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Why my flow doesn't triggers, when data changes in RoomDao

I'm using RoomDao with kotlin coroutines and Flow. What I'm trying to do is collect one Training with all its Exercises with all Repetitions per Exercise. Exercises and Repetitions are Flows, cuz this values can be changed and I want to observe them.
The problem is that when I updating exercises, getTrainingExerciseLinksBy doesn't triggers, and I don't know, why. Here is my code in UseCase:
suspend fun getTrainingWithExercisesAndRepetitionsBy(trainingId: Long): Flow<UiTrainingWithExercisesAndRepetitions> {
/// This method returns Flow<List<TrainingExerciseLink>>
return trainingExerciseLinksRepository.getTrainingExerciseLinksBy(trainingId).flatMapConcat { trainingExerciseLinks ->
trainingExerciseLinks.map { trainingExerciseLink ->
/// This method returns Flow<List<ExerciseRepetition>>
repetitionsRepository.getExerciseRepetitionsBy(trainingExerciseLink.id).map { repetitions ->
/// do some other selects for collecting data about exercise in one training
}.flowOn(Dispatchers.IO)
}.zipFlows()
}.flowOn(Dispatchers.IO)
}
In my ViewModel I'm observing this method like this:
viewModelScope.launch {
useCase.getTrainingWithExercisesAndRepetitionsBy(trainingId)
.distinctUntilChanged()
.collect {
_exercisesListLiveData.value = it.exercises
_trainingListLiveData.value = it.trainingData
}
}
What is wrong with this code?
UPD:
In my DAO I'm using Flows for subscribing on database's updates, like this:
#Dao
abstract class TrainingExerciseLinkDao {
#Query("select * from TrainingExerciseLink where trainingId = :trainingId")
abstract fun getTrainingExerciseLinksBy(trainingId: Long): Flow<List<TrainingExerciseLink>>
}
and ExerciseRepetitionsDao:
#Dao
abstract class ExerciseRepetitionDao {
#Query("select * from ExerciseRepetitionEntity where trainingExerciseId = :trainingExerciseId")
abstract fun getExerciseRepetitionsBy(trainingExerciseId: Long): Flow<List<ExerciseRepetitionEntity>>
}

Actually I found the answer, so maybe somebody will jump in the same gap and this thread will be helpful.
The problem in my code was that I used flatMapConcat. This operator waits emits from original Flow and from flatMapped Flow at one time, so in this case it will trigger callback. To fix this, flatMapLatest should be used. You can read more about difference between this operators here.
So my code now looks like this:
suspend fun getTrainingWithExercisesAndRepetitionsBy(trainingId: Long): Flow<UiTrainingWithExercisesAndRepetitions> {
/// This method returns Flow<List<TrainingExerciseLink>>
/// Here is main change: flatMapConcat -> flatMapLatest
return trainingExerciseLinksRepository.getTrainingExerciseLinksBy(trainingId).flatMapLatest { trainingExerciseLinks ->
trainingExerciseLinks.map { trainingExerciseLink ->
/// This method returns Flow<List<ExerciseRepetition>>
repetitionsRepository.getExerciseRepetitionsBy(trainingExerciseLink.id).map { repetitions ->
/// do some other selects for collecting data about exercise in one training
}.flowOn(Dispatchers.IO)
}.zipFlows()
}.flowOn(Dispatchers.IO)
}

You are using it wrong , as when database updates your getTrainingWithExercisesAndRepetitionsBy does not know,
to get over this issue use flows in your dao like this example as Room supports Flow then
viewModelScope.launch {
viewModel.yourFunctionThatGetsDataFromRepository(trainingId)
.distinctUntilChanged()
.collect {
_exercisesListLiveData.value = it.exercises
_trainingListLiveData.value = it.trainingData
}
}
and if more you can refer this example

