PagedListAdapter

Kotlin |Java

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

kotlin.Any
↳	androidx.recyclerview.widget.RecyclerView.Adapter<VH>
	↳	androidx.paging.PagedListAdapter

RecyclerView.Adapter base class for presenting paged data from androidx.paging.PagedLists in a RecyclerView.

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

While using a LiveData is an easy way to provide data to the adapter, it isn't required - you can use submitList when new lists are available.

PagedListAdapter listens to PagedList loading callbacks as pages are loaded, and uses DiffUtil on a background thread to compute fine grained updates as new PagedLists are received.

Handles both the internal paging of the list as more data is loaded, and updates in the form of new PagedLists.

A complete usage pattern with Room would look like this:

@Dao
interface UserDao {
    @Query("SELECT * FROM user ORDER BY lastName ASC")
    public abstract DataSource.Factory<Integer, User> usersByLastName();
}

class MyViewModel extends ViewModel {
    public final LiveData<PagedList<User>> usersList;
    public MyViewModel(UserDao userDao) {
        usersList = new LivePagedListBuilder&lt;>(
        userDao.usersByLastName(), /* page size */20).build();
    }
}

class MyActivity extends AppCompatActivity {
    @Override
    public void onCreate(Bundle savedState) {
        super.onCreate(savedState);
        MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);
        RecyclerView recyclerView = findViewById(R.id.user_list);
        UserAdapter&lt;User> adapter = new UserAdapter();
        viewModel.usersList.observe(this, pagedList -> adapter.submitList(pagedList));
        recyclerView.setAdapter(adapter);
    }
}

class UserAdapter extends PagedListAdapter<User, UserViewHolder> {
    public UserAdapter() {
        super(DIFF_CALLBACK);
    }
    @Override
    public void onBindViewHolder(UserViewHolder holder, int position) {
        User user = getItem(position);
        if (user != null) {
            holder.bindTo(user);
        } else {
            // Null defines a placeholder item - PagedListAdapter will automatically invalidate
            // this row when the actual object is loaded from the database
            holder.clear();
        }
    }
    public static final DiffUtil.ItemCallback&lt;User> DIFF_CALLBACK =
            new DiffUtil.ItemCallback<User>() {
        @Override
        public boolean areItemsTheSame(@NonNull User oldUser, @NonNull User newUser) {
            // User properties may have changed if reloaded from the DB, but ID is fixed
            return oldUser.getId() == newUser.getId();
        }
        @Override
        public boolean areContentsTheSame(@NonNull User oldUser, @NonNull User newUser) {
            // NOTE: if you use equals, your object must properly override Object#equals()
            // Incorrectly returning false here will result in too many animations.
            return oldUser.equals(newUser);
        }
    }
}

Advanced users that wish for more control over adapter behavior, or to provide a specific base class should refer to AsyncPagedListDiffer, which provides the mapping from paging events to adapter-friendly callbacks.

Summary

Protected constructors
`<init>(diffCallback: DiffUtil.ItemCallback<T>)` Creates a PagedListAdapter with default threading and androidx.recyclerview.widget.ListUpdateCallback.
`<init>(config: AsyncDifferConfig<T>)`

Public methods
open Unit	`addLoadStateListener(listener: (LoadType, LoadState) -> Unit)` Add a LoadState listener to observe the loading state of the current PagedList.
open Int	`getItemCount()`
open Unit	`onCurrentListChanged(currentList: PagedList<T>?)` Called when the current PagedList is updated.
open Unit	`onCurrentListChanged(previousList: PagedList<T>?, currentList: PagedList<T>?)` Called when the current PagedList is updated.
open Unit	`removeLoadStateListener(listener: (LoadType, LoadState) -> Unit)` Remove a previously registered LoadState listener.
open Unit	`submitList(pagedList: PagedList<T>?)` Set the new list to be displayed.
open Unit	`submitList(pagedList: PagedList<T>?, commitCallback: Runnable?)` Set the new list to be displayed.
ConcatAdapter	`withLoadStateFooter(footer: LoadStateAdapter<*>)` Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND as a list item at the start of the presented list.
ConcatAdapter	`withLoadStateHeader(header: LoadStateAdapter<*>)` Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.APPEND 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 methods
open T?	`getItem(position: Int)`

