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How to keep a Mutable List with the previous data added in a view Model class in Kotlin?

I have two classes: one is the viewModel (ShoesViewMode.ktl) to keep the data and the other is the Fragment to show the data.(ShoesList.kt )
ShoesList has a mutableList of words and I recover it from the ShoesList to show in a scrollview.
I get a new word from an EditText from a Fragment -> Click on Save button -> Pass this word through nave Args to ShoesDetails -> save it in the ShoesViewModel -> Recover it and show in the Fragment.
The problem is that every time I add a new word, the list doesn't keep the last one added. It's like if the mutableList was always recreated.
I would like to go back the screen and add a new word, and a new word and see the previous words added in the list.
How can I keep the words added previously?
ShoesViewModel.kt
class ShoesViewModel(_newShoe: String?=null): ViewModel() {
private var _shoesList = MutableLiveData<MutableList<String>>()
init {
//receives the score when the class is instanciated
_shoesList.value = mutableListOf(
"trade",
"calendar",
"sad",
"desk",
"guitar",
"home",
"railway",
"zebra",
"jelly",
"car",
"crow",
"trade",
"bag",
"roll"
)
}
val shoesList: LiveData<MutableList<String>>
get() = _shoesList
fun save (newShoe: String){
_shoesList.value?.add(newShoe)
}
ShoesList. kt // FRAGMENT to show data
val shoesListArgs by navArgs()
viewModelFactory = ShoeViewModelFactory(shoesListArgs.newShoe)
viewModel = ViewModelProvider(this, viewModelFactory).get(ShoesViewModel::class.java)
//get the view Model //pass to the variable in the xml
binding.shoesViewModel = viewModel
binding.setLifecycleOwner(this)
viewModel.save(shoesListArgs.newShoe) //save new Shoe to the List
//keeps track of shoesList. This is an OBSERVER
viewModel.shoesList.observe(viewLifecycleOwner, Observer{ shoesList ->
loadShoes(shoesList)
})
//actig to floating button
binding.buttonFloating.setOnClickListener{ view:View ->
view.findNavController().navigate(ShoesListDirections.actionShoesListToShoesDetails())
}
return binding.root
}
private fun loadShoes(list:MutableList<String>){
for(shoe in list){
val newTextViewShoe = TextView(context)
newTextViewShoe.text = shoe // add TextView to LinearLayout
binding.linearlayoutShoelist.addView(newTextViewShoe)
}
}
}
I save a new word, the Fragment changes and list shows the new word. When I go back to the screen to save a new word, it saves the new word, but the previous on disappears.

In method save You need:
fun save(newShoe: String) {
if (shoeList.value.isNullOrEmpty){
shoeList.value = mutableListOf(newShoe)
}
else {
shoesList.value = shoesList.value.add(newShoe)
}
}
Your problem is that You are trying to set data to the list rather than livedata by calling livedata.value.add(). Your value here is getValue() method, that does nothing but gives you value. If You need to update a value in livedata, then You go:
liveData.value = newValue
Whether this means setValue() method. Additionally, if You want to set data from another thread than main, use postValue():
liveData.postValue(newValue)