Retrofit call, blocked in MessageQueue

I would like to have a system to call API (Retrofit) with cache (in Room), with just coroutines (without LiveData and NetworkBoundResource).
So worflow is:
Check data in db
if present show it
if not:
Call API
Save data in db
show data
Problem app blocked in "Call API" step, here the stack
nativePollOnce:-1, MessageQueue (android.os) next:326, MessageQueue
(android.os) loop:160, Looper (android.os) main:6669, ActivityThread
(android.app) invoke:-1, Method (java.lang.reflect) run:493,
RuntimeInit$MethodAndArgsCaller (com.android.internal.os) main:858,
ZygoteInit (com.android.internal.os)
Retrofit service:
interface ProductService {
#GET("products")
suspend fun getProducts(): Response<List<Product>>
}
DAO Room:
#Dao
interface ProductDao {
#Query("SELECT * FROM Product ORDER BY price")
suspend fun getProducts(): List<Product>
#Transaction
#Insert(entity = Product::class)
suspend fun insertProducts(products: List<Product>)
}
My fragment:
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
productService = createProductService()
productDao = MyDatabase.getDatabase(requireContext()).productDao()
CoroutineScope(Dispatchers.Main).launch {
getProducts()
}
}
private suspend fun getProducts() {
progressBar.visibility = View.VISIBLE
recyclerViewProducts.visibility = View.GONE
var products = withContext(Dispatchers.IO){ productDao.getProducts() }
if(products.isEmpty()) {
val response = withContext(Dispatchers.IO) { productService.getProducts() }
if(response.isSuccessful && response.body() != null) {
products = response.body()!!
withContext(Dispatchers.IO) { productDao.insertProducts(products) }
}
}
withContext(Dispatchers.Main) {
progressBar.visibility = View.GONE
recyclerViewProducts.visibility = View.VISIBLE
recyclerViewProducts.apply {
layoutManager = LinearLayoutManager(context)
// set the custom adapter to the RecyclerView
adapter = ProductsAdapter(products, this#ListProductFragment)
}
}
}

This is not clean architecture. You should have a Database layer (that you have) and a Repository and a Viewmodel. So when framgnet is created calls viewmodel to observe data from repository that also observe data from Db. If data from db is empty then it creates the api call in a coroutine scope and in the same thread it saves data to DB. So automatically viewmodel gets informed for new data

I recommend using MVVM Design Pattern. You must do what you want in the repository pattern.
The repository pattern is a design pattern that isolates data sources
from the rest of the app. A repository mediates between data sources
(such as persistent models, web services, and caches) and the rest of
the app. The diagram below shows how app components such as activities
that use LiveData might interact with data sources by way of a
repository. To implement a repository, you use a repository class,
such as the VideosRepository class that you create in the next task.
The repository class isolates the data sources from the rest of the
app and provides a clean API for data access to the rest of the app.
Using a repository class is a recommended best practice for code
separation and architecture. Advantages of using a repository. A
repository module handles data operations and allows you to use
multiple backends. In a typical real-world app, the repository
implements the logic for deciding whether to fetch data from a network
or use results that are cached in a local database. This helps make
your code modular and testable. You can easily mock up the repository
and test the rest of the code.
I suggest you check it out for the error code.
Control

How can I get the query result at once when I use Room?