Inherited functions

From class Adapter

`Unit`	`bindViewHolder(@NonNull holder: VH, position: Int)` This method internally calls `onBindViewHolder(ViewHolder, int)` to update the `ViewHolder` contents with the item at the given position and also sets up some private fields to be used by RecyclerView. Adapters that merge other adapters should use `bindViewHolder(ViewHolder, int)` when calling nested adapters so that RecyclerView can track which adapter bound the `ViewHolder` to return the correct position from `ViewHolder#getBindingAdapterPosition()` method. They should also override the `findRelativeAdapterPositionIn(Adapter, ViewHolder, int)` method.
`VH`	`createViewHolder(@NonNull parent: ViewGroup, viewType: Int)` This method calls `onCreateViewHolder(ViewGroup, int)` to create a new `ViewHolder` and initializes some private fields to be used by RecyclerView.
`Int`	`findRelativeAdapterPositionIn(@NonNull adapter: RecyclerView.Adapter<out RecyclerView.ViewHolder!>, @NonNull viewHolder: RecyclerView.ViewHolder, localPosition: Int)` Returns the position of the given `ViewHolder` in the given `Adapter`. If the given `Adapter` is not part of this `Adapter`, `RecyclerView#NO_POSITION` is returned.
`Long`	`getItemId(position: Int)` Return the stable ID for the item at `position`. If `hasStableIds()` would return false this method should return `NO_ID`. The default implementation of this method returns `NO_ID`.
`Int`	`getItemViewType(position: Int)` Return the view type of the item at `position` for the purposes of view recycling. The default implementation of this method returns 0, making the assumption of a single view type for the adapter. Unlike ListView adapters, types need not be contiguous. Consider using id resources to uniquely identify item view types.
`RecyclerView.Adapter.StateRestorationPolicy`	`getStateRestorationPolicy()` Returns when this Adapter wants to restore the state.
`Boolean`	`hasObservers()` Returns true if one or more observers are attached to this adapter.
`Boolean`	`hasStableIds()` Returns true if this adapter publishes a unique `long` value that can act as a key for the item at a given position in the data set. If that item is relocated in the data set, the ID returned for that item should be the same.
`Unit`	`notifyDataSetChanged()` Notify any registered observers that the data set has changed. There are two different classes of data change events, item changes and structural changes. Item changes are when a single item has its data updated but no positional changes have occurred. Structural changes are when items are inserted, removed or moved within the data set. This event does not specify what about the data set has changed, forcing any observers to assume that all existing items and structure may no longer be valid. LayoutManagers will be forced to fully rebind and relayout all visible views. `RecyclerView` will attempt to synthesize visible structural change events for adapters that report that they have `stable IDs` when this method is used. This can help for the purposes of animation and visual object persistence but individual item views will still need to be rebound and relaid out. If you are writing an adapter it will always be more efficient to use the more specific change events if you can. Rely on `notifyDataSetChanged()` as a last resort.
`Unit`	`notifyItemChanged(position: Int)` Notify any registered observers that the item at `position` has changed. Equivalent to calling `notifyItemChanged(position, null);`. This is an item change event, not a structural change event. It indicates that any reflection of the data at `position` is out of date and should be updated. The item at `position` retains the same identity.
`Unit`	`notifyItemChanged(position: Int, @Nullable payload: Any?)` Notify any registered observers that the item at `position` has changed with an optional payload object. This is an item change event, not a structural change event. It indicates that any reflection of the data at `position` is out of date and should be updated. The item at `position` retains the same identity. Client can optionally pass a payload for partial change. These payloads will be merged and may be passed to adapter's `onBindViewHolder(ViewHolder, int, List)` if the item is already represented by a ViewHolder and it will be rebound to the same ViewHolder. A notifyItemRangeChanged() with null payload will clear all existing payloads on that item and prevent future payload until `onBindViewHolder(ViewHolder, int, List)` is called. Adapter should not assume that the payload will always be passed to onBindViewHolder(), e.g. when the view is not attached, the payload will be simply dropped.
