Develop Reference

Saving and deleting data from firestore async [duplicate]

I have created an app with Kotlin and Firebase Firestore. Now I need to implement coroutines as there is so much work on the main thread. But I'm also a beginner so it's something new to me. I've watched some tutorials on this but I didn't find complete tutorials on Firestore with coroutines. So I need some help to implement coroutines in my app In such parts like these (I tried by myself but didn't get it).
Retrieving posts from Firestore.
private fun retrievePosts() {
FirebaseFirestore.getInstance().collection("Posts")
.orderBy("timeStamp", Query.Direction.DESCENDING)
.get()
.addOnSuccessListener { queryDocumentSnapshots ->
postList?.clear()
for (documentSnapshot in queryDocumentSnapshots) {
val post = documentSnapshot.toObject(Post::class.java)
postList?.add(post)
}
postAdapter?.notifyDataSetChanged()
postAdapter?.setOnPostClickListener(this)
if (isRefreshed) {
swipe_refresh_home?.setRefreshing(false)
isRefreshed = false
}
swipe_refresh_home?.visibility = VISIBLE
progress_bar_home?.visibility = GONE
}.addOnFailureListener { e ->
Log.d(TAG, "UserAdapter-retrieveUsers: ", e)
swipe_refresh_home?.visibility = VISIBLE
progress_bar_home?.visibility = GONE
}
}
Getting user data into an adapter
private fun userInfo( fullName: TextView, profileImage: CircleImageView,
about: TextView, uid: String,
userLocation: TextView, itemRoot: LinearLayout ) {
val userRef = FirebaseFirestore.getInstance().collection("Users").document(uid)
userRef.get()
.addOnSuccessListener {
if (it != null && it.exists()) {
val user = it.toObject(User::class.java)
Glide.with(mContext).load(user?.getImage()).placeholder(R.drawable.default_pro_pic).into(profileImage)
fullName.text = user?.getFullName().toString()
about.text = user?.getAbout()
if (user?.getLocation() != ""){
userLocation.visibility = VISIBLE
userLocation.text = user?.getLocation()
}
if (profileImage.drawable == null){
itemRoot.visibility = GONE
}
else{
itemRoot.visibility = VISIBLE
}
}
}
}
And this Save post button in an adapter.
private fun savedPost(postId: String, saveButton: ImageView?) {
FirebaseFirestore.getInstance().collection("Users").document(currentUserID)
.collection("Saved Posts").document(postId)
.get()
.addOnSuccessListener {
if (it.exists()) {
saveButton?.setImageResource(drawable.ic_bookmark)
} else {
saveButton?.setImageResource(drawable.bookmark_post_ic)
}
}
}

As I see your code, you are using the following query:
val queryPostsByTimestamp = FirebaseFirestore.getInstance().collection("Posts")
.orderBy("timeStamp", Query.Direction.DESCENDING)
Most probably to get a list of Post objects from your "Posts" collection.
In order to use Kotlin Coroutines, don't forget to add the following dependencies in the Gradle (app) file:
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.3.9"
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1"
implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.3.1"
I'll provide you a solution using the MVVM architecture pattern. So we'll use a repository class and a ViewModel class. For the asynchronous calls to Firestore, we'll use Flow.
For the response that we get from the database call, we need a sealed class that looks like this:
sealed class Response<out T> {
class Loading<out T>: Response<T>()
data class Success<out T>(
val data: T
): Response<T>()
data class Failure<out T>(
val errorMessage: String
): Response<T>()
}
Assuming that you have a "Post" class, let's create in the repository class the following function:
fun getPostsFromFirestore() = flow {
emit(Loading())
emit(Success(queryPostsByTimestamp.get().await().documents.mapNotNull { doc ->
doc.toObject(Post::class.java)
}))
}. catch { error ->
error.message?.let { errorMessage ->
emit(Failure(errorMessage))
}
}
So we'll emit an object according to the state. When first-time calling the function, we emit a loading state using emit(Loading(), when we get the data we emit the List<Post> and if we get an error, we emit the error message using Failure(errorMessage).
Now we need to call this function, from the ViewModel class:
fun getPosts() = liveData(Dispatchers.IO) {
repository.getPostsFromFirestore().collect { response ->
emit(response)
}
}
With the above function, we collect the data that we get from the getPostsFromFirestore() function call, and we emit the result further as a LiveData object so it can be observed in the activity/fragment like this:
private fun getPosts() {
viewModel.getPosts().observe(this, { response ->
when(response) {
is Loading -> //Load a ProgessBar
is Success -> {
val postList = response.data
//Do what you need to do with your list
//Hide the ProgessBar
}
is Failure -> {
print(response.errorMessage)
//Hide the ProgessBar
}
}
})
}
That's pretty much of it!

