(). This implementation used the "get()" method which I got to know later on does not fetch real-time updates. On trying to implement the code similarly using onSnapshotListener gives an error (which was quite expected to be honest, because get() and this methods return quite different things). Does anyone have any idea how to implement this?
How the code is currently:
suspend fun fetchHubList(): ArrayList<HubModel>? = try {
val hubList = ArrayList<HubModel>()
hubsListCollection.get().await().map { document ->
if (document != null) {
Log.d(TAG, "Data fetch successful!")
Log.d(TAG, "the document id is ${document.id}")
val temp = HubModel(document.get("hubName").toString(),
document.id.toString(),
document.get("isAdmin") as Boolean)
hubList.add(temp)
// hubList.add(document.toObject(HubModel::class.java))
} else {
Log.d(TAG, "No such document")
}
}
And what I want to implement here (and which is totally erroneous):
suspend fun fetchHubList(): ArrayList<HubModel>? = try {
val hubList = ArrayList<HubModel>()
hubsListCollection.addSnapshotListener().await().map { document ->
if (document != null) {
Log.d(TAG, "Data fetch successful!")
Log.d(TAG, "the document id is ${document.id}")
val temp = HubModel(document.get("hubName").toString(),
document.id.toString(),
document.get("isAdmin") as Boolean)
hubList.add(temp)
// hubList.add(document.toObject(HubModel::class.java))
} else {
Log.d(TAG, "No such document")
}
}
I use this function in my ViewModel class to create a LiveData wrapped ArrayList:
val hubList = MutableLiveData<ArrayList<HubModel>>()
private val hubListDao = HubListDao()
init {
viewModelScope.launch {
hubList.value = hubListDao.fetchHubList()
}
}
Thanks in advance!

You don't need addSnapshotListener, just use get:
hubsListCollection.get().await()
In order to observe changes in your collection you can extend LiveData:
class CafeLiveData(
private val documentReference: DocumentReference
) : LiveData<Cafe>(), EventListener<DocumentSnapshot> {
private var snapshotListener: ListenerRegistration? = null
override fun onActive() {
super.onActive()
snapshotListener = documentReference.addSnapshotListener(this)
}
override fun onInactive() {
super.onInactive()
snapshotListener?.remove()
}
override fun onEvent(result: DocumentSnapshot?, error: FirebaseFirestoreException?) {
val item = result?.let { document ->
document.toObject(Cafe::class.java)
}
value = item!!
}
}
And expose it from your view model:
fun getCafe(id: String): LiveData<Cafe> {
val query = Firebase.firestore.document("cafe/$id")
return CafeLiveData(query)
}

As #FrankvanPuffelen already mentioned in his comment, there is no way you can use ".await()" along with "addSnapshotListener()", as both are two totally different concepts. One is used to get data only once, while the second one is used to listen to real-time updates. This means that you can receive a continuous flow of data from the reference you are listening to.
Please notice that ".await()" is used in Kotlin with suspend functions. This means that when you call ".await()", you start a separate coroutine, which is a different thread that can work in parallel with other coroutines if needed. This is called async programming because ".await()" starts the coroutine execution and waits for its finish. In other words, you can use ".await()" on a deferred value to get its eventual result, if no Exception is thrown. Unfortunately, this mechanism doesn't work with real-time updates.
When it comes to Firestore, you can call ".await()" on a DocumentReference object, on a Query object, or on a CollectionReference object, which is actually a Query without filters. This means that you are waiting for the result/results to be available. So you can get a document or multiple documents from such calls. However, the following call:
hubsListCollection.addSnapshotListener().await()
Won't work, as "addSnapshotListener()" method returns a ListenerRegistration object.
I want to use a snapshot listener to listen to changes that might occur in my database to update my RecyclerView
In this case, you should consider using a library called Firebase-UI for Android. In this case, all the heavy work will be done behind the scenes. So there is no need for any coroutine or ".await()" calls, everything is synched in real-time.
If you don't want to use either Kotlin Coroutines, nor Firebase-UI Library, you can use LiveData. A concrete example can be seen in my following repo:
https://github.com/alexmamo/FirestoreRealtimePagination/blob/master/app/src/main/java/ro/alexmamo/firestorerealtimepagination/ProductListLiveData.java
Where you can subclass LiveData class and implement EventListener the interface.

