Develop Reference

ViewPager (RecyclerView) with custom item count doesn't update data properly

I've created an adapter (extending ListAdapter with DiffUtil.ItemCallback) for my RecyclerView. It's an ordinary adapter with several itemViewTypes, but it should be smth like cyclic, if API sends flag and dataset size is > 1 (made by overriding getItemCount() to return 1000 when conditions == true).
When I change app locale through app settings, my fragment recreates, data loads asynchronously (reactively, several times in a row, from different requests, depending on several rx fields, which causes data set to be a combination of data on different languages just after locale is changed (in the end all dataset is correctly translated btw) (make it more like synchronous is not possible because of feature specifics)), posting its values to LiveData, which triggers updates of recycler view, the problem appears:
After last data set update some of the views (nearest to currently displayed and currently displayed) appear not to be translated.
Final data set, which is posted to LiveData is translated correctly, it even has correct locale tag in its id. Also after views are recycled and we return back to them - they are also correct.
DiffUtil is computed correctly also (I've tried to return only false in item callbacks and recycler view still didn't update its view holders correctly).
When itemCount == list.size everything works fine.
When adapter is pretending to be cyclic and itemCount == 1000 - no.
Can somebody explain this behaviour and help to figure out how to solve this?
Adapter Code Sample:
private const val TYPE_0 = 0
private const val TYPE_1 = 1
class CyclicAdapter(
val onClickedCallback: (id: String) -> Unit,
val onCloseClickedCallback: (id: String) -> Unit,
) : ListAdapter<IViewData, RecyclerView.ViewHolder>(DataDiffCallback()) {
var isCyclic: Boolean = false
set(value) {
if (field != value) {
field = value
}
}
override fun getItemCount(): Int {
return if (isCyclic) {
AdapterUtils.MAX_ITEMS // 1000
} else {
currentList.size
}
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
return when (viewType) {
TYPE_0 -> Type0.from(parent)
TYPE_1 -> Type1.from(parent)
else -> throw ClassCastException("View Holder for ${viewType} is not specified")
}
}
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
when (holder) {
is Type0 -> {
val item = getItem(
AdapterUtils.actualPosition(
position,
currentList.size
)
) as ViewData.Type0
holder.setData(item, onClickedCallback)
}
is Type1 -> {
val item = getItem(
AdapterUtils.actualPosition(
position,
currentList.size
)
) as ViewData.Type1
holder.setData(item, onClickedCallback, onCloseClickedCallback)
}
}
}
override fun getItemViewType(position: Int): Int {
return when (val item = getItem(AdapterUtils.actualPosition(position, currentList.size))) {
is ViewData.Type0 -> TYPE_0
is ViewData.Type1 -> TYPE_1
else -> throw ClassCastException("View Type for ${item.javaClass} is not specified")
}
}
class Type0 private constructor(itemView: View) :
RecyclerView.ViewHolder(itemView) {
fun setData(
viewData: ViewData.Type0,
onClickedCallback: (id: String) -> Unit
) {
(itemView as Type0View).apply {
acceptData(viewData)
setOnClickedCallback { url ->
onClickedCallback(viewData.id,)
}
}
}
companion object {
fun from(parent: ViewGroup): Type0 {
val view = Type0View(parent.context).apply {
layoutParams =
LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)
}
return Type0(view)
}
}
}
class Type1 private constructor(itemView: View) :
RecyclerView.ViewHolder(itemView) {
fun setData(
viewData: ViewData.Type1,
onClickedCallback: (id: String) -> Unit,
onCloseClickedCallback: (id: String) -> Unit
) {
(itemView as Type1View).apply {
acceptData(viewData)
setOnClickedCallback { url ->
onClickedCallback(viewData.id)
}
setOnCloseClickedCallback(onCloseClickedCallback)
}
}
companion object {
fun from(parent: ViewGroup): Type1 {
val view = Type1View(parent.context).apply {
layoutParams =
LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)
}
return Type1(view)
}
}
}
}
ViewPager Code Sample:
class CyclicViewPager #JvmOverloads constructor(
context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0
) : FrameLayout(context, attrs, defStyleAttr),
ICyclicViewPager {
private val cyclicViewPager: ViewPager2
private lateinit var onClickedCallback: (id: String) -> Unit
private lateinit var onCloseClickedCallback: (id: String) -> Unit
private lateinit var adapter: CyclicAdapter
init {
LayoutInflater
.from(context)
.inflate(R.layout.v_cyclic_view_pager, this, true)
cyclicViewPager = findViewById(R.id.cyclic_view_pager)
(cyclicViewPager.getChildAt(0) as RecyclerView).apply {
addItemDecoration(SpacingDecorator().apply {
dpBetweenItems = 12
})
clipToPadding = false
clipChildren = false
overScrollMode = RecyclerView.OVER_SCROLL_NEVER
}
cyclicViewPager.offscreenPageLimit = 3
}
override fun initialize(
onClickedCallback: (id: String) -> Unit,
onCloseClickedCallback: (id: String) -> Unit
) {
this.onClickedCallback = onClickedCallback
this.onCloseClickedCallback = onCloseClickedCallback
adapter = CyclicAdapter(
onClickedCallback,
onCloseClickedCallback,
).apply {
stateRestorationPolicy = RecyclerView.Adapter.StateRestorationPolicy.PREVENT_WHEN_EMPTY
}
cyclicViewPager.adapter = adapter
}
override fun setState(viewPagerState: CyclicViewPagerState) {
when (viewPagerState.cyclicityState) {
is CyclicViewPagerState.CyclicityState.Enabled -> {
adapter.submitList(viewPagerState.pages) {
adapter.isCyclic = true
cyclicViewPager.post {
cyclicViewPager.setCurrentItem(
// Setting view pager item to +- 500
AdapterUtils.getCyclicInitialPosition(
adapter.currentList.size
), false
)
}
}
}
is CyclicViewPagerState.CyclicityState.Disabled -> {
if (viewPagerState.pages.size == 1 && adapter.isCyclic) {
cyclicViewPager.setCurrentItem(0, false)
adapter.isCyclic = false
}
adapter.submitList(viewPagerState.pages)
}
}
}
}
Adapter Utils Code:
object AdapterUtils {
const val MAX_ITEMS = 1000
fun actualPosition(position: Int, listSize: Int): Int {
return if (listSize == 0) {
0
} else {
(position + listSize) % listSize
}
}
fun getCyclicInitialPosition(listSize: Int): Int {
return if (listSize > 0) {
MAX_ITEMS / 2 - ((MAX_ITEMS / 2) % listSize)
} else {
0
}
}
}
Have tried not to use default itemView variable of RecyclerView (became even worse).
Tried to make diff utils always return false, to check if it calculates diff correctly (yes, correctly)
Tried to add locale tags to ids of data set items (didn't help to solve)
Tried to post empty dataset on locale change before setting new data to it (shame on me, shouldn't even think about it)
Tried do add debounce to rx to make it wait a bit before update (didn't help)
UPD: When I call adapter.notifyDatasetChanged() manually, which is not the preferred way, everything works fine, so the question is why ListAdapter doesn't dispatch notify callbacks properly in my case?