I hope to get the total of all records with Room database at once. But, normally Room use background thread to query record asynchronously.
If I use getTotalOfVoiceAsLiveData() in Code A, it will return LiveData<Long>, you know that LiveData variable is lazy, maybe the result is null.
If I use getTotalOfVoice() in Code A, I will get error because I can't use return in viewModelScope.launch{ }.
How can I get the total of all records at once with Room database?
Code A
class HomeViewModel(val mApplication: Application, private val mDBVoiceRepository: DBVoiceRepository) : AndroidViewModel(mApplication) {
fun getTotalOfVoice():Long {
viewModelScope.launch {
return mDBVoiceRepository.getTotalOfVoice() //It will cause error
}
}
fun getTotalOfVoiceAsLiveData(): LiveData<Long>{
return mDBVoiceRepository.getTotalOfVoiceAsLiveData() //It's lazy, maybe the result is null.
}
}
class DBVoiceRepository private constructor(private val mDBVoiceDao: DBVoiceDao){
suspend fun getTotalOfVoice() = mDBVoiceDao.getTotalOfVoice()
fun getTotalOfVoiceAsLiveData() = mDBVoiceDao.getTotalOfVoiceAsLiveData()
}
#Dao
interface DBVoiceDao{
#Query("SELECT count(id) FROM voice_table")
suspend fun getTotalOfVoice(): Long
//When Room queries return LiveData, the queries are automatically run asynchronously on a background thread.
#Query("SELECT count(id) FROM voice_table")
fun getTotalOfVoiceAsLiveData(): LiveData<Long>
}
Add content
To Tobi: Thanks!
Why it is important to you to get the data directly?
I need to generate a filename based the total of the records, such as "untitled0", "untitled1", "untitled2"...
If I can get the query result at once, I can use the following code easyly.
Added again
I hope to record a voice by filename based the total of query records when I click Start button. You know the total of records will change when a reocrd is added or deleted!
Code B
fun getTotalOfVoice():Long {
//Get the query result at once
...
}
fun createdFileanme(){
return "untitled"+getTotalOfVoice().toString()
}
btnStart.setOnClickListener{
recordVoice(createdFileanme()) //I will record voice by filename
}
fun addRecord(){
...
}
fun deleteRecord(){
...
}
New added content
Thanks!
I think 'You should also move all of that into the viewmodel class, without LiveData ' is good way, you can see Image A and How can I get the value of a LivaData<String> at once in Android Studio? .
Do you agree with it?
Image A

Question: at once meaning synchronous or what ? if yes, what happens if the function to get the result has to take a longer time? like network call? well you can decide to do that on another thread.
What I think is for you to use a mutable Object and use the postValue function to dispatch the result to the observers. It should look something like below:
class HomeViewModel(val mApplication: Application, private val mDBVoiceRepository: DBVoiceRepository) : AndroidViewModel(mApplication) {
private val voices = MutableLiveData<Long>()
fun getTotalOfVoiceAsLiveData(): LiveData<Long> {
voices.postValue(mDBVoiceRepository.getTotalOfVoiceAsLiveData().value)
return voices;
}
}
Making use of it in your Fragment will look like below:
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
if (activity != null) {
val viewModel = ViewModelProvider(requireActivity())
viewModel.get(HomeViewModel::class.java).getTotalOfVoiceAsLiveData().observe(viewLifecycleOwner, Observer { voices: Long ? ->
voices // Sound of music ? be very free to use ...
})
}
}
Happy Coding.

I hope to get the result at once, but LiveData is lazy
Sorry to tell, but this is how the Room interface is designed.
You are right with the lazyness of the returned LiveData object. But this allows you to handle it on a different thread without having to manually handle different threads.
Based on your new information!
You basically have two options:
A) you could do the following:
load data from Room via LivaData
add observer that stores the current total amount
when the button is clicked you just read the local copy
In your View: (only one observer and one clickListener)
val totalVoiceCount: long
val viewModel = ViewModelProvider(requireActivity()).get(HomeViewModel::class.java)
viewModel.getTotalOfVoiceAsLiveData().observe(viewLifecycleOwner, Observer { totalOfVoice : Long ? ->
if (totalOfVoice != null)
totalVoiceCount = totalOfVoice
})
btnStart.setOnClickListener{
viewModel.recordVoice(totalVoiceCount)
}
In your ViewModel: (the logic and everything else)
fun recordVoice(totalVoiceCount : long){
val fileName = createdFileanme(totalVoiceCount)
// create your recording // depending on how you do this, it probably runs on a background thread anyways
}
fun createdFileName(totalVoiceCount : long){
return "untitled"+ String.valueOf(totalVoiceCount)
}
This works reliably because the LiveData has enough time to update the local copy of totalVoiceCount before the user has the chance to click the button.
B) Based on the answer in your parallel question you can of course outsource even more to a background thread. Then you also have the option to call the DAO query with a non-livedata return (as room returns non-livedata queries only on background threads). Is it worth to implement the threading suggestion of Ridcully? Not possible to answer without knowing what else is going on simultaneously... To me it seems like an overkill, but he is right that the more you do on background threads the better for your refresh rate..