DiffUtil not refreshing view in Observer call android kotlin

Hey I am using diff util with ListAdapter. The updating of list works but I can only see those new values by scrolling the list, I need to view the updates even without recycling the view (when scrolling) just like notifyItemChanged(). I tried everything inside this answer ListAdapter not updating item in RecyclerView only working for me is notifyItemChanged or setting adapter again. I am adding some code. Please someone know how to fix this problem?
Data and Enum class
data class GroupKey(
val type: Type,
val abc: Abc? = null,
val closeAt: String? = null
)
data class Group(
val key: GroupKey,
val value: MutableList<Item?> = ArrayDeque()
)
enum class Type{
ONE,
TWO
}
data class Abc(
val qq: String? = null,
val bb: String? = null,
val rr: RType? = null,
val id: String? = null
)
data class RType(
val id: String? = null,
val name: String? = null
)
data class Item(
val text: String? = null,
var abc: Abc? = null,
val rr: rType? = null,
val id: String? = null
)
viewmodel.kt
var list: MutableLiveData<MutableList<Group>?> = MutableLiveData(ArrayDeque())
fun populateList(){
// logic to call api
list.postValue(data)
}
fun addItemTop(){
// logic to add item on top
list.postValue(data)
}
inside view model I am filling data by api call inside viewmodel function and return value to list. Also another function which item is inserting at top of list so that's why is used ArrayDeque
Now I am adding nested reyclerview diff util callback.
FirstAdapter.kt
class FirstAdapter :
ListAdapter<Group, RecyclerView.ViewHolder>(comp) {
companion object {
private val comp = object : DiffUtil.ItemCallback<Group>() {
override fun areItemsTheSame(oldItem: Group, newItem: Group): Boolean {
return oldItem == newItem
}
override fun areContentsTheSame(oldItem: Group, newItem: Group): Boolean {
return ((oldItem.value == newItem.value) && (oldItem.key == newItem.key))
}
}
}
......... more function of adapter
}
FirstViewHolder
val adapter = SecondAdapter()
binding.recyclerView.adapter = adapter
adapter.submitList(item.value)
SecondAdapter.kt
class SecondAdapter : ListAdapter<Item, OutgoingMessagesViewHolder>(comp) {
companion object {
private val comp = object : DiffUtil.ItemCallback<Item>() {
override fun areItemsTheSame(oldItem: Item, newItem: Item): Boolean {
return oldItem.id == newItem.id
}
override fun areContentsTheSame(oldItem: Item, newItem: Item): Boolean {
return ((oldItem.rr == newItem.rr) &&
(oldItem.text == oldItem.text) && (oldItem.abc == newItem.abc))
}
}
}
..... more function
}
Activity.kt
viewModel.list.observe(this, { value ->
submitList(value)
})
private fun submitList(list: MutableList<Group>?) {
adapter?.submitList(list)
// adapter?.notifyDataSetChanged()
}
I am 100% sure that my list is updating and my observer is calling when my new list is added. I debug that through debug view. But problem is I can only see those new values by scrolling the list, I need to view the updates even without recycling the view (when scrolling) just like notifyItemChanged()
UPDATE
viewmodel.kt
class viewModel : BaseViewModel(){
var list: MutableLiveData<MutableList<Group>?> = MutableLiveData()
//... more variables...
fun fetchData(context: Context) {
viewModelScope.launch {
val response = retroitApiCall()
response.handleResult(
onSuccess = { response ->
list.postValue(GroupData(response?.items, context))
},
onError = { error ->
Log.e("error" ,"$error")
}
)
}
}
}
internal fun GroupData(items: List<CItem>?, context: Context): MutableList<Group> {
val result: MutableList<Group> = MutableList()
items?.iterator()?.forEach { item ->
// adding item in list by add function and then return list.
return result
}
private fun addItemOnTop(text: String) {
list.value?.let { oldlist ->
// logic to add items on top of oldlist variable
if(top != null){
oldlist.add(0,item)
}else{
val firstGroup = oldlist[0]
firstGroup.value.add(item)
}
list.postValue(oldlist)
}
}
}
I am using sealed class something like this but not this one Example. And Something similar to these when call api Retrofit Example. Both link I am giving you example. What I am using in my viewmodel.

