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How should I go about implementing MVVM architecture pattern in my project?

I know this is a very documented topic, but I couldn't find a way to implement it in my project, even after spending hours trying to figure it out.
My root problem is that I have a RecyclerView with an Adapter whose content isn't updating as I'd like. I'm a beginner in Android, so I didn't implement any MVVM or such architecture, and my project only contains a repository, fetching data from Firebase Database, and passing it to a list of ShowModel, a copy of said list being used in my Adapter to display my shows (In order to filter/sort them without modifying the list with all shows).
However, when adding a show to the database from another Activity, my Adapter isn't displaying the newly added show (as detailed here)
I was told to use LiveData and ViewModel, but even though I started understanding how it works after spending time researching it, I don't fully get how I should use it in order to implement it in my project.
Currently I have the following classes:
The Adapter:
class ShowAdapter(private val context: MainActivity, private val layoutId: Int, private val textNoResult: TextView?) : RecyclerView.Adapter<ShowAdapter.ViewHolder>(), Filterable {
var displayList = ArrayList(showList)
class ViewHolder(view : View) : RecyclerView.ViewHolder(view){
val showName: TextView = view.findViewById(R.id.show_name)
val showMenuIcon: ImageView = view.findViewById(R.id.menu_icon)
}
#SuppressLint("NewApi")
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
val view = LayoutInflater.from(parent.context).inflate(layoutId, parent, false)
return ViewHolder(view)
}
#SuppressLint("NewApi", "WeekBasedYear")
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val currentShow = displayList[position]
val index = holder.adapterPosition
holder.showName.text = currentShow.name
holder.itemView.setOnClickListener{ // Display show content
val intent = Intent(context, DetailsActivity::class.java)
intent.putExtra("position", index)
startActivity(context, intent, null)
}
holder.showMenuIcon.setOnClickListener{
val popupMenu = PopupMenu(context, it)
popupMenu.menuInflater.inflate(R.menu.show_management_menu, popupMenu.menu)
popupMenu.show()
popupMenu.setOnMenuItemClickListener {
when(it.itemId){
R.id.edit -> { // Edit show
val intent = Intent(context, AddShowActivity::class.java)
intent.putExtra("position", index)
startActivity(context, intent, null)
return#setOnMenuItemClickListener true
}
R.id.delete -> { // Delete show
val repo = ShowRepository()
repo.deleteShow(currentShow)
displayList.remove(currentShow)
notifyItemRemoved(index)
return#setOnMenuItemClickListener true
}
else -> false
}
}
}
}
override fun getItemCount(): Int = displayList.size
// Sorting/Filtering methods
}
The fragment displaying the adapter:
class HomeFragment : Fragment() {
private lateinit var context: MainActivity
private lateinit var verticalRecyclerView: RecyclerView
private lateinit var buttonAddShow: Button
private lateinit var showsAdapter: ShowAdapter
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
val view = inflater.inflate(R.layout.fragment_home, container, false)
context = getContext() as MainActivity
buttonAddShow = view.findViewById(R.id.home_button_add_show)
buttonAddShow.setOnClickListener{ // Starts activity to add a show
startActivity(Intent(context, AddShowActivity::class.java))
}
verticalRecyclerView = view.findViewById(R.id.home_recycler_view)
showsAdapter = ShowAdapter(context, R.layout.item_show, null)
verticalRecyclerView.adapter = showsAdapter
return view
}
}
The MainActivity:
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
loadFragment(HomeFragment())
}
private fun loadFragment(fragment: Fragment){
val repo = ShowRepository()
if(showsListener != null) databaseRef.removeEventListener(showsListener!!)
repo.updateData{
val transaction = supportFragmentManager.beginTransaction()
transaction.replace(R.id.fragment_container, fragment)
transaction.addToBackStack(null)
if(supportFragmentManager.isStateSaved)transaction.commitAllowingStateLoss()
else transaction.commit()
}
}
}
The repository:
class ShowRepository {
object Singleton{
val databaseRef = FirebaseDatabase.getInstance().getReference("shows")
val showList = arrayListOf<ShowModel>()
var showsListener: ValueEventListener? = null
}
fun updateData(callback: () -> Unit){
showsListener = databaseRef.addValueEventListener(object : ValueEventListener {
override fun onDataChange(snapshot: DataSnapshot) {
showList.clear()
for(ds in snapshot.children){
val show = ds.getValue(ShowModel::class.java)
if(show != null) showList.add(show)
}
callback()
}
override fun onCancelled(p0: DatabaseError) { }
})
}
fun insertShow(show: ShowModel){
databaseRef.child(show.id).setValue(show)
}
fun deleteShow(show: ShowModel){
databaseRef.child(show.id).removeValue()
}
}
From what I understand of LiveData and ViewModel, what I should do is creating a ShowViewModel containing a MutableLiveData<List<ShowModel>> containing the shows, and then observe it in my HomeFragment and update the adapter depending on the changes happening. However, everytime I start something to implement it, I encounter a situation where I'm lost and don't know what I should do, which leads me back to square one once again. I've been trying this for more than a week without progressing even a little bit, and that's why I'm here, hoping for some insight.
Sorry for the silly question and the absurd amount of informations, and hoping someone will be able to help me understand what I do wrong/should do.

