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).

How to observe a list from Adapter in Android [Kotlin]

In my case I am trying to get a list of String from my Adapter and use it in my Fragment but upon debugging using Logs I found that the list is getting updated inside the onBindViewHolder but not outside it. So when I try to access the list from my Fragment I am getting an empty list of String.
I have spent few hours trying to figure this but can't find a feasible solution.
My Approach: I am thinking of an approach to save this list in a room table and then query it back in the Fragment. Though it may solve the issue but is it the only way? Are there any other ways to achieve this result?
My Adapter
class FloorProfileDialogAdapter() : RecyclerView.Adapter<FloorProfileDialogAdapter.MyViewHolder>() {
var floors = emptyList<String>()
inner class MyViewHolder(val binding: ScheduleFloorDialogItemBinding) :
RecyclerView.ViewHolder(binding.root)
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
val inflater = LayoutInflater.from(parent.context)
val binding = ScheduleFloorDialogItemBinding.inflate(inflater, parent, false)
return MyViewHolder(binding)
}
private val checkedFloors: MutableList<String> = mutableListOf()
//List of uniquely selected checkbox to be observed from New Schedule Floor Fragment
var unique: List<String> = mutableListOf()
#SuppressLint("SetTextI18n")
override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
val currentFloor = floors[position]
Timber.d("Current floor: $currentFloor")
holder.binding.floorCheckBox.text = "Floor $currentFloor"
//Checks the checked boxes and updates the list
holder.binding.floorCheckBox.setOnCheckedChangeListener { buttonView, isChecked ->
if (buttonView.isChecked) {
Timber.d("${buttonView.text} checked")
checkedFloors.add(buttonView.text.toString())
} else if (!buttonView.isChecked) {
Timber.d("${buttonView.text} unchecked")
checkedFloors.remove(buttonView.text)
}
unique = checkedFloors.distinct().sorted()
Timber.d("List: $unique")
}
}
fun returnList(): List<String> {
Timber.d("$unique")
return unique
}
override fun getItemCount(): Int {
return floors.size
}
#SuppressLint("NotifyDataSetChanged")
fun getAllFloors(floorsReceived: List<String>) {
Timber.d("Floors received : $floorsReceived")
this.floors = floorsReceived
notifyDataSetChanged()
}
}
Fragment code where I am trying to read it
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
//Chosen Floors
val chosenFloors = floorProfileDialogAdapter.returnList()
Timber.d("Chosen floors : $chosenFloors")
}
Note: The list I am trying to receive is var unique: List<String> = mutableListOf. I tried to get it using the returnList() but the log in that function shows that list is empty. Similarly the Log in fragment shows that it received an empty list.
Edit 1 :
Class to fill the Adapter Floors using getAllFloors()
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val floorList: MutableList<String> = mutableListOf()
var profileName: String? = ""
profileName = args.profileName
//Profile name received
Timber.d("Profile name : $profileName")
//Getting list of all floors
createProfileViewModel.totalFloors.observe(viewLifecycleOwner) {
Timber.d("List of floors received : $it")
val intList = it.map(String::toInt)
val maxFloorValue = intList.last()
var count = 0
try {
while (count <= maxFloorValue) {
floorList.add(count.toString())
count++
}
} catch (e: Exception) {
Timber.d("Exception: $e")
}
floorProfileDialogAdapter.getAllFloors(floorList)
Timber.d("Floor List : $floorList")
}