You can return Deferred<Long> from viewModelScope.async. I recommend you to use:
val deferred = viewModelScope.async(Dispatchers.IO) {
return#async mDBVoiceRepository.getTotalOfVoice()
}
val value = deferred.await()
await() is suspend
Edit:
If you want to get a getter which will use in your activity or fragment
you need to write a suspend function like this:
suspend fun getTotalOfVoice(): Long {
return viewModelScope.async(Dispatchers.IO) {
return#async mDBVoiceRepository.getTotalOfVoice()
}.await()
}
But mvvm pattern allows you to create LiveData inside your ViewModel, which gives your fragment an observer.
In view model:
private val _totalOfVoiceLD: MutableLiveData<Long> = MutableLiveData()
val totalOfVoiceLD: LiveData<Long>
get() = _totalOfVoiceLD
fun updateTotalOfVoice() {
viewModelScope.launch(Dispatchers.IO) {
val totalOfVoice = mDBVoiceRepository.getTotalOfVoice()
_totalOfVoiceLD.postValue(totalOfVoice)
}
}
and in your fragment:
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel.totalOfVoiceLD.observe(viewLifecycleOwner, Observer { totalOfVoice ->
totalOfVoiceTextView.text = totalOfVoice.toString()
})
}

You can use coroutineContext.async to get data from DB and wait for getting it's response with data by using .await function for a async dispatch.
suspend fun getAllVoices() : Long{
val awatingResults = viewModelScope.async(Dispatchers.IO) {
mDBVoiceRepository.getTotalOfVoice()
}
val records = awatingResults.await()
return records
}
It is necessary to call a Suspend function from a coroutine and
async.await() is always called in a suspended function so,
val voiceLiveData: MutableLiveData<Long> = MutableLiveData()
fun getAllVoicesFromDB() {
viewModelScope.launch(Dispatchers.IO) {
voiceLiveData.postValue(mDBVoiceRepository.getTotalOfVoice())
}
}
Now call it where ever you want to get your voice data from database and also remember do your further work inside your voiceLiveData observer where you get your response of voices :)

Live data is designed to be lazy, when the value of the live data changes internally it emits and wherever you are observing it, the onChange function will be invoked. It is designed to fire and forget.
Because room uses background thread to run the query.
You can't expect live data to behave like sharedpreference where you store key value pair.
If you want to achieve something like that.
I would suggest you to use
Paper Db or Realm.

If you need your Room result synchronously, your code should be execute in IO thread. In case of coroutines, you can use Dispatchers.IO. Your code can be changed to this to pass the error.
class HomeViewModel(val mApplication: Application, private val mDBVoiceRepository: DBVoiceRepository) : AndroidViewModel(mApplication) {
fun getTotalOfVoice():Long {
viewModelScope.launch(Dispatchers.IO) { // here
return mDBVoiceRepository.getTotalOfVoice()
}
}
}

If you must run the queries in the main thread, then:
Allow android room to execute queries in main thread.
val dbInstance = Room
.databaseBuilder(ctx, YourDBClass::class.java, "YourDBName")
.allowMainThreadQueries()
.build()
Define the dao method as follows
#Dao
interface DBVoiceDao{
#Query("SELECT count(id) FROM voice_table")
fun getTotalOfVoice(): Long
}
Access the method in the repository
fun getTotalOfVoice():Long {
return dao.getTotalOfVoice()
}