(this ended up longer than I meant it to be - hope it's not too much! There's a lot to learn, but you don't have to make it super complicated at first)
Broadly, working backwards, it should go like this:
Adapter
displays whatever the Fragment tells it to (some kind of setData function that updates its internal list and refreshes)
passes events to the Fragment (deleteItem(item), showDetails(item) etc.) - don't have the Adapter doing things like starting Activites, that's not its responsibility
Fragment
grabs a reference to any ViewModels (only certain components like Fragments and Activities can actually "own" them)
observes any LiveData (or collects Flows if you're doing it that way) on the VM, and updates stuff in the UI in response
e.g. model.shows.observe(viewLifecycleOwner) { shows -> adapter.setData(shows) }
handles UI events and calls methods on the VM in response, e.g. click listeners, events from the Adapter
ViewModel
acts as a go-between for the UI (the Fragment) and the data layer (the repository)
exposes methods for handling events like deleting items, interacts with the data layer as required (e.g. calling the appropriate delete function)
exposes data state for the UI to observe, so it can react to changes/updates (e.g. a LiveData containing the current list of shows that the data layer has provided)
That's the basic setup - the VM exposes data which the UI layer observes and reacts to, by displaying it. The UI layer also produces events (usually down to user interaction) which are passed to the VM. You can read more about this general approach in this guide about app architecture - it's worth reading because not only is it recommended as a way to build apps, a lot of the components you use in modern Android are designed with this kind of approach in mind (like the reactive model of wiring stuff up).
You could handle the Adapter events like this:
// in your Adapter
var itemDeletedListener: ((Item) -> Unit)? = null
// when the delete event happens for an item
itemDeletedListener?.invoke(item)
// in your Fragment
adapter.itemDeletedListener = { viewModel.deleteItem(it) }
which is easier than implementing an interface, and lets you wire up your Adapter similar to doing setOnClickListener on a button. Notice we're passing the actual Item object here instead of a list index - generally this is easier to work with, you don't need to maintain multiple copies of a list just so you can look up an index given to you by something else. Passing a unique ID can make sense though, especially if you're working with a database! But usually the object itself is more useful and consistent
The data layer is the tricky bit - the ViewModel needs to communicate with that to get the current state. Say you delete an item - you then need to get the current, updated list of shows. You have three approaches:
Call the delete function, immediately after fetch the current data, and set it on the appropriate LiveData
This can work, but it's not very reactive - you're doing one action, then immediately doing another because you know your data is stale. It would be better if the new data just arrived automatically and you could react to that by pushing it out. The other issue is that calling the delete function might not have an immediate effect - if you fetch the current data, nothing might have changed yet. It's better if the data layer is responsible for announcing updates.
This is the simplest approach though, and probably a good start! You could run this task in a coroutine (viewModelScope.launch { // delete and fetch and update LiveData }) so any slowness doesn't block the current thread.
Have the data layer's functions return the current, updated data that results
Similar to above, you're just sort of pushing the fetching into the data layer. This requires all those functions to be written to return the current state, which could take a while! And depending on what data you want, this might be impossible - if you have an active query on some data, how does the function know what specific data to return?
Make the ViewModel observe the data it wants, so when the data layer updates, you get the results automatically
This is the recommended reactive approach - again it's that two-way idea. The VM calling a function on the data layer is completely separate from the VM receiving new data. One thing just happens as a natural consequence of the other, they don't need to be tied together. You just need to wire them up right!
How do you actually do that though? If you're working with something like Room, that's already baked in. Queries can return async data providers like LiveData or Flows - your VM just needs to observe those and expose the results, or just expose them directly. That way, when a table is updated, any queries (like the current shows) push a new value, and the observers receive it and do whatever they need to do, like telling the Adapter to display the data. It all Just Works once it's wired up.
Since you have your own repo, you need to expose your own data sources. You could have a currentShows LiveData or (probably preferably) the flow equivalent, StateFlow. When the repo initialises, and when any data is changed, it updates that currentShows data. Anything observing that (e.g. the VM, the Fragment through a LiveData/Flow that the VM exposes) will automatically get the new values. So broadly:
// Repo
// this setup is exactly the same as your typical LiveData, except you need an initial value
private val _currentShows = MutableStateFlow<List<Show>>(emptyList()) // or whatever default
val currentShows: StateFlow<List<Show>> = _currentShows
fun deleteItem(item: Item) {
// do the deletion
// get the updated show list
_currentShows.value = updatedShowList
}
// ViewModel
// one way of doing things - you have a lot of options! This literally just exposes
// the state from the data layer, and turns it into a LiveData (if you want that)
val currentShows = repo.currentShows.asLiveData()
// Fragment
// wire things up so you handle new data as it arrives
viewModel.currentShows.observe(viewLifecycleOwner) { shows -> adapter.setData(shows) }
That's basically it. I've skimmed over a lot because honestly, there's a lot to learn with this - especially about Flows and coroutines if you're not already familiar with those. But hopefully that gives you an overview of the general idea, and don't be afraid to take shortcuts (like just updating your data in the ViewModel by setting its LiveData values) while you're learning and getting the hang of it. Definitely give that app architecture guide a read, and also the guides for ViewModels and LiveData. It'll start to click when you get the general idea!