I don't know Firebase, so I may miss something, but generally speaking you don't need a special support in the library to use it with coroutines. If you start a background coroutine and then execute your above code in it, then Firebase will probably run within your coroutine without any problems.
The only problematic part could be listeners. Some libs invoke callbacks in the thread that was used to execute them, but some dispatch callbacks to a specific thread. In the case of Firebase it seems by default it runs listeners in the main thread. If this is not what you want, you can pass an executor to run callbacks within coroutines as well, e.g.:
.addOnSuccessListener(Dispatchers.Default.asExecutor()) { ... }

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.

Firebase: How to check if document write was successful

I want to check if my database write was successful in order to show the user an error message.
My current approach doesn't work as it says "Type mismatch, required Unit found EmailStatus"
Current approach
class EmailRepositoryImpl : EmailRepository {
private val db = Firebase.firestore
override fun sendEmail(email: Email): EmailStatus<Nothing> {
db.collection("emails").document().set(email).addOnCompleteListener {
if (it.isSuccessful) return#addOnCompleteListener EmailStatus.Success<Nothing>
if (it.isCanceled) return#addOnCompleteListener EmailStatus.Error(it.exception!!)
}
}
}
Status Sealed Class
sealed class EmailStatus<out T> {
data class Success<out T>(val data: T) : EmailStatus<T>()
data class Error(val exception: Exception) : EmailStatus<Nothing>()
}
Is it even possible to write something like this? As far as I know there is a generic firebase error type but I didn't found anything related to kotlin or android...
I appreciate every help, thank you
Edit
I've tried getting my document, but I am just getting null: (When I use the listener approach, everything works fine)
Interface
interface EmailRepository {
suspend fun getEmail(): Flow<EmailEntity?>
}
Interface Implementation
override suspend fun getEmail(): Flow<EmailEntity?> = flow {
val result = db.collection("emailprice").document("Email").get().await()
emit(result.toObject<EmailEntity>())
}
ViewModel
private val emailEntity = liveData<EmailEntity?>(Dispatchers.IO) {
emailRepository.getCalibratePrice()
}

The problem is that addOnCompleteListener callback does not return anything (Unit) and you are trying to return an EmailStatus from that scope.
You have three approaches:
Create an interface that will populate the value and return that EmailStatus down to your caller layer
Use Coroutines to suspend this function when the async call to firebase is done and then return that value
Use Flow to offer the data when it's ready to process
I think the easiest way to do this one shot operation is to use Coroutines; I have written an article about that.

Okay, this is the final solution, thanks to #Gastón Saillén and #Doug Stevenson :
EmailRepository
interface EmailRepository {
fun sendEmail(email: Email): Flow<EmailStatus<Unit>>
}
EmailRepository Implementation
class EmailRepositoryImpl #Inject constructor(
private val db: FirebaseFirestore
) : EmailRepository {
override fun sendEmail(email: Email)= flow<EmailStatus<Unit>> {
db.collection("emails").add(email).await()
emit(EmailStatus.success(Unit))
}.catch {
emit(EmailStatus.failed(it.message.toString()))
}.flowOn(Dispatchers.Main)
}
ViewModel
fun sendEmail(): LiveData<EmailStatus<Unit>> {
val newEmail = createEmail()
return emailRepository.sendEmail(newEmail).asLiveData()
}
Fragment
btn.setOnClickListener {
viewModel.sendEmail().observe(viewLifecycleOwner) {
when(it) {
is EmailStatus.Success -> {
valid = true
navigateTo(next, bundleNext)
Toast.makeText(requireContext(), "Success", Toast.LENGTH_SHORT).show()
}
is EmailStatus.Failure -> {
valid = false
Toast.makeText(requireContext(), "Failed ${it.message}", Toast.LENGTH_SHORT).show()
}
}
}
}
The only problem I currently have is that my "faield state" does not work like it should.
It should fail, if the user has no internet access. Currently, the write to the db never fails and Firebase just waits till the user has internet access. The problem here is that when I click multiple times, the write is executed multiple times. But I think I have to implement a bit more logic here and the above written code is fine like it currently is.