I don't know what's going on, but I can tell you two things that caught my attention.
First Adapter:
override fun areItemsTheSame(oldItem: Group, newItem: Group): Boolean {
return oldItem == newItem
}
You're not comparing if the items are the same, you're comparing the items and their contents are the same. Don't you have an Id like you did in your second adapter?
I'd probably check oldItem.key == newItem.key.
Submitting the List
As indicated in the answer you linked, submitList has a very strange logic where it compares if the reference of the actual list is the same, and if it is, it does nothing.
In your question, you didn't show where the list comes from (it's observed through what appears to be liveData or RXJava), but the souce of where the list is constructed is not visible.
In other words:
// P S E U D O C O D E
val item1 = ...
val item2 = ...
val list1 = mutableListOf(item1, item2)
adapter.submitList(list1) // works fine
item1.xxx = ""
adapter.submitList(list1) // doesn't work well.
WHY?
Unfortunately, submitList's source code shows us that if the reference to the list is the same, the diff is not calculated. This is really not on the adapter, but rather on AsyncListDiffer, used by ListAdapter internally. It is this differ's responsibility to trigger the calculation(s). But if the list references are the same, it doesn't, and it silently ignores it.
My suspicion is that you're not creating a new list. This rather undocumented and silent behavior hurts more than it helps, because more often than not, developers aren't expecting to duplicate a list supplied to an object whose purpose and promise is to offer the ability to "magically" (and more importantly, automatically) calculate its differences between the previous.
I understand why they did it, but I would have at the very least emitted a log WARNING, indicating you're supplying the same list. Or, if you want to avoid polluting the already polluted logCat, then at least be much more explicit about it in its official documentation.
The only hint is this simple phrase:
you can use submitList(List) when new lists are available.
The key here being the word new lists. So not the same list with new items, but simply a new List reference (regardless of whether the items are the same or not).
What should you try?
I'd start by modifying your submitList method:
private fun submitList(list: MutableList<Group>?) {
adapter?.submitList(list.toMutableList())
}
For Java users out there:
adapter.submitList(new ArrayList(oldList));
The change is to create a copy of the list you receive: list.ToMutableList(). This way the AsyncListDiffer's check for list equality will return false and the code will continue.
UPDATE / DEBUG
Unfortunately, I don't know what is going on with your code; I assure you that ListAdapter works, as I use it myself on a daily basis; If you think you've found a case where there are problems with it, I suggest you create a small prototype and publish it on github or similar so we can reproduce it.
I would start by using debug/breakpoints in key areas:
ViewModel; write down the reference fromthe list you "return".
DiffUtil methods, is diffUtil being called?
Your submitList() method, is the list reference the same as the one you had in your ViewModel?
etc.
You need to dig a bit deeper until you find out who is not doing what.
On Deep vs Shallow copy and Java and whatever...
Please keep in mind, ListAdapter (through AsyncDiff) checks if the reference to the list is the same. In other words, if you have a list val x = mutableListOf(...) and you give this to the adapter, it will work the 1st time.
If you then modify the list...
val x = mutableListOf(...)
adapter.submitList(x)
x.clear()
adapter.submitList(x)
This will NOT WORK correctly, because to the eyes of the Adapter both lists are the same (they actually are the same list).
The fact that the list is mutable is irrelevant. (I still frown upon the mutable list; why does submitList accept a mutable list if you cannot mutate it and submit it again, escapes my knowledge but I would not have approved that Pull Request like so) It would have avoided most problems if they only took a non-mutable list, therefore implying you must supply a new list every time if you mutate it. Anyway...
as I was saying, duplicating a list is simple, in either Kotlin or Java there are multiple variations:
val newListWithSameContents = list1.toList()
List newListWithSameContents = ArrayList(list1);
now if list1 has an item...
list1.add("hello")
When you copy list1 into newList... The reference to "Hello" (the string) is the same. If String were mutable (it's not, but assume it is), and you modified that string somehow... you would be modifying both strings at the same time or rather, the same string, referenced in both lists.
data class Thing(var id: Int)
val thing = Thing(1)
val list1: MutableList<Thing> = mutableListOf(thing)
val list2: MutableList<Thing> = list1.toMutableList()
println(list1)
println(list2)
// This prints
[Thing(id=1)]
[Thing(id=1)]
Now modify the thing...
thing.id = 2
println(list1)
println(list2)
As expected, both lists, pointing to the same object:
[Thing(id=2)]
[Thing(id=2)]
This was a shallow copy because the items were not copied. They still point to the same thing in memory.
ListAdapter/DiffUtil do not care if the objects are the same in that regard (depending how you implemented your diffutil that is); but they certainly care if the lists are the same. As in the above example.
I hope this clarifies what is needed for ListAdapter to dispatch updates. If it fails to do so, then check if you're effectively doing the right thing.

LiveData - How to observe changes to List inside object?

I have a Composable, a ViewModel and an object of a User class with a List variable in it. Inside the ViewModel I define a LiveData object to hold the User object and in the Composable I want to observe changes to the List inside the User object but it doesn't seem to work very well.
I understand when you change the contents of a List its reference is the same so the List object doesn't change itself, but I've tried copying the list, and it doesn't work; copying the whole User object doesn't work either; and the only way it seems to work is if I create a copy of both. This seems too far-fetched and too costly for larger lists and objects. Is there any simpler way to do this?
The code I have is something like this:
Composable
#Composable
fun Greeting(viewModel: ViewModel) {
val user = viewModel.user.observeAsState()
Column {
// TextField and Button that calls viewModel.addPet(petName)
LazyColumn {
items(user.value!!.pets) { pet ->
Text(text = pet)
}
}
}
}
ViewModel
class ViewModel {
val user: MutableLiveData<User> = MutableLiveData(User())
fun addPet(petName: String){
val sameList = user.value!!.pets
val newList = user.value!!.pets.toMutableList()
newList.add(petName)
sameList.add(petName) // This doesn't work
user.value = user.value!!.copy() // This doesn't work
user.value!!.pets = newList // This doesn't work
user.value = user.value!!.copy(pets = newList) // This works BUT...
}
}
User
data class User(
// Other variables
val pets: MutableList<String> = mutableListOf()
)