ListAdapter Diff does not dispatch updates on same list instance, but neither on different list from LiveData

it is a known issue that ListAdapter (actually the AsyncListDiffer from its implementation) does not update the list if the new list only has modified items but has the same instance. The updates do not work on new instance list either if you use the same objects inside.
For all of this to work, you have to create a hard copy of the entire list and objects inside.
Easiest way to achieve this:
items.toMutableList().map { it.copy() }
But I am facing a rather weird issue. I have a parse function in my ViewModel that finally posts the items.toMutableList().map { it.copy() } to the LiveData and gets observes in the fragment. Even with the hard copy, DiffUtil does not work. If I move the hard copy inside the fragment, then it works.
To get this easier, if I do this:
IN VIEW MODEL:
[ ... ] parse stuff here
items.toMutableList().map { it.copy() }
restaurants.postValue(items)
IN FRAGMENT:
restaurants.observe(viewLifecycleOwner, Observer { items ->
adapter.submitList(items)
... then, it doesn't work. But if I do this:
IN VIEW MODEL:
[ ... ] parse stuff here
restaurants.postValue(items)
IN FRAGMENT:
restaurants.observe(viewLifecycleOwner, Observer { items ->
adapter.submitList(items.toMutableList().map { it.copy() })
... then it works.
Can anybody explain why this doesn't work?
In the mean time, I have opened an issue on the Google Issue Tracker because maybe they will fix the AsyncListDiffer not updating same instance lists or items. It defeats the purpose of the new adapter. The AsyncListDiffer SHOULD ALWAYS accept same instance lists or items, and fully update using the diff logic that the user customises in the adapter.