Firestore query on Kotlin from Non-Activity class [duplicate]

I'm building an app for a friend and I use Firestore. What I want is to display a list of favorite places but for some reason, the list is always empty.
I cannot get the data from Firestore. This is my code:
fun getListOfPlaces() : List<String> {
val places = ArrayList<String>()
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
for (document in task.result) {
val name = document.data["name"].toString()
places.add(name)
}
}
}
return list;
}
If I try to print, let's say the size of the list in onCreate function, the size is always 0.
Log.d("TAG", getListOfPlaces().size().toString()); // Is 0 !!!
I can confirm Firebase is successfully installed.
What am I missing?

This is a classic issue with asynchronous web APIs. You cannot return something now, that hasn't been loaded yet. With other words, you cannot simply return the places list as a result of a method because it will always be empty due the asynchronous behavior of the onComplete function. Depending on your connection speed and the state, it may take from a few hundred milliseconds to a few seconds before that data is available.
But not only Cloud Firestore loads data asynchronously, almost all of modern other web APIs do, since it may take some time to get the data. But let's take an quick example, by placing a few log statements in the code, to see more clearly what I'm talking about.
fun getListOfPlaces() : List<String> {
Log.d("TAG", "Before attaching the listener!");
val places = ArrayList<String>()
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
Log.d("TAG", "Inside onComplete function!");
for (document in task.result) {
val name = document.data["name"].toString()
places.add(name)
}
}
}
Log.d("TAG", "After attaching the listener!");
return list;
}
If we run this code will, the output in your logcat will be:
Before attaching the listener!
After attaching the listener!
Inside onComplete function!
This is probably not what you expected, but it explains precisely why your places list is empty when returning it.
The initial response for most developers is to try and "fix" this asynchronous behavior, which I personally recommend against it. Here is an excelent article written by Doug Stevenson that I'll highly recommend you to read.
A quick solve for this problem would be to use the places list only inside the onComplete function:
fun readData() {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
//Do what you need to do with your list
}
}
}
If you want to use the list outside, there is another approach. You need to create your own callback to wait for Firestore to return you the data. To achieve this, first you need to create an interface like this:
interface MyCallback {
fun onCallback(value: List<String>)
}
Then you need to create a function that is actually getting the data from the database. This method should look like this:
fun readData(myCallback : MyCallback) {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
myCallback.onCallback(list)
}
}
}
See, we don't have any return type anymore. In the end just simply call readData() function in your onCreate function and pass an instance of the MyCallback interface as an argument like this:
readData(object: MyCallback {
override fun onCallback(value: List<String>) {
Log.d("TAG", list.size.toString())
}
})
If you are using Kotlin, please check the other answer.

Nowadays, Kotlin provides a simpler way to achieve the same result as in the case of using a callback. This answer is going to explain how to use Kotlin Coroutines. In order to make it work, we need to add the following dependency in our build.gradle file:
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.2.1"
This library that we use is called Module kotlinx-coroutines-play-services and is used for the exact same purpose. As we already know, there is no way we can return a list of objects as a result of a method because get() returns immediately, while the callback from the Task it returns will be called sometime later. That's the reason why we should wait until the data is available.
When calling "get()" on the Task object that is returned, we can attach a listener so we can get the result of our query. What we need to do now is to convert this into something that is working with Kotlin Coroutines. For that, we need to create a suspend function that looks like this:
private suspend fun getListOfPlaces(): List<DocumentSnapshot> {
val snapshot = placesRef.get().await()
return snapshot.documents
}
As you can see, we have now an extension function called await() that will interrupt the Coroutine until the data from the database is available and then return it. Now we can simply call it from another suspend method like in the following lines of code:
private suspend fun getDataFromFirestore() {
try {
val listOfPlaces = getListOfPlaces()
} catch (e: Exception) {
Log.d(TAG, e.getMessage()) //Don't ignore potential errors!
}
}

The reason for having a empty list got perfectly answered by Alex Mamo above.
I just like to present the same thing without needing to add an extra interface.
In Kotlin you could just implement it like so:
fun readData(myCallback: (List<String>) -> Unit) {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
myCallback(list)
}
}
}
and then use it like so:
readData() {
Log.d("TAG", it.size.toString())
})

Firebase repository MVVM pattern [duplicate]

I'm building an app for a friend and I use Firestore. What I want is to display a list of favorite places but for some reason, the list is always empty.
I cannot get the data from Firestore. This is my code:
fun getListOfPlaces() : List<String> {
val places = ArrayList<String>()
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
for (document in task.result) {
val name = document.data["name"].toString()
places.add(name)
}
}
}
return list;
}
If I try to print, let's say the size of the list in onCreate function, the size is always 0.
Log.d("TAG", getListOfPlaces().size().toString()); // Is 0 !!!
I can confirm Firebase is successfully installed.
What am I missing?

