Develop Reference

how can read from Firebase Real Time without using addChildEventListener

in my case when login to the application
need to get user profile from realTime Database by UID
in docs must be using addValueEventListener to read from RealTime DB
//how can get value direct from real-time firebase if I have id for obj
fun loginByEmail(email: String, password: String) {
firebaseAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener { task ->
if (task.isSuccessful) {
getUserValueFromDB(firebaseAuth.currentUser!!.uid, email)
}
}
}
//My problem is here is not access to this method
private fun getUserValueFromDB(uid: String, email: String) { //todo
databaseReference.child("Users").addValueEventListener(object : ValueEventListener {
override fun onDataChange(snapshot: DataSnapshot) {
for (snap in snapshot.children) {
var data = snap.getValue(User::class.java)
if (data?.id == uid) {
prefs.userImage = data.image!!
GlobalScope.launch {
_loginStatus.emit(true)
}
}
}
}
override fun onCancelled(error: DatabaseError) {
TODO("Not yet implemented")
}
})
}

If you want to read the data from one specific user from your database, you can do so by using a query.
private fun getUserValueFromDB(uid: String, email: String) {
var query = databaseReference.child("Users").orderByChild("id").equalTo(uid)
query.addValueEventListener(object : ValueEventListener {
override fun onDataChange(snapshot: DataSnapshot) {
...
The rest of your code can stay the same, you'll just be loading a lot less data.

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.

callbackFlow not returning anything android kotlin

I want to implement firebase realtime database with coroutines, so I need to use flow because firebase just accept callbacks. the problem is the .collect{} block never gets executed
here is my code
#ExperimentalCoroutinesApi
override suspend fun getProduct(barcode: String): ProductItem? {
return withContext(Dispatchers.Default) {
println("Hi")
var item: ProductItem? = null
productFlow(barcode).collect {
//this never gets called
print("Getting product")
item = it
}
println("Ending product request ${item?.name}")
Log.i("GetProduct",item?.name)
item
}
}
#ExperimentalCoroutinesApi
private fun productFlow(barcode: String): Flow<ProductItem?> = callbackFlow {
val database = FirebaseDatabase.getInstance()
val productRef = database.getReference("products/$barcode")
val callback = object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
for(snapshot in dataSnapshot.children){
Log.i("Source", snapshot.value.toString())
}
val product = dataSnapshot.getValue(ProductItem::class.java)
Log.i("Source",product?.name) //everything is good until here
sendBlocking(dataSnapshot.getValue(ProductItem::class.java)) //after this i dont get anything on the collect{} block
}
override fun onCancelled(databaseError: DatabaseError) {
println("cancelling")
sendBlocking(null)
}
}
try {
productRef.addListenerForSingleValueEvent(callback)
} catch (e: FirebaseException) {
println("Firebase exception")
sendBlocking(null)
}
awaitClose{
println("Closing")
productRef.removeEventListener(callback)
}
}

First I would suggest to use the catch method to check if there is an error or not. Second, for callbackflow I remember using offer() instead of sendBlocking

Retrieve data from Firebase on Android Studio

Can someone help me fix it?
Following code works without any error, however, it does not retrieve data from Firebase and show in the TextView.
private fun viewData() {
val postReference = FirebaseDatabase.getInstance().getReference("dataID")
val dbView= findViewById<TextView>(R.id.txtFdbData)
val postListener = object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
val post = dataSnapshot.getValue(Post::class.java)
dbView.text=post?.postName
}
override fun onCancelled(databaseError: DatabaseError) {
}
}
postReference.addValueEventListener(postListener)
Toast.makeText(this,"Retrieved",Toast.LENGTH_LONG).show()
}
Above code is called when I tap the button 'btnView'
viewButton = findViewById(R.id.btnView)
viewButton.setOnClickListener {
viewData()
}
When I hit the button it shows the toast message 'Retrieved' and the default value given in the TextView (txtFdbData) is deleted (or may be replaced with an empty string?, I do not know).
Following is the post Class
data class Post (
val postName: String="",
val postDescription:String="")
I am working on Android Studio, using Kotlin and Firebase Realtime Database.

You query to database return list of items. So loop through it and try to get Post. Check below:
override fun onDataChange(dataSnapshot: DataSnapshot) {
dataSnapshot.children.forEach {childSnapshot ->
val post = childSnapshot.getValue(Post::class.java)
dbView.text=post?.postName
}
}

getReference("dataID") is not your data node it is your parent node.
Then you have to access their children using getChildren() method.
Change you on data change method with this.
override fun onDataChange(dataSnapshot: DataSnapshot) {
for (postSnapshot : dataSnapshot.getChildren()) {
val post = postSnapshot .getValue(Post::class.java)
dbView.text=post?.postName
}
}

Android App Development - Firebase Database

I'm building an activity in an app where users can upload a post, and other users in the app can view the post.
I built a function called loadPost()
This function will take what's there in the database and load it in my View.
Unfortunately, I'm facing an error with the
.addValueEventListener(ValueEventListener{
It's telling me that the interface needs a constructor.
here's the function:
//*******************************************************
//Load posts to the screen from the database
//****************************************************
fun loadPost(){
myRef.child("posts")
.addValueEventListener(ValueEventListener{
override fun onDataChange(#NonNull dataSnapshot: DataSnapshot?) {
try {
postElements.clear()
postElements.add(Post("0","him","url","add"))
postElements.add(Post("0","him","url","ads"))
//Hashmap : Key and value (represents the post and node)
var td= dataSnapshot!!.value as HashMap<String,Any>
for(key in td.keys){
var post= td[key] as HashMap<String,Any>
postElements.add(Post(key,
post["text"] as String,
post["postImage"] as String
,post["userUID"] as String))
}
adpater!!.notifyDataSetChanged() //notify when there's an update in the adapter
}catch (ex:Exception){}
}
override fun onCancelled(p0: DatabaseError?) {
}
})
}

this is how the constructor of a ValueEventListener should look alike:
object : ValueEventListener {
override fun onDataChange(dataSnapshot: DataSnapshot) {
}
}
which means, that you have to replace this one line
.addValueEventListener(ValueEventListener {
with:
.addValueEventListener(object : ValueEventListener {
see https://youtrack.jetbrains.com/issue/KT-7770 (concerning the code-converter)