I made a quick sample using DiffUtil.Callback and ListAdapter<T, K> (so I called submitList(...) on the adapter), and had no issues.
Then I modified the adapter to be a normal RecyclerView.Adapter and constructed an AsyncDiffUtil inside of it (using the same DiffUtil.Callback from above).
The architecture is:
Activity -> Fragment (contains RecyclerView).
Adapter
ViewModel
"Fake Repository" that simply holds a val source: MutableList<Thing> = mutableListOf()
Model
I've created a Thing object: data class Thing(val name: String = "", val age: Int = 0).
For readability I added typealias Things = List<Thing> (less typing). ;)
Repository
It's fake in the sense that items are created like:
private fun makeThings(total: Int = 20): List<Thing> {
val things: MutableList<Thing> = mutableListOf()
for (i in 1..total) {
things.add(Thing("Name: $i", age = i + 18))
}
return things
}
But the "source" is a mutableList of (the typealias).
The other thing the repo can do is "simulate" a modification on a random item. I simply create a new data class instance, since it's obviously all immutable data types (as they should be). Remember this is just simulating a real change that may have come from an API or DB.
fun modifyItemAt(pos: Int = 0) {
if (source.isEmpty() || source.size <= pos) return
val thing = source[pos]
val newAge = thing.age + 1
val newThing = Thing("Name: $newAge", newAge)
source.removeAt(pos)
source.add(pos, newThing)
}
ViewModel
Nothing fancy here, it talks and holds the reference to the ThingsRepository, and exposes a LiveData:
private val _state = MutableLiveData<ThingsState>(ThingsState.Empty)
val state: LiveData<ThingsState> = _state
And the "state" is:
sealed class ThingsState {
object Empty : ThingsState()
object Loading : ThingsState()
data class Loaded(val things: Things) : ThingsState()
}
The viewModel has two public methods (Aside from the val state):
fun fetchData() {
viewModelScope.launch(Dispatchers.IO) {
_state.postValue(ThingsState.Loaded(repository.fetchAllTheThings()))
}
}
fun modifyData(atPosition: Int) {
repository.modifyItemAt(atPosition)
fetchData()
}
Nothing special, just a way to modify a random item by position (remember this is just a quick hack to test it).
So FetchData, launches the async code in IO to "fetch" (in reality, if the list is there, the cached list is returned, only the 1st time the data is "made" in the repo).
Modify data is simpler, calls modify on the repo and fetch data to post the new value.
Adapter
Lots of boilerplate... but as discussed, it's just an Adapter:
class ThingAdapter(private val itemClickCallback: ThingClickCallback) :
RecyclerView.Adapter<RecyclerView.ViewHolder>() {
The ThingClickCallback is just:
interface ThingClickCallback {
fun onThingClicked(atPosition: Int)
}
This Adapter now has an AsyncDiffer...
private val differ = AsyncListDiffer(this, DiffUtilCallback())
this in this context is the actual adapter (needed by the differ) and DiffUtilCallback is just a DiffUtil.Callback implementation:
internal class DiffUtilCallback : DiffUtil.ItemCallback<Thing>() {
override fun areItemsTheSame(oldItem: Thing, newItem: Thing): Boolean {
return oldItem.name == newItem.name
}
override fun areContentsTheSame(oldItem: Thing, newItem: Thing): Boolean {
return oldItem.age == newItem.age && oldItem.name == oldItem.name
}
nothing special here.
The only special methods in the adapter (aside from onCreateViewHolder and onBindViewHolder) are these:
fun submitList(list: Things) {
differ.submitList(list)
}
override fun getItemCount(): Int = differ.currentList.size
private fun getItem(position: Int) = differ.currentList[position]
So we ask the differ to do these for us and expose the public method submitList to emulate a listAdapter#submitList(...), except we delegate to the differ.
Because you may be wondering, here's the ViewHolder:
internal class ViewHolder(itemView: View, private val callback: ThingClickCallback) :