The issue with ListAdapter is that it doesn't clearly state that you need to supply a new list for it to function.
In other words, the documentation says: (and I quote the source code):
/**
* Submits a new list to be diffed, and displayed.
* <p>
* If a list is already being displayed, a diff will be computed on a background thread, which
* will dispatch Adapter.notifyItem events on the main thread.
*
* #param list The new list to be displayed.
*/
public void submitList(#Nullable List<T> list) {
mDiffer.submitList(list);
}
The key word being new list.
However, as you can see there, all the adapter does is defer to the DiffUtil and calls submitList there.
So when you look at the actual source code of the AsyncListDiffer you will notice it does, at the beginning of its code block:
if (newList == mList) {
// nothing to do (Note - still had to inc generation, since may have ongoing work)
if (commitCallback != null) {
commitCallback.run();
}
return;
}
In other words, if the new list (reference) is the same as the old one, regardless of their contents, don't do anything.
This may sound cool but it means that if you have this code, the adapter will not really update:
(pseudo...)
var list1 = mutableListOf(...)
adapter.submitList(list1)
list1.add(...)
adapter.submitList(list1)
The reason is list1 is the same reference your adapter has, so the differ exits prematurely, and doesn't dispatch any changes to the adapter.
Quite obscure, I know.
The solution, as pointed in many SO answers is to create a copy of the list itself.
Most users do
var list1 = mutableListOf(...)
adapter.submitList(list1)
var list2 = list1.toMutableList()
list2.add(...)
adapter.submitList(list2)
The call to toMutableList() creates a new list containing the items of list1 and so the comparison above if (newList == mList) { should now be false and the normal code should execute.
UPDATE
Keep in mind that a lot of developers make the mistake of...
var list = mutableListOf...
adapter.submitList(list)
list.add(xxx)
adapter.submitList(list.toList())
This doesn't work, because the new list you create, is referencing the same objects the adapter has. This means that both lists list and list.toList() are pointing to the same things despite being two instances of an ArrayList.
But the side-effect is that DiffUtil compares the items and they are the same, so no diff is dispatched to the adapter either.
The correct sequence is...
val list = mutableListOf(...)
adapter.submitList(list.toList())
// Make a copy first, so we can alter it as we please without the *current list held by the adapter* from being affected.
var modified = list.toMutableList()
modified.add(...)
adapter.submitList(modified)