Emit coroutine Flow from Room while backfilling via network request

I have my architecture like so:
Dao methods returning Flow<T>:
#Query("SELECT * FROM table WHERE id = :id")
fun itemById(id: Int): Flow<Item>
Repository layer returning items from DB but also backfilling from network:
(* Need help here -- this is not working as intended **)
fun items(): Flow<Item> = flow {
// Immediately emit values from DB
emitAll(itemDao.itemById(1))
// Backfill DB via network request without blocking coroutine
itemApi.makeRequest()
.also { insert(it) }
}
ViewModel layer taking the flow, applying any transformations, and converting it into a LiveData using .asLiveData():
fun observeItem(): LiveData<Item> = itemRepository.getItemFlow()
.map { // apply transformation to view model }
.asLiveData()
Fragment observing LiveData emissions and updating UI:
viewModel.item().observeNotNull(viewLifecycleOwner) {
renderUI(it)
}
The issue I'm having is at step 2. I can't seem to figure out a way to structure the logic so that I can emit the items from Flow immediately, but also perform the network fetch without waiting.
Since the fetch from network logic is in the same suspend function it'll wait for the network request to finish before emitting the results downstream. But I just want to fire that request independently since I'm not interested in waiting for a result (when it comes back, it'll update Room and I'll get the results naturally).
Any thoughts?
EDIT
Marko's solution works well for me, but I did attempt a similar approach like so:
suspend fun items(): Flow<List<Cryptocurrency>> = coroutineScope {
launch {
itemApi.makeRequest().also { insert(it) }
}
itemDao.itemById(1)
}

It sounds like you're describing a background task that you want to launch. For that you need access to your coroutine scope, so items() should be an extension function on CoroutineScope:
fun CoroutineScope.items(): Flow<Item> {
launch {
itemApi.makeRequest().also { insert(it) }
}
return flow {
emitAll(itemDao.itemById(1))
}
}
On the other hand, if you'd like to start a remote fetch whose result will also become a part of the response, you can do it as follows:
fun items(): Flow<Item> = flow {
coroutineScope {
val lateItem = async { itemApi.makeRequest().also { insert(it) } }
emitAll(itemDao.itemById(1))
emit(lateItem.await())
}
}

Room with Flowable: initialize database if it's empty

I have following #Dao, that provides Flowable<User> stream:
#Dao
interface UsersDao {
#Query("SELECT * FROM users")
fun loadUsers(): Flowable<List<User>>
}
I want the subscriber of the stream to receive updates of the database as soon as some change happens there. Subscribing to Room's Flowable I will get that feature out of the box.
What I want is following: if database is empty I want to perform a web request and save users into database. The subscriber will automatically receive new updates that had just happened.
Now I want the client of the repository not to be aware all of the initialization logics: all he does - he performs usersRepository.loadUsers(). And all of these magic should take place inside the repository class:
class UsersRepository #Inject constructor(
private val api: Api,
private val db: UsersDao
) {
fun loadUsers(): Flowable<List<User>> {
...
}
}
Of course I can use following approach:
fun loadUsers(): Flowable<List<User>> {
return db.loadTables()
.doOnSubscribe {
if (db.getCount() == 0) {
val list = api.getTables().blockingGet()
db.insert(list)
}
}
}
But I would like to construct the stream without using side-effects (doOn... operators). I've tried composing() but that didn't help much. Been stuck on this for a while.

You could apply some conditional flatMaps:
#Dao
interface UsersDao {
#Query("SELECT * FROM users")
fun loadUsers(): Flowable<List<User>>
#Query("SELECT COUNT(1) FROM users")
fun userCount() : Flowable<List<Integer>>
#Insert // I don't know Room btw.
fun insertUsers(List<User> users) : Flowable<Object>
}
interface RemoteUsers {
fun getUsers() : Flowable<List<User>>
}
fun getUsers() : Flowable<List<User>> {
return
db.userCount()
.take(1)
.flatMap({ counts ->
if (counts.isEmpty() || counts.get(0) == 0) {
return remote.getUsers()
.flatMap({ users -> db.insertUsers(users) })
.ignoreElements()
.andThen(db.loadUsers())
}
return db.loadUsers()
})
}
Disclaimer: I don't know Room so please adapt the example above as the features of it allow.

Assuming your insert() call is async and also handles updates, you could do something like this:
fun loadUsers(): Flowable<List<User>> = userDao.getAllUsers().switchIfEmpty { api.getAllUsers().doOnNext { userDao.insert(it) } }
You could also use some:
fun loadUsers(): Flowable<List<User>> = userDao.getAllUsers().flatMap { it-> if (it.isEmpty()) api.getAllUsers().doOnNext { userDao.insert(it) } else Flowable.just(it)}
Advice:
You should consider the case when the data is stale, therefore you need to go another way around, do a network request and database call at the same time. Whichever observable finish first, take the result and display it. Updating database should be right after network call is done.