When you first set up your Fragment and create your Adapter, unique is empty:
var unique: List<String> = mutableListOf()
(if you have some checked state you want to save and restore, you'll have to initialise this with your checked data)
In onViewCreated, during Fragment setup, you get a reference to this (empty) list:
// Fragment onViewCreated
val chosenFloors = floorProfileDialogAdapter.returnList()
// Adapter
fun returnList(): List<String> {
return unique
}
So chosenFloors is a reference to this initial entry list. But when you actually update unique in onBindViewHolder
unique = checkedFloors.distinct().sorted()
you're replacing the current list with a new list object. You're not updating the existing list (even though you made it a MutableList). So you never actually add anything to that empty list you started with, and chosenFloors is left pointing at a list that contains nothing, while the Adapter has discarded it and unique holds a completely different object.
The solution there is to make unique a val (so you can't replace it) and just change its contents, e.g.
unique.clear()
unique += checkedFloors.distinct().sorted()
But I don't feel like that's your problem. Like I pointed out, that list is initially empty anyway, and you're grabbing it in your Fragment during initialisation just so you can print out its contents, as though you expect it to contain something at that point. Unless you initialise it with some values, it's gonna be empty.
If you're not already storing/restoring them, you'll need to handle that! I posted some code to do that on another answer so I'll just link that instead of repeating myself. That code is storing indices though, not text labels like you're doing. Indices are much cleaner and avoid errors - the text is more of a display thing, a property of the item the specific (and unique) index refers to. (But you can store a string array in SharedPreferences if you really want to.)
Also you're not actually updating your ViewHolder to display the checked state for the current item in onBindViewHolder. So whatever ViewHolder you happen to have been given (there's only a few of them for the list, they get reused) it's just showing whatever its checkbox was last set to, by you poking at it. Check an item, then scroll the list and see what happens!
So you need to check or uncheck the box so it's correct for the item you're displaying. This is pretty easy if you're storing the checked items by indices:
// explicitly set the checked state, depending on whether the item at 'position' is checked
holder.binding.floorCheckBox.checked = checkedItems[position]
You can work out something similar for your text label approach, but again I wouldn't recommend doing things that way.

is there any alternate for arraylist.remove(element) function?

my recycler view reloads with spinner value, selected data is stored in a global arraylist, and here is my recycler view code to do the selection and deselection. Selection and deselection works just fine, but when i select and change spinner value, and then come to back original spinner value, where items are already selected, deselction happens, but item is not removed from the global arraylist. While debugging i found that cursor reaches there, but i dont know why .remove() isnt working. Is there any alternative for it or am i doing wrong? Is there anything i should know that why isnt the item removed.
.
.
.
class RecyclerViewAdapter(val dataList:ArrayList<ModelClass>,val onItemClicked: (Int) -> Unit):RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder>() {
object ob {
val dataSelected = ArrayList<ModelClass>()
val hm = HashMap<ModelClass,String>()
}
fun setData(listModel: List<ModelClass>) {
dataList.clear()
dataList.addAll(listModel)
notifyDataSetChanged()
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
val binding = ItemViewBinding.inflate(
LayoutInflater.from(parent.context), parent, false
)
return ViewHolder(binding, parent.context)
}
#SuppressLint("ResourceAsColor")
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
holder.bindItems(dataList[position])
}
override fun getItemCount() = dataList.size
inner class ViewHolder(
val binding: ItemViewBinding,
val context: Context
) : RecyclerView.ViewHolder(binding.root) {
var count=0
#SuppressLint("ResourceAsColor")
fun restore() {
for (i in 0 until ob.dataSelected.size) {
for (j in 0 until dataList.size) {
if (ob.dataSelected[i].sku_code == (dataList[j]).sku_code) {
if (adapterPosition == j) {
itemView.isSelected = true
itemView.setBackgroundColor(R.color.black)
count=count+1
println("****")
}
}
}
}
if(!itemView.isSelected){
itemView.isSelected = false
itemView.setBackgroundResource(0)
}
}
#SuppressLint("ResourceAsColor", "ResourceType")
fun bindItems(data: ModelClass) = with(binding) {
binding.itemQuant.text = data.item_quant
binding.itemName.text = data.item_name
binding.mfgName.text = data.mfg
binding.quantity.text = data.item_stock.toString()
count=0
restore()
itemView.setOnClickListener {
count += 1
var isPresent:Int
if (count%2 == 0){
isPresent=1
}
else
{
isPresent=0
}
if (isPresent == 1) {
it.setBackgroundResource(0) //works
ob.dataSelected.remove(dataList[adapterPosition]) //doesnt work if spinner value is changed and changed back. works while still on same screen.
} else {
if (isPresent == 0) {
it.setBackgroundColor(R.color.black)
ob.dataSelected.add(dataList[adapterPosition])
}
// onItemClicked.invoke(adapterPosition)
}
}
}
}
}