Communication between view and ViewModel in MVVM with LiveData

What is a proper way to communicate between the ViewModel and the View, Google architecture components give use LiveData in which the view subscribes to the changes and update itself accordingly, but this communication not suitable for single events, for example show message, show progress, hide progress etc.
There are some hacks like SingleLiveEvent in Googles example but it work only for 1 observer.
Some developers using EventBus but i think it can quickly get out of control when the project grows.
Is there a convenience and correct way to implement it, how do you implement it?
(Java examples welcome too)

Yeah I agree, SingleLiveEvent is a hacky solution and EventBus (in my experience) always lead to trouble.
I found a class called ConsumableValue a while back when reading the Google CodeLabs for Kotlin Coroutines, and I found it to be a good, clean solution that has served me well (ConsumableValue.kt):
class ConsumableValue<T>(private val data: T) {
private var consumed = false
/**
* Process this event, will only be called once
*/
#UiThread
fun handle(block: ConsumableValue<T>.(T) -> Unit) {
val wasConsumed = consumed
consumed = true
if (!wasConsumed) {
this.block(data)
}
}
/**
* Inside a handle lambda, you may call this if you discover that you cannot handle
* the event right now. It will mark the event as available to be handled by another handler.
*/
#UiThread
fun ConsumableValue<T>.markUnhandled() {
consumed = false
}
}
class MyViewModel : ViewModel {
private val _oneShotEvent = MutableLiveData<ConsumableValue<String>>()
val oneShotEvent: LiveData<ConsumableValue<String>>() = _oneShotData
fun fireEvent(msg: String) {
_oneShotEvent.value = ConsumableValue(msg)
}
}
// In Fragment or Activity
viewModel.oneShotEvent.observe(this, Observer { value ->
value?.handle { Log("TAG", "Message:$it")}
})
In short, the handle {...} block will only be called once, so there's no need for clearing the value if you return to a screen.

What about using Kotlin Flow?
I do not believe they have the same behavior that LiveData has where it would alway give you the latest value. Its just a subscription similar to the workaround SingleLiveEvent for LiveData.
Here is a video explaining the difference that I think you will find interesting and answer your questions
https://youtu.be/B8ppnjGPAGE?t=535

try this:
/**
* Used as a wrapper for data that is exposed via a LiveData that represents an event.
*/
open class Event<out T>(private val content: T) {
var hasBeenHandled = false
private set // Allow external read but not write
/**
* Returns the content and prevents its use again.
*/
fun getContentIfNotHandled(): T? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
/**
* Returns the content, even if it's already been handled.
*/
fun peekContent(): T = content
}
And wrapper it into LiveData
class ListViewModel : ViewModel {
private val _navigateToDetails = MutableLiveData<Event<String>>()
val navigateToDetails : LiveData<Event<String>>
get() = _navigateToDetails
fun userClicksOnButton(itemId: String) {
_navigateToDetails.value = Event(itemId) // Trigger the event by setting a new Event as a new value
}
}
And observe
myViewModel.navigateToDetails.observe(this, Observer {
it.getContentIfNotHandled()?.let { // Only proceed if the event has never been handled
startActivity(DetailsActivity...)
}
})
link reference: Use an Event wrapper

For showing/hiding progress dialogs and showing error messages from a failed network call on loading of the screen, you can use a wrapper that encapsulates the LiveData that the View is observing.
Details about this method are in the addendum to app architecture:
https://developer.android.com/jetpack/docs/guide#addendum
Define a Resource:
data class Resource<out T> constructor(
val state: ResourceState,
val data: T? = null,
val message: String? = null
)
And a ResourceState:
sealed class ResourceState {
object LOADING : ResourceState()
object SUCCESS : ResourceState()
object ERROR : ResourceState()
}
In the ViewModel, define your LiveData with the model wrapped in a Resource:
val exampleLiveData = MutableLiveData<Resource<ExampleModel>>()
Also in the ViewModel, define the method that makes the API call to load the data for the current screen:
fun loadDataForView() = compositeDisposable.add(
exampleUseCase.exampleApiCall()
.doOnSubscribe {
exampleLiveData.setLoading()
}
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{
exampleLiveData.setSuccess(it)
},
{
exampleLiveData.setError(it.message)
}
)
)
In the View, set up the Observer on creation:
viewModel.exampleLiveData.observe(this, Observer {
updateResponse(it)
})
Here is the example updateResponse() method, showing/hiding progress, and showing an error if appropriate:
private fun updateResponse(resource: Resource<ExampleModel>?) {
resource?.let {
when (it.state) {
ResourceState.LOADING -> {
showProgress()
}
ResourceState.SUCCESS -> {
hideProgress()
// Use data to populate data on screen
// it.data will have the data of type ExampleModel
}
ResourceState.ERROR -> {
hideProgress()
// Show error message
// it.message will have the error message
}
}
}
}

You can easily achieve this by not using LiveData, and instead using Event-Emitter library that I wrote specifically to solve this problem without relying on LiveData (which is an anti-pattern outlined by Google, and I am not aware of any other relevant alternatives).
allprojects {
repositories {
maven { url "https://jitpack.io" }
}
}
implementation 'com.github.Zhuinden:event-emitter:1.0.0'
If you also copy the LiveEvent class , then now you can do
private val emitter: EventEmitter<String> = EventEmitter()
val events: EventSource<String> get() = emitter
fun doSomething() {
emitter.emit("hello")
}
And
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel = getViewModel<MyViewModel>()
viewModel.events.observe(viewLifecycleOwner) { event ->
// ...
}
}
// inline fun <reified T: ViewModel> Fragment.getViewModel(): T = ViewModelProviders.of(this).get(T::class.java)
For rationale, you can check out my article I wrote to explain why the alternatives aren't as valid approaches.
You can however nowadays also use a Channel(UNLIMITED) and expose it as a flow using asFlow(). That wasn't really applicable back in 2019.