After taking a look at your sample in GitHub, I was able to reproduce the issue. With only about 30-40 minutes of playing with it, I can say that I'm not 100% sure what component is not updating.
Things I've noticed.
The onBindViewHolder method is not called when you change the locale (except maybe the 1st time?).
I do not understand why the need to post to the adapter after you've submitted the list in the callback:
cyclicViewPager.setCurrentItem(
// Setting view pager item to +- 500
AdapterUtils.getCyclicInitialPosition(
adapter.currentList.size
), false
)
Why ? This means the user loses their current position.
Why not keep the existing?
I noticed you do cyclicViewPager.offscreenPageLimit = 3 this effectively disables the RecyclerView "logic" for handling changes, and uses instead the usual ViewPager state adapter logic of "prefetching/keeping" N (3 in your case) pages in "advance".
At first I thought this was causing issues, but removing it (which sets it to -1 which is the default and the "use RecyclerView" value, didn't make a big change (though I did notice some changes here and there, as in it would sometimes update the current one -but not the next ones within 2~3 pages).
The documentation says:
Set the number of pages that should be retained to either side of the currently visible page(s). Pages beyond this limit will be recreated from the adapter when needed. Set this to OFFSCREEN_PAGE_LIMIT_DEFAULT to use RecyclerView's caching strategy.
So I would have imagined that the default value would be aided by the ListAdapter and its DiffUtil. Doesn't seem to be the case.
What I did try (among a few other things) was to see if the issue was in the actual adapter (or at least the viewPager dependency on its adapter). I ran out of time (work!) but I noticed that if you do:
override fun setState(viewPagerState: CyclicViewPagerState) {
when (viewPagerState.cyclicityState) {
is CyclicViewPagerState.CyclicityState.Enabled -> {
// call initialize again, to recreate the adapter
initialize(this.onClickedCallback, this.onCloseClickedCallback)
adapter.submitList(viewPagerState.pages) {
adapter.isCyclic = true
// Setting vp item to ... (code omitted for brevity)
}
This works. It's theoretically less efficient as you're recreating the whole adapter, but in your example you're effectively creating an ENTIRE new set of data changing every ID, so in terms of performance, I'd argue this is more efficient as there's no need to recalculate changes and dispatch them, since to the eyes of the Diff Util, all the rows are different. By recreating the adapter, well... the VP has to reinit anyway.
I noticed this worked fine in your example.
I went ahead and added two more things, because the "silly" adapter cannot reliably tell you which position is the current... you can naively save it:
In CyclicViewPager:
var currentPos: Int = 0
init {
...
this.cyclicViewPager.registerOnPageChangeCallback(object : OnPageChangeCallback() {
override fun onPageSelected(position: Int)
currentPos = position
}
})
}
And then
is CyclicViewPagerState.CyclicityState.Enabled -> {
initialize(this.onClickedCallback, this.onCloseClickedCallback)
adapter.submitList(viewPagerState.pages) {
adapter.isCyclic = true
if (adapter.currentList.size <= currentPos) {
cyclicViewPager.setCurrentItem(currentPos, false)
} else {
cyclicViewPager.setCurrentItem(
// Setting view pager item to +- 500
AdapterUtils.getCyclicInitialPosition(
adapter.currentList.size
), false
)
}
}
}
This does work, but of course, you're recreating the entire VP adapter again, so it may not be desired.
At this point, I'd either need to spend much more time trying to figure out which part of VP, RV, or its dependencies is not "dispatching" the correct data. My guess would be somewhere around some silly ViewPager optimization combined with Android terribly unreliable View system, not picking a message in the queue; but I may be also terribly wrong ;)
I hope someone smarter and/or with more coffee in their system can find out a simpler solution.
(all in all, I found the sample project relatively easy to navigate, but the design of your data a bit convoluted, but... as it was a sample, it's hard to tell what "real-life" data structures you really have).

