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PagingDataAdapter

abstract class PagingDataAdapter<T : Any, VH : RecyclerView.ViewHolder> : RecyclerView.Adapter

kotlin.Any
   ↳ androidx.recyclerview.widget.RecyclerView.Adapter
     ↳ androidx.paging.PagingDataAdapter

RecyclerView.Adapter base class for presenting paged data from PagingDatas in a RecyclerView.

This class is a convenience wrapper around AsyncPagingDataDiffer that implements common default behavior for item counting, and listening to update events.

To present a Pager, use collectLatest to observe Pager.flow and call submitData whenever a new PagingData is emitted.

If using RxJava and LiveData extensions on Pager, use the non-suspending overload of submitData, which accepts a Lifecycle.

PagingDataAdapter listens to internal PagingData loading events as pages are loaded, and uses DiffUtil on a background thread to compute fine grained updates as updated content in the form of new PagingData objects are received.

State Restoration: To be able to restore RecyclerView state (e.g. scroll position) after a configuration change / application recreate, PagingDataAdapter calls RecyclerView.Adapter.setStateRestorationPolicy with RecyclerView.Adapter.StateRestorationPolicy.PREVENT upon initialization and waits for the first page to load before allowing state restoration. Any other call to RecyclerView.Adapter.setStateRestorationPolicy by the application will disable this logic and will rely on the user set value.

import androidx.paging.PagingDataAdapter
import androidx.recyclerview.widget.DiffUtil

val USER_COMPARATOR = object : DiffUtil.ItemCallback<User>() {
    override fun areItemsTheSame(oldItem: User, newItem: User): Boolean =
        // User ID serves as unique ID
        oldItem.userId == newItem.userId

    override fun areContentsTheSame(oldItem: User, newItem: User): Boolean =
        // Compare full contents (note: Java users should call .equals())
        oldItem == newItem
}

class UserAdapter : PagingDataAdapter<User, UserViewHolder>(USER_COMPARATOR) {
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): UserViewHolder {
        return UserViewHolder.create(parent)
    }

    override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
        val repoItem = getItem(position)
        // Note that item may be null, ViewHolder must support binding null item as placeholder
        holder.bind(repoItem)
    }
}

Summary

Public constructors

<T : Any, VH : RecyclerView.ViewHolder> PagingDataAdapter(
    diffCallback: DiffUtil.ItemCallback<T>,
    mainDispatcher: CoroutineContext,
    workerDispatcher: CoroutineContext
)

Public functions

Unit

Add a CombinedLoadStates listener to observe the loading state of the current PagingData.

Unit

Add a listener which triggers after the pages presented to the UI are updated, even if the actual items presented don't change.

open Int
final Long
getItemId(position: Int)

Note: getItemId is final, because stable IDs are unnecessary and therefore unsupported.

T?
@MainThread
peek(index: @IntRange(from = 0) Int)

Returns the presented item at the specified position, without notifying Paging of the item access that would normally trigger page loads.

Unit

Refresh the data presented by this PagingDataAdapter.

Unit

Remove a previously registered CombinedLoadStates listener.

Unit

Remove a previously registered listener for new PagingData generations completing initial load and presenting to the UI.

Unit

Retry any failed load requests that would result in a LoadState.Error update to this PagingDataAdapter.

final Unit
setHasStableIds(hasStableIds: Boolean)

Stable ids are unsupported by PagingDataAdapter.

open Unit
ItemSnapshotList<T>

Returns a new ItemSnapshotList representing the currently presented items, including any placeholders if they are enabled.

suspend Unit
submitData(pagingData: PagingData<T>)

Present a PagingData until it is invalidated by a call to refresh or PagingSource.invalidate.

Unit
submitData(lifecycle: Lifecycle, pagingData: PagingData<T>)

Present a PagingData until it is either invalidated or another call to submitData is made.

ConcatAdapter

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.APPEND as a list item at the start of the presented list.

ConcatAdapter

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND as a list item at the end of the presented list.

ConcatAdapter
withLoadStateHeaderAndFooter(
    header: LoadStateAdapter<*>,
    footer: LoadStateAdapter<*>
)

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND and LoadType.APPENDs as list items at the start and end respectively.

Protected functions

T?
@MainThread
getItem(position: @IntRange(from = 0) Int)

Returns the presented item at the specified position, notifying Paging of the item access to trigger any loads necessary to fulfill prefetchDistance.

Public properties

Flow<CombinedLoadStates>

A hot Flow of CombinedLoadStates that emits a snapshot whenever the loading state of the current PagingData changes.

Flow<Unit>

A hot Flow that emits after the pages presented to the UI are updated, even if the actual items presented don't change.