RxJava - Use flowables async way

Re-edit : I will use the result of this method to initialize the visibility of some buttons in my view
The accepted answer seems good in theory but the return value of first method is Flowable> instead of Flowable. Hence I cannot pass it as a parameter to subscription since it requires Flowable but it is Flowable>
Question Before Edit
I am using RxJava to observe a method I am required to call from SDK. Using this method I am trying to make an assertion about the existence of something but I do not know how long the call will take so it is hard for me to say terminate the subscription after x seconds.
override fun doesExist(): Boolean {
var doesExist = false
var subscription : Subscription
val flowable = Flowable.just(SDK.searchContact("contact"))
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(object : FlowableSubscriber<Flowable<List<Contact>>> {
override fun onSubscribe(s: Subscription) {
subscription = s
}
override fun onNext(t: Flowable<List<Contact>>?) {
doesExist = true
}
override fun onComplete() {
Log.d("tagtagcomplete", "tagtagcomplete")
}
override fun onError(e: Throwable?) {
Log.d("tagtagerror", "tagtagerror")
}
return doesExist
}
So what I want to do is return true if I won't get any result from searchContract method and return false if I get a result. While using observables, I can create a subscription object and call it's methods but I could not figure out how to do it right way with flowables.
My confusion is the following: Method to sdk returns a Flowable<List<Contact>>> but in my opinion I need to check if a contact exists only once and stop
Right now my method does not go inside onError, onNext or onComplete. It just returns doesExist

I reedit my answer,following return type is you need.
fun doesExist(): Flowable<Single<Boolean>> {
return Flowable.just(Single.just(SDK.searchContact("contact")).map{ it.isEmpty()})
}

Usage of RXJava map in Kotlin?

Why is this complaining about a miss-type? map supposed to transfor the value of an observable to another, but it's expecting me to return an observable from the map
override fun fetchExercises(): Observable<List<Exercise>> {
return FirebaseDatabase.getInstance()
.getReference("exercises")
.observeSingleEvent()
.map { snapshot: DataSnapshot? -> object: List<Exercise>
// Error here
return listOf<Exercise>()
}
}
fun DatabaseReference.observeSingleEvent(): Observable<DataSnapshot> {
return Observable.create(object : Observable.OnSubscribe<DataSnapshot> {
override fun call(subscriber: Subscriber<in DataSnapshot>) {
val valueEventListener = object: ValueEventListener {
override fun onDataChange(snapshot: DataSnapshot?) {
subscriber.onNext(snapshot)
subscriber.onCompleted()
}
override fun onCancelled(error: DatabaseError?) {
subscriber.onError(FirebaseDatabaseThrowable(error))
}
}
addListenerForSingleValueEvent(valueEventListener)
}
})
}

#zsmb13 has pointed out the correct answer right there. Here, I would like to comment on some mistake made in your lambda expression.
.map { snapshot: DataSnapshot? -> object: List<Exercise>
// Error here
return listOf<Exercise>()
}
object: List<Exercise> here is not a valid syntax since the body goes after an -> sign inside the lambda expression. We do not specify the return type inside the body.
Also, the return here means to return from fetchExercises() as described in Kotlin docs:
A return statement without a label always returns from the function declared with the fun keyword. This means that a return inside a lambda expression will return from the enclosing function, whereas a return inside an anonymous function will return from the anonymous function itself.
As what #zsmb13 said, return is not needed in most of the case. If you really want it(for flow control), you can use qualified returns:
//This is answer from #zsmb13
.map { snapshot: DataSnapshot? ->
return#map listOf<Exercise>()
}

This part of your code
.map { snapshot: DataSnapshot? -> object: List<Exercise>
says that the type of the snapshot parameter that this lambda receives is DataSnapshot? -> object: List<Exercise> (which actually isn't a valid function type, that would be DataSnapshot? -> List<Exercise>).
I believe what you actually wanted to do is the following, a snapshot parameter that just has the type DataSnapshot:
.map { snapshot: DataSnapshot? ->
listOf<Exercise>()
}
The return type of the lambda doesn't have to be specified here, it will just be inferred. You also don't need to use the return keyword in a lambda, because the last expression in it is what's returned.