RecyclerView.ViewHolder(itemView) {
private val title: TextView = itemView.findViewById(R.id.thingName)
private val age: TextView = itemView.findViewById(R.id.thingAge)
fun bind(data: Thing) {
title.text = data.name
age.text = data.age.toString()
itemView.setOnClickListener { callback.onThingClicked(adapterPosition) }
}
}
Don't be too harsh, I know i passed the click listener directly, I only had about 1 hour to do all this, but nothing special, the layout it's just two text views (age and name) and we set the whole row clickable to pass the position to the callback. Nothing special here either.
Last but not least, the Fragment.
Fragment
class ThingListFragment : Fragment() {
private lateinit var viewModel: ThingsViewModel
private var binding: ThingsListFragmentBinding? = null
private val adapter = ThingAdapter(object : ThingClickCallback {
override fun onThingClicked(atPosition: Int) {
viewModel.modifyData(atPosition)
}
})
...
It has 3 member variables. The ViewModel, the Binding (I used ViewBinding why not it's just 1 liner in gradle), and the Adapter (which takes the Click listener in the ctor for convenience).
In this impl., I simply call the viewmodel with "modify item at position (X)" where X = the position of the item clicked in the adapter. (I know this could be better abstracted but this is irrelevant here).
there's only two other implemented methods in this fragment...
onDestroy:
override fun onDestroy() {
super.onDestroy()
binding = null
}
(I wonder if Google will ever accept their mistake with Fragment's lifecycle that we still have to care for this).
Anyway, the other is unsurprisingly, onCreateView.
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val root = inflater.inflate(R.layout.things_list_fragment, container, false)
binding = ThingsListFragmentBinding.bind(root)
viewModel = ViewModelProvider(this).get(ThingsViewModel::class.java)
viewModel.state.observe(viewLifecycleOwner) { state ->
when (state) {
is ThingsState.Empty -> adapter.submitList(emptyList())
is ThingsState.Loaded -> adapter.submitList(state.things)
is ThingsState.Loading -> doNothing // Show Loading? :)
}
}
binding?.thingsRecyclerView?.adapter = adapter
viewModel.fetchData()
return root
}
Bind the thing (root/binding), get the viewModel, observe the "state", set the adapter in the recyclerView, and call the viewModel to start fetching data.
That's all.
How does it work then?
The app starts, the fragment is created, subscribes to the VM state LiveData, and triggers the Fetch of data.
The ViewModel calls the repo, which is empty (new), so makeItems is called the list now has items and cached in the repo's "source" list. The viewModel receives this list asynchronously (in a coroutine) and posts the LiveData state.
The fragment receives the state and posts (submit) to the Adapter to finally show something.
When you "click" on an Item, ViewHolder (which has a click listener) triggers the "call back" towards the fragment which receives a position, this is then passed onto the Viewmodel and here the data is mutated in the Repo, which again, pushes the same list, but with a different reference on the clicked item that was modified. This causes the ViewModel to push a new LIveData state with the same list reference as before, towards the fragment, which -again- receives this, and does adapter.submitList(...).
The Adapter asynchronously calculates this and the UI updates.
It works, I can put all this in GitHub if you want to have fun, but my point is, while the concerns about the AsyncDiffer are valid (and may be or been true), this doesn't seem to be my (super limited) experience.
Are you using this differently?
When I tap on any row, the change is propagated from the Repository
UPDATE: forgot to include the doNothing function:
val doNothing: Unit
get() = Unit
I've used this for a while, I normally use it because it reads better than XXX -> {} to me. :)