RecyclerView inside WindowManager does not update

I try to create a notification overlay inside my application that can show notifications about certain application wise important events.
I decided to use a RecyclerView which will be drown directly on WindowManager.
This works fine for showing initial items, however the the items don't get updated.
Below is my implementation. So when I call start the not1 and not2 are shown, but when removeNotification function get called, the notification is actually being removed and a correct list is being submitted to the adapter, but the view on screen does not update.
However if I add windowManager.updateViewLayout(recyclerView, layoutParams) after submitList inside removeNotification, everything seems to work as expected, but this time I am loosing RV animations..
As this is the first time I work with WindowManager directly, I am quite confused. Can someone help me to figure out what's going on and how can I achieve what I want to, if only that's possible.
class NotificationOverlay(private val context: Context) {
private val windowManager: WindowManager =
context.getSystemService(Context.WINDOW_SERVICE) as WindowManager
private val layoutParams = WindowManager.LayoutParams().apply {
gravity = android.view.Gravity.TOP or android.view.Gravity.CENTER_HORIZONTAL
width = WindowManager.LayoutParams.MATCH_PARENT
height = WindowManager.LayoutParams.WRAP_CONTENT
format = PixelFormat.TRANSLUCENT
dimAmount = 0.5f
flags = WindowManager.LayoutParams.FLAG_DIM_BEHIND
type = WindowManager.LayoutParams.TYPE_APPLICATION_PANEL
}
private val notifications = mutableListOf<NotificationItem>().also {
it.addAll(listOf(
NotificationItem(title = "not 1", message = "first notification"),
NotificationItem(title = "not 2", message = "second notification")
))
}
private val notificationsAdapter = NotificationAdapter(::removeNotification)
private val recyclerView = RecyclerView(context).apply {
layoutManager = LinearLayoutManager(context, LinearLayoutManager.VERTICAL, false)
adapter = notificationsAdapter
}
private fun removeNotification(notification: NotificationItem){
notifications.remove(notification)
notificationsAdapter.submitList(notifications)
if(notificationsAdapter.currentList.isEmpty()){
windowManager.removeView(recyclerView)
}
}
fun show(){
windowManager.addView(recyclerView, layoutParams)
notificationsAdapter.submitList(notifications)
}
}
Edited
Well, I found out that the issue is not in WindowManager but rather in DiffUtils, but cannot understand what's wrong with it, as it is very simple one, and I implemented such DiffUtils a lot of times, anyways, I'll post the code here if you could have any idea on why this does not work:
class NotificationAdapter(private val onCloseClicked: (NotificationItem) -> Unit):
ListAdapter<NotificationItem, NotificationAdapter.NotificationViewHolder>(DiffCallback()) {
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): NotificationViewHolder {
val binding = ItemNotificationOverlayBinding.inflate(LayoutInflater.from(parent.context), parent, false)
return NotificationViewHolder(binding, onCloseClicked)
}
override fun onBindViewHolder(holder: NotificationViewHolder, position: Int) {
holder.bind(currentList[position])
}
class NotificationViewHolder(private val itemBinding: ItemNotificationOverlayBinding, private val onCloseClicked: (NotificationItem) -> Unit): RecyclerView.ViewHolder(itemBinding.root) {
fun bind(item: NotificationItem) {
itemBinding.title.text = item.title
itemBinding.message.text = item.message
itemBinding.close.setOnClickListener {
onCloseClicked.invoke(item)
}
}
}
class DiffCallback : DiffUtil.ItemCallback<NotificationItem>() {
override fun areItemsTheSame(oldItem: NotificationItem, newItem: NotificationItem) =
oldItem.id == newItem.id
override fun areContentsTheSame(oldItem: NotificationItem, newItem: NotificationItem) =
oldItem == newItem
}
}
What can every be wrong in such a simple construction? I am going crazy already and want to throw away this DiffUtils and implement the old school notifyItemRemoved
Edited 2
So the answer offered by #IamFr0ssT fixed the issue. I dig a bit deeper to see why this happens and the reason is in androidx.recyclerview.widget.AsyncListDiffer class in main submitList function. It is doing the following check there:
if (newList == mList) {
// nothing to do (Note - still had to inc generation, since may have ongoing work)
if (commitCallback != null) {
commitCallback.run();
}
return;
}
so my diff was never being even tried to be calculated as I was submitting the same reference of the list.
what the additional toList() function suggested by #IamFr0ssT did, is created a new instance of the list thus making the differ to calculate my diff. If you go deeper inside toList() function, it eventually creates a new instance of an ArrayList based on provided list ...return ArrayList(this)
So well, this issue had nothing to do with WindowManager just the DiffUtil