MutableLiveData will only notify view when it value changes, e.g. when you place other value which is different from an old one. That's why user.value = user.value!!.copy(pets = newList) works.
MutableLiveData cannot know when one of the fields was changed, when they're simple basic types/classes.
But you can make pets a mutable state, in this case live data will be able to notify about changes. Define it like val pets = mutableStateListOf<String>().
I personally not a big fan of live data, and code with value!! looks not what I'd like to see in my project. So I'll tell you about compose way of doing it, in case your project will allow you to use it. You need to define both pets as a mutable state list of strings, and user as a mutable state of user.
I suggest you read about compose states in the documentation carefully.
Also note that in my code I'm defining user with delegation, and pets without delegation. You can use delegation only in view model, and inside state holders you cannot, othervise it'll become plain objects at the end.
#Composable
fun TestView() {
val viewModel = viewModel<TestViewModel>()
Column {
// TextField and Button that calls viewModel.addPet(petName)
var i by remember { mutableStateOf(0) }
Button(onClick = { viewModel.addPet("pet ${i++}") }) {
Text("add new pet")
}
LazyColumn {
items(viewModel.user.pets) { pet ->
Text(text = pet)
}
}
}
}
class User {
val pets = mutableStateListOf<String>()
}
class TestViewModel: ViewModel() {
val user by mutableStateOf(User())
fun addPet(petName: String) {
user.pets.add(petName)
}
}

Jetpack Compose works best with immutable objects, making a copy with modern Android and ART is not the issue that it was in the past.
However, if you do not want to make a whole copy of your object, you could add a dummy int to it and then mutate that int when you also mutate the list, but I strongly urge you to consider immutability and instantiate a new User object instead.

Live Data with ArrayList not observing (Kotlin)?

I am using one of Android Jetpack Component ViewModel + Live data in my project it works fine for me when using normal data such as string and Int but when it comes to arrayList it won't observe anything
Here's my code
class MainActivityModel : ViewModel() {
private var dataObservable = MutableLiveData<ArrayList<Int>>()
init {
dataObservable.value = arrayListOf(1,2,3,4,5)
}
fun getInt(): LiveData<ArrayList<Int>> = dataObservable
fun addInt(i:Int) {
dataObservable.value!![i] = dataObservable.value!![i].plus(1)
}
}

A LiveData won't broadcast updates to observers unless its value is completely reassigned with a new value. This does not reassign the value:
dataObservable.value!![i] = dataObservable.value!![i].plus(1)
What it does is retain the existing array, but add an array element. LiveData doesn't notify its observables about that. The actual array object has to be reassigned.
If you want to reassign a new array value and notify all observers, reassign a new array altogether, like this:
dataObservable.value = dataObservable.value!![i].plus(1)
Assigning dataObservable.value will call the LiveData setValue() method, and notify observers of the new value passed to setValue() .

If you modify your complex observable object outside of main thread you need to use postValue
dataObservable?.value[i] += 1
dataObservable.apply {
postValue(value)
}

Refreshing MutableLiveData of list of items

I'm using LiveData and ViewModel from the architecture components in my app.
I have a list of items that is paginated, I load more as the user scrolls down. The result of the query is stored in a
MutableLiveData<List<SearchResult>>
When I do the initial load and set the variable to a new list, it triggers a callback on the binding adapter that loads the data into the recyclerview.
However, when I load the 2nd page and I add the additional items to the list, the callback is not triggered. However, if I replace the list with a new list containing both the old and new items, the callback triggers.
Is it possible to have LiveData notify its observers when the backing list is updated, not only when the LiveData object is updated?
This does not work (ignoring the null checks):
val results = MutableLiveData<MutableList<SearchResult>>()
/* later */
results.value.addAll(newResults)
This works:
val results = MutableLiveData<MutableList<SearchResult>>()
/* later */
val list = mutableListOf<SearchResult>()
list.addAll(results.value)
list.addAll(newResults)
results.value = list

I think the extension is a bit nicer.
operator fun <T> MutableLiveData<ArrayList<T>>.plusAssign(values: List<T>) {
val value = this.value ?: arrayListOf()
value.addAll(values)
this.value = value
}
Usage:
list += anotherList;

According to MutableLiveData, you need to use postValue or setValue in order to trigger the observers.