While doing
items.toMutableList().map { it.copy() }
restaurants.postValue(items)
you are creating a new list but items remains the same. You have to store that new list into a variable or passing that operation directly as a param to postItem.

How to refresh the UI of a fragment using LiveData after Firestore data loads

I'm making an Android app using Kotlin with Firebase products. I have successful connections with Firestore and can successfully retrieve the data I want, but I am having difficulty displaying it within a RecyclerView.
When the application loads, and after a user has logged in, my Firestore queries use the UID of the user to get a list of their assignments. Using logs I can see that this occurs without issue as the home screen loads. Within the home screen fragment I have data binding for the RecyclerView and setup my ViewModel to have the fragment observe the returned Firestore data.
I believe it is a misunderstanding on my part on exactly how LiveData works because if I tap the bottom nav icon for the home screen to trigger a refresh of the UI then the list populates and I can use the app as desired. Therefore my observer/LiveData must not be setup properly as it is not automatically refreshing once the data has changed (null list to not null list).
As I'm new to programming I'm sure I've fallen into a number of pitfalls and done a few things incorrectly, but I've been searching through StackOverflow and YouTube for help on this issue for months now. Unfortunately I don't have all of the links saved to every video and every post.
I've tried tweaking the ViewModel and the Repository/Database class (singleton) to different effects and currently I'm at my best version with only a single tap required to refresh the UI. Previously it took multiple taps.
from the Database class
private val assignments = MutableLiveData<List<AssignmentModel>>()
private fun getUserAssignments(c: ClassModel) {
val assignmentQuery = assignmentRef.whereEqualTo("Class_ID", c.Class_ID)
assignmentQuery.addSnapshotListener { documents, _ ->
documents?.forEach { document ->
val a = document.toObject(AssignmentModel::class.java)
a.Assignment_ID = document.id
a.Class_Title = c.Title
a.Formatted_Date_Due = formatAssignmentDueDate(a)
assignmentMap[a.Assignment_ID] = a
}
}
}
fun getAssignments() : LiveData<List<AssignmentModel>> {
assignments.value = assignmentMap.values.toList().filter {
if (it.Date_Due != null) it.Date_Due!!.toDate() >= Calendar.getInstance().time else true }
.sortedBy { it.Date_Due }
return assignments
}
from the ViewModel
class AssignmentListViewModel internal constructor(private val myDatabase: Database) : ViewModel() {
private var _assignments: LiveData<List<AssignmentModel>>? = null
fun getAssignments() : LiveData<List<AssignmentModel>> {
var liveData = _assignments
if (liveData == null) {
liveData = myDatabase.getAssignments()
_assignments = liveData
}
return liveData
}
}
from the Fragment
class AssignmentList : Fragment() {
private lateinit var model: AssignmentListViewModel
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val binding = AssignmentListBinding.inflate(inflater, container, false)
val factory = InjectorUtils.provideAssignmentListViewModelFactory()
model = ViewModelProvider(this, factory).get(AssignmentListViewModel::class.java)
val assignmentAdapter = AssignmentAdapter()
binding.assignmentRecycler.adapter = assignmentAdapter
updateUI(assignmentAdapter)
return binding.root
}
private fun updateUI(adapter: AssignmentAdapter) {
model.getAssignments().observe(this, Observer { assignments ->
if (assignments.isNotEmpty()) adapter.submitList(assignments)
})
}
}
Again, I expect the RecyclerView to populate automatically once the data from Firestore appears, but it doesn't. The screen remains empty until I tap the home screen button.
These snippets show the most recent changes I've made. Originally I had the Firestore query function returning the LiveData directly. I also had a much simpler ViewModel of something like fun getAssignments() = myDatabase.getAssignments().
Thanks for any and all help and advice.