You are passing the MutableList notifications to the adapter, and the adapter is not making a copy of the list, it is just using the same list.
When you edit the list in your removeNotification callback, you are editing the list that the adapter is using.
Because of that, when the diff is being calculated, it is comparing the list that it thinks is currently displayed, but is not, to itself. Thus no diff and no notifyItemRemoved or other events.
What you can do to fix it, I think, is just call .toList() on the mutable list when you call submitList():
class NotificationOverlay(private val context: Context) {
...
private fun removeNotification(notification: NotificationItem){
notifications.remove(notification)
notificationsAdapter.submitList(notifications.toList())
if(notificationsAdapter.currentList.isEmpty()){
windowManager.removeView(recyclerView)
}
}
fun show(){
windowManager.addView(recyclerView, layoutParams)
notificationsAdapter.submitList(notifications.toList())
}
}
Also, how do you get NotificationItem.id? It should be different for each entry.

Um.. I think you should try to manually notify your RecyclerView to redraw with notifyDataSetChanged()
If it doesn't work ListAdapter you are using as adapter does diff computation and dispatches the result to the RecyclerView. Maybe diff is not correct and the adapter does not see a difference between the old list and the new one - in this case it won't update UI. You may try to check diff behaviour and maybe comparing behavior of your data to change it.

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.

Add Drag and Drop on RecyclerView with DiffUtil

I have a list that gets updated from a Room Database. I receive the updated data from Room as a new list and I then pass it to ListAdapter's submitList to get animations for the changes.
list.observe(viewLifecycleOwner, { updatedList ->
listAdapter.submitList(updatedList)
})
Now, I want to add a drag and drop functionality for the same RecyclerView. I tried to implement it using ItemTouchHelper. However, the notifyItemMoved() is not working as ListAdapter updates its content through the submitList().
override fun onMove(
recyclerView: RecyclerView,
viewHolder: RecyclerView.ViewHolder,
target: RecyclerView.ViewHolder
): Boolean {
val from = viewHolder.bindingAdapterPosition
val to = target.bindingAdapterPosition
val list = itemListAdapter.currentList.toMutableList()
Collections.swap(list, from, to)
// does not work for ListAdapter
// itemListAdapter.notifyItemMoved(from, to)
itemListAdapter.submitList(list)
return false
}
The drag and drop now works fine but only when dragged slowly, when the dragging gets fast enough, I get different and inconsistent results.
What could be the reason for this? What is the best way that I can achieve a drag and drop functionality for my RecyclerView which uses ListAdapter?