This is a classic issue with asynchronous web APIs. You cannot return something now, that hasn't been loaded yet. With other words, you cannot simply return the places list as a result of a method because it will always be empty due the asynchronous behavior of the onComplete function. Depending on your connection speed and the state, it may take from a few hundred milliseconds to a few seconds before that data is available.
But not only Cloud Firestore loads data asynchronously, almost all of modern other web APIs do, since it may take some time to get the data. But let's take an quick example, by placing a few log statements in the code, to see more clearly what I'm talking about.
fun getListOfPlaces() : List<String> {
Log.d("TAG", "Before attaching the listener!");
val places = ArrayList<String>()
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
Log.d("TAG", "Inside onComplete function!");
for (document in task.result) {
val name = document.data["name"].toString()
places.add(name)
}
}
}
Log.d("TAG", "After attaching the listener!");
return list;
}
If we run this code will, the output in your logcat will be:
Before attaching the listener!
After attaching the listener!
Inside onComplete function!
This is probably not what you expected, but it explains precisely why your places list is empty when returning it.
The initial response for most developers is to try and "fix" this asynchronous behavior, which I personally recommend against it. Here is an excelent article written by Doug Stevenson that I'll highly recommend you to read.
A quick solve for this problem would be to use the places list only inside the onComplete function:
fun readData() {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
//Do what you need to do with your list
}
}
}
If you want to use the list outside, there is another approach. You need to create your own callback to wait for Firestore to return you the data. To achieve this, first you need to create an interface like this:
interface MyCallback {
fun onCallback(value: List<String>)
}
Then you need to create a function that is actually getting the data from the database. This method should look like this:
fun readData(myCallback : MyCallback) {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
myCallback.onCallback(list)
}
}
}
See, we don't have any return type anymore. In the end just simply call readData() function in your onCreate function and pass an instance of the MyCallback interface as an argument like this:
readData(object: MyCallback {
override fun onCallback(value: List<String>) {
Log.d("TAG", list.size.toString())
}
})
If you are using Kotlin, please check the other answer.

Nowadays, Kotlin provides a simpler way to achieve the same result as in the case of using a callback. This answer is going to explain how to use Kotlin Coroutines. In order to make it work, we need to add the following dependency in our build.gradle file:
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.2.1"
This library that we use is called Module kotlinx-coroutines-play-services and is used for the exact same purpose. As we already know, there is no way we can return a list of objects as a result of a method because get() returns immediately, while the callback from the Task it returns will be called sometime later. That's the reason why we should wait until the data is available.
When calling "get()" on the Task object that is returned, we can attach a listener so we can get the result of our query. What we need to do now is to convert this into something that is working with Kotlin Coroutines. For that, we need to create a suspend function that looks like this:
private suspend fun getListOfPlaces(): List<DocumentSnapshot> {
val snapshot = placesRef.get().await()
return snapshot.documents
}
As you can see, we have now an extension function called await() that will interrupt the Coroutine until the data from the database is available and then return it. Now we can simply call it from another suspend method like in the following lines of code:
private suspend fun getDataFromFirestore() {
try {
val listOfPlaces = getListOfPlaces()
} catch (e: Exception) {
Log.d(TAG, e.getMessage()) //Don't ignore potential errors!
}
}

The reason for having a empty list got perfectly answered by Alex Mamo above.
I just like to present the same thing without needing to add an extra interface.
In Kotlin you could just implement it like so:
fun readData(myCallback: (List<String>) -> Unit) {
placesRef.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
val list = ArrayList<String>()
for (document in task.result) {
val name = document.data["name"].toString()
list.add(name)
}
myCallback(list)
}
}
}
and then use it like so:
readData() {
Log.d("TAG", it.size.toString())
})