When troubleshooting this issue, I'd recommend starting by looking at two things.
Take a look at where/when you're updating your LiveData
The goal is whenever the data in Firebase updates, your assignments LiveData updates your UI. Something like:
Firestore updates
Firestore triggers SnapshotListener
SnapshotListener updates LiveData
LiveData observer updates UI
So in your snapshot listener, you should be updating your LiveData, which is what I think you're missing. So it would be something like:
// Where you define your SnapshotListner
assignmentQuery.addSnapshotListener { documents, _ ->
// Process the data
documents?.forEach { document ->
val a = document.toObject(AssignmentModel::class.java)
a.Assignment_ID = document.id
a.Class_Title = c.Title
a.Formatted_Date_Due = formatAssignmentDueDate(a)
assignmentMap[a.Assignment_ID] = a
}
// Update your LiveData
assignments.value = assignmentMap.values.toList().filter {
if (it.Date_Due != null) it.Date_Due!!.toDate() >= Calendar.getInstance().time else true }
.sortedBy { it.Date_Due }
}
Now every time your Firestore updates, your LiveData will update and your UI should update.
Given the code change, getAssignments() can just return assignments. You can do this using a Kotlin backing property, covered here:
private val _assignments = MutableLiveData<List<AssignmentModel>>()
val assignments: LiveData<List<AssignmentModel>>
get() = _assignments
As for why it's not working at the moment, right now you call getAssignments() once on start up. This will filter an empty assignmentMap.values (I believe - might be worth checking), because when it's called, Firebase hasn't finished getting you any data. And when Firebase does get it's new data, it triggers the listener, but you don't update the LiveData.
Mind where you're setting up your listeners/observers
A tricky thing with LiveData observers and Firebase listeners is to make sure you only set them up once.
For your Firebase listener, you should be setting up the listener when you initialize your database and not every single time you call getUserAssignments. Then you wouldn't need all the null checking in the ViewModel, which essentially ensures that at least the ViewModel won't call getUserAssignments twice....but if you have other classes interacting with your database, they might call getUserAssignments multiple times and then you have tons of extra listeners.
Also, make sure you detach your listener.
One way to handle this is described in Doug Stevenson's talk Firebase and Android Jetpack: Fit Like a Glove - the talk includes a demo code here. The part that's related to this is how he handles LiveData -- notice how the class includes adding and removing the listener. The TL;DR is that he's using LiveData's lifecycle awareness to automatically do Firebase listener setup and cleanup. How that's done is a bit complicated, so I'd suggest watching the talk from here.
For your LiveData, setup/tear down looks correct since it's getting setup in onCreateView (and torn down automatically via the fact it's lifecycle aware). I might rename updateUI to something like setupUIObservation, since updateUI sounds like something you call multiple times. As with the Firebase listeners, you want to make sure you're not setting up the same LiveData observer more than once.
Hope that helps!

How to create LiveData which emits a single event and notifies only last subscribed observer?

I created live data which emits a single event as in this example.
My question is next:
How to notify only last subscribed observer when the value in the LiveData changes?
What comes to my mind is to store observers in the linked list in SingleLiveData class and then to call super.observe only if a passed observer is the same as the last element of the list.
I'm not sure if this is the best approach.
I want to use this mechanism to propagate FAB click events from activity to the fragments which are shown inside of the ViewPager. Fragments are dynamically added to view pager adapter, so let's say that we know the order of the fragments.

In the end, I found a workaround for this problem. I had to move away from the live data that emits a single event since it couldn't behave the way I needed it to behave.
Instead of this, I used simple mutable live data which emits an event object which wraps a data as in the last paragraph of this article by Jose Alcérreca.
I'm showing fragments in a view pager so I have only one visible fragment at the time.
So my view model looks like this:
class ActionViewModel : ViewModel() {
private val onCreateLiveData: MutableLiveData<Event<String>> = MutableLiveData()
fun observeOnCreateEvent(): LiveData<Event<String>> = onCreateLiveData
fun onCreateCollectionClick(message: String) {
this.onCreateLiveData.value = Event(message)
}
}
Event wrapper class implementation looks like 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
}
In fragments now we can observe events like this:
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
actionViewModel = ViewModelProviders.of(requireActivity()).get(ActionViewModel::class.java)
actionViewModel.observeOnCreateEvent()
.observe(this, Observer {
it?.takeIf { userVisibleHint }?.getContentIfNotHandled()?.let {
//DO what ever is needed
}
})
}
Fragment userVisibleHint property will return true if the fragment is currently visible to the user. Since we are only showing one fragment at the time this works for us. This means that the fragment will only access the event data if it is visible.
Also, implementation of the Event wrapper allows only one read of the value, so that every next time Observer gets this event, its value will be null and we'll ignore it.
Conclusion: This way we are simulating a single event live data which notifies only last subscribed observer.