So I made a quick test (this whole thing doesn't fit in a comment so I'm writing an answer)
My Activity contains the adapter, RV, and observes a viewModel. When the ViewModel pushes the initial list from the repo via LiveData, I save a local copy of the list in mutable form (just for the purpose of this test) so I can quickly mutate it on the fly.
This is my "onMove" implementation:
val from = viewHolder.bindingAdapterPosition
val to = target.bindingAdapterPosition
list[from] = list[to].also { list[to] = list[from] }
adapter.submitList(list)
return true
I also added this log to verify something:
Log.d("###", "onMove from: $from (${list[from].id}) to: $to (${list[to].id})")
And I noticed it.. works. But because I'm returning true (you seem to be returning false).
Now... unfortunately, if you drag fast up and down, this causes the list to eventually become shuffled:
E.g.: Let's suppose there are 10 items from 0-9.
You want to grab item 0 and put it between item 1 and 2.
You start Dragging item 0 at position 0, and move it a bit down so now it's between 1 and 2, the new item position in the onMove method is 1 (so far, you're still dragging). Now if you slowly drag further (to position 2), the onMove method is from 1 to 2, NOT from 0 to 2. This is because I returned "true" so every onMove is a "finished operation". This is fine, since the operations are slow and the ListAdapter has time to update and calculate stuff.
But when you drag fast, the operations go out of sync before the adapter has time to catch up.
If you return false instead (like you do) then you get various other effects:
The RecyclerView Animations don't play (while you drag) since the viewHolders haven't been "moved" yet. (you returned false)
The onMove method is then spammed every time you move your finger over a viewHolder, since the framework wants to perform this move again... but the list is already modified...
So you'd get something like (similar example above, 10 items, moving the item 0)> let's say each item has an ID that corresponds to its position+1 (in the initial state, so item at position 0 has id 1, item at position 1 has id 2, etc.)
This is what the log shows while I slowly drag item 0 "down":
(format is `from: position(id of item from) to: position(id of item to)
onMove from: 0 (1) to: 1 (2) // Initial drag of first item down to 2nd item.
onMove from: 0 (2) to: 1 (1) // now the list is inverted, notice the IDs.
onMove from: 0 (1) to: 1 (2) // Back to square one.
onMove from: 0 (2) to: 1 (1) // and undo-again...
I just cut it there, but you can see how it's bouncing all over the place back and forth. I believe this is because you return false but modify the list behind the scenes, so it's getting confused. on one side of the equation the "data" says one thing, (and so does the diff util), but on the other, the adapter is oblivious to this change, at least "yet" until the computations are done, which, as you guessed, when you drag super fast, is not enough time.
Unfortunately, I don't have a good answer (today) as to what would the best approach be. Perhaps, not relying on the ListAdapter's behavior and implementing a normal adapter, where you have better list/source control of the data and when to call submitList and when to simply notifyItemChanged or moved between two positions may be a better alternative for this use-case.
Apologies for the useless answer.

I ended up implementing a new adapter and use it instead of ListAdapter, as mentioned on Martin Marconcini's answer. I added two separate functions. One for receiving updates from Room database (replacement for submitList from ListAdapter) and another for every position change from drag
MyListAdapter.kt
class MyListAdapter(list: ArrayList<Item>) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
// save instance instead of creating a new one every submit
// list to save some allocation time. Thanks to Martin Marconcini
private val diffCallback = DiffCallback(list, ArrayList())
fun submitList(updatedList: List<Item>) {
diffCallback.newList = updatedList
val diffResult = DiffUtil.calculateDiff(diffCallback)
list.clear()
list.addAll(updatedList)
diffResult.dispatchUpdatesTo(this)
}
fun itemMoved(from: Int, to: Int) {
Collections.swap(list, from, to)
notifyItemMoved(from, to)
}
}
DiffCallback.kt
class DiffCallback(
val oldList: List<Item>,
var newList: List<Item>
) : DiffUtil.Callback() {
override fun getOldListSize(): Int {
return oldList.size
}
override fun getNewListSize(): Int {
return newList.size
}
override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
val oldItem = oldList[oldItemPosition]
val newItem = newList[newItemPosition]
return oldItem.id == newItem.id
}
override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
val oldItem = oldList[oldItemPosition]
val newItem = newList[newItemPosition]
return compareContents(oldItem, newItem)
}
}
Call itemMoved every position change:
override fun onMove(
recyclerView: RecyclerView,
viewHolder: RecyclerView.ViewHolder,
target: RecyclerView.ViewHolder
): Boolean {
val from = viewHolder.bindingAdapterPosition
val to = target.bindingAdapterPosition
itemListAdapter.itemMoved(from, to)
// Update database as well if needed
return true
}
When receiving updates from Room database:
You may also want to check if currently dragging using onSelectedChanged if you are also updating your database every position change to prevent unnecessary calls to submitList
list.observe(viewLifecycleOwner, { updatedList ->
listAdapter.submitList(updatedList)
})