Inherited functions

From androidx.recyclerview.widget.RecyclerView.Adapter

Public constructors

PagingDataAdapter

<T : Any, VH : RecyclerView.ViewHolder> PagingDataAdapter(
    diffCallback: DiffUtil.ItemCallback<T>,
    mainDispatcher: CoroutineContext = Dispatchers.Main,
    workerDispatcher: CoroutineContext = Dispatchers.Default
)

Public functions

addLoadStateListener

fun addLoadStateListener(listener: (CombinedLoadStates) -> Unit): Unit

Add a CombinedLoadStates listener to observe the loading state of the current PagingData.

As new PagingData generations are submitted and displayed, the listener will be notified to reflect the current CombinedLoadStates.

import androidx.paging.LoadState

val adapter = UserPagingAdapter()
adapter.addLoadStateListener {
    // show a retry button outside the list when refresh hits an error
    retryButton.isVisible = it.refresh is LoadState.Error

    // swipeRefreshLayout displays whether refresh is occurring
    swipeRefreshLayout.isRefreshing = it.refresh is LoadState.Loading

    // show an empty state over the list when loading initially, before items are loaded
    emptyState.isVisible = it.refresh is LoadState.Loading && adapter.itemCount == 0
}
Parameters
listener: (CombinedLoadStates) -> Unit

LoadStates listener to receive updates.

addOnPagesUpdatedListener

fun addOnPagesUpdatedListener(listener: () -> Unit): Unit

Add a listener which triggers after the pages presented to the UI are updated, even if the actual items presented don't change.

An update is triggered from one of the following:

Parameters
listener: () -> Unit

called after pages presented are updated.

getItemCount

open fun getItemCount(): Int

getItemId

final fun getItemId(position: Int): Long

Note: getItemId is final, because stable IDs are unnecessary and therefore unsupported.

PagingDataAdapter's async diffing means that efficient change animations are handled for you, without the performance drawbacks of RecyclerView.Adapter.notifyDataSetChanged. Instead, the diffCallback parameter of the PagingDataAdapter serves the same functionality - informing the adapter and RecyclerView how items are changed and moved.

peek

@MainThread
fun peek(index: @IntRange(from = 0) Int): T?

Returns the presented item at the specified position, without notifying Paging of the item access that would normally trigger page loads.

Parameters
index: @IntRange(from = 0) Int

Index of the presented item to return, including placeholders.

Returns
T?

The presented item at position index, null if it is a placeholder.

refresh

fun refresh(): Unit

Refresh the data presented by this PagingDataAdapter.

refresh triggers the creation of a new PagingData with a new instance of PagingSource to represent an updated snapshot of the backing dataset. If a RemoteMediator is set, calling refresh will also trigger a call to RemoteMediator.load with LoadType to allow RemoteMediator to check for updates to the dataset backing PagingSource.

Note: This API is intended for UI-driven refresh signals, such as swipe-to-refresh. Invalidation due repository-layer signals, such as DB-updates, should instead use PagingSource.invalidate.

import androidx.appcompat.app.AppCompatActivity
import androidx.paging.LoadState

class MyActivity : AppCompatActivity() {
    private lateinit var binding: MyActivityBinding
    private val pagingAdapter = UserPagingAdapter()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = MyActivityBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.recyclerView.adapter = pagingAdapter
        pagingAdapter.addLoadStateListener { loadStates ->
            binding.swipeRefreshLayout.isRefreshing = loadStates.refresh is LoadState.Loading
        }

        binding.swipeRefreshLayout.setOnRefreshListener {
            pagingAdapter.refresh()
        }
    }
}
See also
invalidate

removeLoadStateListener

fun removeLoadStateListener(listener: (CombinedLoadStates) -> Unit): Unit

Remove a previously registered CombinedLoadStates listener.

Parameters
listener: (CombinedLoadStates) -> Unit

Previously registered listener.

removeOnPagesUpdatedListener

fun removeOnPagesUpdatedListener(listener: () -> Unit): Unit

Remove a previously registered listener for new PagingData generations completing initial load and presenting to the UI.

Parameters
listener: () -> Unit

Previously registered listener.

retry

fun retry(): Unit

Retry any failed load requests that would result in a LoadState.Error update to this PagingDataAdapter.

Unlike refresh, this does not invalidate PagingSource, it only retries failed loads within the same generation of PagingData.

LoadState.Error can be generated from two types of load requests:

setHasStableIds

final fun setHasStableIds(hasStableIds: Boolean): Unit

Stable ids are unsupported by PagingDataAdapter. Calling this method is an error and will result in an UnsupportedOperationException.