If you're using Kotlin, you can replace LiveData with Flow. StateFlow can be used to replace regular LiveData, while SharedFlow can be used for stateless events. It will also provide you null safety and all the operators and configurations that come with Flow.
The migration is described here among other places. Here's a basic example:
ViewModel:
interface MyViewModel {
val myData: StateFlow<MyData>
val myEvents: SharedFlow<MyEvent>
}
class MyViewModelImpl: MyViewModel {
override val myData = MutableStateFlow(MyData())
override val myEvents = MutableSharedFlow<MyEvent>(replay = 0, extraBufferCapacity = 1, BufferOverflow.DROP_OLDEST)
/*
* Do stuff
*/
}
Activity:
lifecycleScope.launch {
myData.collect {
// handle stateful data
}
}
lifecycleScope.launch {
myEvents.collect {
// handle stateless events
}
}
Note that lifecycleScope requires the appropriate ktx dependency.
Herer's some more reading about Flow in Android.

I found solution for me in LD extension:
fun <T> LiveData<T>.observeAsEvent(owner: LifecycleOwner, observer: Observer<in T>) {
var previousKey: Any? = value?: NULL
observe(owner) { value ->
if (previousKey == NULL || previousKey != value) {
previousKey = value
observer.onChanged(value)
}
}
}
private const val NULL = "NULL"
Usage for this:
viewModel.resultLiveData.observeAsEvent(viewLifecycleOwner) {
...
}

I crafted a solution, feel free to take a look
https://github.com/ueen/LiveEvent

I've created a library to handle the most common cases that we might encounter while working with event-driven data scenarios
https://github.com/javaherisaber/LiveX
It contains the following types of classes:
LiveData
LiveEvent
OneShotLiveEvent
SingleLiveEvent
Multiple observers can register, all of them receive the event based on lifecycle
Multiple observers can register, each one receive the event only once
Only one observer can register and receive the event only once
Multiple observers can register, only the first one receive the event

How to test reading from/updating file in MVP?

I'm trying to change my app from having no design pattern to using MVP.
Originally I had the following code:
override fun onCreateInputView(): View {
//favoritesData is an instance variable, same with "Gson", "parser", "favorites", and "stringArrayListType"
favoritesData = File(filesDir, "favorites_data.json")
if (favoritesData.exists()) {
favorites = Gson.fromJson(parser.parse(FileReader(favoritesData)), stringArrayListType)
}
}
and
fun updateFavoritesFile() {
favoritesData.writeText(Gson.toJson(favorites))
}
After trying to use MVP I changed the code to:
class AnimeFaceKeyboardPresenter(val view : AnimeFaceKeyboardView, private val model : KeyboardModel = KeyboardModel()) : Presenter {
override fun onCreateInputView() {
model.favorites = view.loadFavoritesFile()
//At some point, call view.updateFavoritesFile(arrayListOf("test","test2"))
}
override fun onStartInputView() {
}
}
and the code in the activity itself to:
override fun loadFavoritesFile() : ArrayList<String> {
val favoritesData = File(filesDir, favoritesFileName)
var favorites = ArrayList<String>()
//"favorites" is no longer an instance variable
if (favoritesData.exists()) {
favorites = Gson.fromJson(parser.parse(FileReader(favoritesData)), stringArrayListType)
}
return favorites
}
override fun updateFavoritesFile(favorites: ArrayList<String>) {
File(filesDir, favoritesFileName).writeText(Gson.toJson(favorites))
}
override fun onCreateInputView(): View {
super.onCreateInputView()
presenter = AnimeFaceKeyboardPresenter(this)
presenter.onCreateInputView()
}
I'm not sure if I'm using MVP correctly, but if I am, how would I go about testing this code. For example - writing a test that calls updateFavoritesFile(arrayListOf("test1","test2")) and uses loadFavoritesFile() to check if the contents is as expected.

Well, you might want to relocate your file read and write to your model (they are associated with data which doesn't really belong in your view).
Then your test consists of instantiating your model object, and testing the methods which can be done without the view and presenter (simplifying the tests).
I would be very tempted to abstract your file as some form of "Repository" object that knows how to read and write strings (but you don't care where or how). You would pass the repository object to your model as a construction property. The advantage of this is that you can create a mock or fake Repository object which you can use to "feed" test data and examine written data, making testing that part of your model a little easier.
Don't forget, your view shouldn't have direct access to your model under MVP .. that would me more like MVC (one of the very few differences between MVP and MVC).