I've tried danartillaga's answer and got a ConcurrentModificationException for the list variable. I use LiveData in the code and it looks like the data was changed during invalidation of the list.
I've tried to keep the ListAdapter implementation and concluded to the following solution:
class MyListAdapter : ListAdapter<Item, RecyclerView.ViewHolder>(MyDiffUtil) {
var modifiableList = mutableListOf<Item>()
private set
fun moveItem(from: Int, to: Int) {
Collections.swap(modifiableList, to, from)
notifyItemMoved(from, to)
}
override fun submitList(list: List<CourseData>?) {
modifiableList = list.orEmpty().toMutableList()
super.submitList(modifiableList)
}
}
and the onMove code from ItemTouchHelper.SimpleCallback:
override fun onMove(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder, target: RecyclerView.ViewHolder): Boolean {
val adapter = recyclerView.adapter as CoursesDownloadedAdapter
val from = viewHolder.bindingAdapterPosition
val to = target.bindingAdapterPosition
val list = adapter.modifiableList
// Change your DB here
adapter.moveItem(from, to)
return true
}
The magic here is saving the modifiableList inside the adapter. ListAdapter stores a link to the list from submitList call, so we can change it externally. During the Drag&Drop the list is changed with Collections.swap and RecyclerView is updated with notifyItemMoved with no DiffCallback calls. But the data inside ListAdapter was changed and the next submitList call will use the updated list to calculate the difference.

IllegalStateException: Range start point not set

I am using Paging library form Jetpack for loading data. In order to allow users to select multiple items in RecyclerView, I have used the RecyclerView Selection library.
Now, the problem is that when the user selects an item and drags down, the app gets crashed after few items are selected. I am getting the below exception:
java.lang.IllegalStateException: Range start point not set.
I don't know what I am missing here. Also, I want to disable drag and select in the SelectionTracker but can't find a solution for that. Any help will be appreciated.
Update
I am attaching the necessary code used for the multi-selection below.
Adapter
fun getItemDetails(): ItemDetailsLookup.ItemDetails<Long> =
object : ItemDetailsLookup.ItemDetails<Long>() {
override fun getPosition(): Int = adapterPosition
override fun getSelectionKey(): Long? = itemId
}
ItemDetailsLookup
class HomeItemDetailsLookup(private val recyclerView: RecyclerView) : ItemDetailsLookup<Long>() {
override fun getItemDetails(event: MotionEvent): ItemDetails<Long>? {
val view = recyclerView.findChildViewUnder(event.x, event.y)
if (view != null) {
return (recyclerView.getChildViewHolder(view) as HomeViewHolder).getItemDetails()
}
return null
}
}
Fragment
selectionTracker = SelectionTracker.Builder<Long>(
"mySelection",
rvHome,
StableIdKeyProvider(rvHome),
HomeItemDetailsLookup(rvHome),
StorageStrategy.createLongStorage()
).build()
homeAdapter.tracker = selectionTracker

While combining paging library and selection library there exists this bug.
No solutions have been found so far.
It happens when paging library calls notifyItemRangeInserted on adapter which cause DefaultSelectionTracker.endRange method trigger that set DefaultSelectionTracker.mRange to null.
Better try updating your libraries to latest alpha and try again