Parameters
hasStableIds: Boolean

Whether items in data set have unique identifiers or not.

Throws
kotlin.UnsupportedOperationException

Always thrown, since this is unsupported by PagingDataAdapter.

setStateRestorationPolicy

open fun setStateRestorationPolicy(
    strategy: RecyclerView.Adapter.StateRestorationPolicy
): Unit

snapshot

fun snapshot(): ItemSnapshotList<T>

Returns a new ItemSnapshotList representing the currently presented items, including any placeholders if they are enabled.

submitData

suspend fun submitData(pagingData: PagingData<T>): Unit

Present a PagingData until it is invalidated by a call to refresh or PagingSource.invalidate.

This method is typically used when collecting from a Flow produced by Pager. For RxJava or LiveData support, use the non-suspending overload of submitData, which accepts a Lifecycle.

Note: This method suspends while it is actively presenting page loads from a PagingData, until the PagingData is invalidated. Although cancellation will propagate to this call automatically, collecting from a Pager.flow with the intention of presenting the most up-to-date representation of your backing dataset should typically be done using collectLatest.

import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.lifecycleScope

class MyFlowActivity : AppCompatActivity() {
    val pagingAdapter = UserPagingAdapter()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val viewModel by viewModels<UserListViewModel>()

        lifecycleScope.launch {
            viewModel.pagingFlow
                .collectLatest { pagingData ->
                    // submitData suspends until loading this generation of data stops
                    // so be sure to use collectLatest {} when presenting a Flow<PagingData>
                    pagingAdapter.submitData(pagingData)
                }
        }
    }
}
See also
Pager

submitData

fun submitData(lifecycle: Lifecycle, pagingData: PagingData<T>): Unit

Present a PagingData until it is either invalidated or another call to submitData is made.

This method is typically used when observing a RxJava or LiveData stream produced by Pager. For Flow support, use the suspending overload of submitData, which automates cancellation via CoroutineScope instead of relying of Lifecycle.

import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity

class MyLiveDataActivity : AppCompatActivity() {
    val pagingAdapter = UserPagingAdapter()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val viewModel by viewModels<UserListViewModel>()

        viewModel.pagingLiveData.observe(this) { pagingData ->
            pagingAdapter.submitData(lifecycle, pagingData)
        }
    }
}
import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity

class MyRxJava2Activity : AppCompatActivity() {
    val pagingAdapter = UserPagingAdapter()
    val disposable = CompositeDisposable()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val viewModel by viewModels<UserListViewModel>()

        viewModel.pagingFlowable
            .autoDispose(this) // Using AutoDispose to handle subscription lifecycle
            .subscribe { pagingData ->
                pagingAdapter.submitData(lifecycle, pagingData)
            }
    }
}
See also
submitData
Pager

withLoadStateFooter

fun withLoadStateFooter(footer: LoadStateAdapter<*>): ConcatAdapter

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.APPEND as a list item at the start of the presented list.

withLoadStateHeader

fun withLoadStateHeader(header: LoadStateAdapter<*>): ConcatAdapter

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND as a list item at the end of the presented list.

withLoadStateHeaderAndFooter

fun withLoadStateHeaderAndFooter(
    header: LoadStateAdapter<*>,
    footer: LoadStateAdapter<*>
): ConcatAdapter

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND and LoadType.APPENDs as list items at the start and end respectively.

Protected functions

getItem

@MainThread
protected fun getItem(position: @IntRange(from = 0) Int): T?

Returns the presented item at the specified position, notifying Paging of the item access to trigger any loads necessary to fulfill prefetchDistance.

Parameters
position: @IntRange(from = 0) Int

Index of the presented item to return, including placeholders.

Returns
T?

The presented item at position, null if it is a placeholder

Public properties

loadStateFlow

val loadStateFlowFlow<CombinedLoadStates>

A hot Flow of CombinedLoadStates that emits a snapshot whenever the loading state of the current PagingData changes.

This flow is conflated, so it buffers the last update to CombinedLoadStates and immediately delivers the current load states on collection.

onPagesUpdatedFlow

val onPagesUpdatedFlowFlow<Unit>

A hot Flow that emits after the pages presented to the UI are updated, even if the actual items presented don't change.

An update is triggered from one of the following:

Note: This is a SharedFlow configured to replay 0 items with a buffer of size 64. If a collector lags behind page updates, it may trigger multiple times for each intermediate update that was presented while your collector was still working. To avoid this behavior, you can conflate this Flow so that you only receive the latest update, which is useful in cases where you are simply updating UI and don't care about tracking the exact number of page updates.