Are you sure it's not being removed? Have you set a breakpoint and checked the contents of the array (or just logged it)? Because it looks like when you're binding a viewholder, you just check if the item is in dataSelected, and if it is you set the view's selected and backgroundColor values. It doesn't look like you change them back if it's not in dataSelected, so that selected appearance "sticks".
It looks like you're trying to reset them with this:
if(!itemView.isSelected){
itemView.isSelected = false
itemView.setBackgroundResource(0)
}
but that only works if itemView.selected hasn't been set to true, which it has if your item is in dataSelected. You need to do it like this:
for (i in 0 until ob.dataSelected.size) {
for (j in 0 until dataList.size) {
// rolling multiple conditions into a single value makes it easier to do an if/else
val selected = ob.dataSelected[i].sku_code == (dataList[j]).sku_code && adapterPosition == j
if (selected) {
itemView.isSelected = true
itemView.setBackgroundColor(R.color.black)
count=count+1
println("****")
} else {
itemView.isSelected = false
itemView.setBackgroundResource(0)
}
}
}
That way you're always updating the contents of the view holder to reflect the state of the current item. That's especially important in RecyclerViews, because the point of those is that the ViewHolders get reused to display different items, and any View attributes you don't set when binding (e.g. colours, checkbox statuses) will just stay on whatever they were last set to when displaying some other item.
btw, you can simplify that looping situation by just checking if anything in dataSelected has the SKU you're looking for:
val itemSku = datalist[adapterPosition].sku_code
val selected = ob.dataSelected.any { it.sku_code == itemSku }
ideally you wouldn't be using adapterPosition to find the current item either - the item you're matching is passed in to bindItems (as data), you could just pass that to your restore function
edit if the items aren't being removed from dataSelected, at a guess it's because the object you're fetching from dataList "doesn't exist" in dataSelected. By "doesn't exist" I mean there isn't an object that equals the one you're trying to remove.
ob.dataSelected.remove(dataList[adapterPosition])
I'm assuming that might be the case because you're not doing straight object comparisons when you're checking if an item is in the selected array, you're specifically comparing their sku_code values instead
if (ob.dataSelected[i].sku_code == (dataList[j]).sku_code)
If you can't just do if (obj.dataSelected[i] == dataList[j]), then remove won't work either - it's the same check. So maybe you need to look into making those objects equal (e.g. using data classes if that works for what you're doing), or use removeAll with a predicate (remove only works with a specific item):
ob.dataSelected.removeAll { it.sku_code == dataList[adapterPosition].sku_code }
You'll still have the problem of the UI not being updated though, like the first part of the answer explains! So you might have two problems here

change this line
ob.dataSelected.add(dataList[adapterPosition])
to this
ob.dataSelected.add(dataList[adapterPosition].copy)
Don't forget to call notfiyDatasetChanged() after that

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.

OnBindViewHolder does not apply after notifyitemmoved () in Android Recyclerview

The code above is the RecyclerViewAdapter, which changes color only when it is the first item, as shown below.
class TestAdapter : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
private val textColor1 = Color.BLACK
private val textColor2 = Color.YELLOW
private val items = ArrayList<String>()
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
val textColor = if(position==0) textColor1 else textColor2
holder.itemView.textView.setTextColor(textColor)
holder.itemView.textView.text = items[position]
}
fun move(from:Int,to:Int){
val item = items[from]
items.remove(item)
items.add(to,item)
notifyItemMoved(from,to)
}
}
In this state I would like to move Value 3 to the first position using the move function. The results I want are shown below.
But in fact, it shows the following results
When using notifyDataSetChanged, I can not see the animation transition effect,
Running the onBindViewHolder manually using findViewHolderForAdapterPosition results in what I wanted, but it is very unstable. (Causing other parts of the error that I did not fix)
fun move(from:Int,to:Int){
val item = items[from]
val originTopHolder = recyclerView.findViewHolderForAdapterPosition(0)
val afterTopHolder = recyclerView.findViewHolderForAdapterPosition(from)
items.remove(item)
items.add(to,item)
notifyItemMoved(from,to)
if(to==0){
onBindViewHolder(originTopHolder,1)
onBindViewHolder(afterTopHolder,0)
}
}
Is there any other way to solve this?

Using the various notifyItemFoo() methods, like moved/inserted/removed, doesn't re-bind views. This is by design. You could call
if (from == 0 || to == 0) {
notifyItemChanged(from, Boolean.FALSE);
notifyItemChanged(to, Boolean.FALSE);
}
in order to re-bind the views that moved.

notifyItemMoved will not update it. According to documentation:
https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter
This is a structural change event. Representations of other existing items in the data set are still considered up to date and will not be rebound, though their positions may be altered.
What you're seeing is expected.
Might want to look into using notifyItemChanged, or dig through the documentation and see what works best for you.