`Unit`	`notifyItemInserted(position: Int)` Notify any registered observers that the item reflected at `position` has been newly inserted. The item previously at `position` is now at position `position + 1`. 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.
`Unit`	`notifyItemMoved(fromPosition: Int, toPosition: Int)` Notify any registered observers that the item reflected at `fromPosition` has been moved to `toPosition`. 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.
`Unit`	`notifyItemRangeChanged(positionStart: Int, itemCount: Int)` Notify any registered observers that the `itemCount` items starting at position `positionStart` have changed. Equivalent to calling `notifyItemRangeChanged(position, itemCount, null);`. This is an item change event, not a structural change event. It indicates that any reflection of the data in the given position range is out of date and should be updated. The items in the given range retain the same identity.
`Unit`	`notifyItemRangeChanged(positionStart: Int, itemCount: Int, @Nullable payload: Any?)` Notify any registered observers that the `itemCount` items starting at position `positionStart` have changed. An optional payload can be passed to each changed item. This is an item change event, not a structural change event. It indicates that any reflection of the data in the given position range is out of date and should be updated. The items in the given range retain the same identity. Client can optionally pass a payload for partial change. These payloads will be merged and may be passed to adapter's `onBindViewHolder(ViewHolder, int, List)` if the item is already represented by a ViewHolder and it will be rebound to the same ViewHolder. A notifyItemRangeChanged() with null payload will clear all existing payloads on that item and prevent future payload until `onBindViewHolder(ViewHolder, int, List)` is called. Adapter should not assume that the payload will always be passed to onBindViewHolder(), e.g. when the view is not attached, the payload will be simply dropped.
`Unit`	`notifyItemRangeInserted(positionStart: Int, itemCount: Int)` Notify any registered observers that the currently reflected `itemCount` items starting at `positionStart` have been newly inserted. The items previously located at `positionStart` and beyond can now be found starting at position `positionStart + itemCount`. 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.
`Unit`	`notifyItemRangeRemoved(positionStart: Int, itemCount: Int)` Notify any registered observers that the `itemCount` items previously located at `positionStart` have been removed from the data set. The items previously located at and after `positionStart + itemCount` may now be found at `oldPosition - itemCount`. 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.
`Unit`	`notifyItemRemoved(position: Int)` Notify any registered observers that the item previously located at `position` has been removed from the data set. The items previously located at and after `position` may now be found at `oldPosition - 1`. 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.
`Unit`	`onAttachedToRecyclerView(@NonNull recyclerView: RecyclerView)` Called by RecyclerView when it starts observing this Adapter. Keep in mind that same adapter may be observed by multiple RecyclerViews.
`Unit`	`onBindViewHolder(@NonNull holder: VH, position: Int)` Called by RecyclerView to display the data at the specified position. This method should update the contents of the `ViewHolder#itemView` to reflect the item at the given position. Note that unlike `android.widget.ListView`, RecyclerView will not call this method again if the position of the item changes in the data set unless the item itself is invalidated or the new position cannot be determined. For this reason, you should only use the `position` parameter while acquiring the related data item inside this method and should not keep a copy of it. If you need the position of an item later on (e.g. in a click listener), use `ViewHolder#getBindingAdapterPosition()` which will have the updated adapter position. Override `onBindViewHolder(ViewHolder, int, List)` instead if Adapter can handle efficient partial bind.
`Unit`	`onBindViewHolder(@NonNull holder: VH, position: Int, @NonNull payloads: MutableList<Any!>)` Called by RecyclerView to display the data at the specified position. This method should update the contents of the `ViewHolder#itemView` to reflect the item at the given position. Note that unlike `android.widget.ListView`, RecyclerView will not call this method again if the position of the item changes in the data set unless the item itself is invalidated or the new position cannot be determined. For this reason, you should only use the `position` parameter while acquiring the related data item inside this method and should not keep a copy of it. If you need the position of an item later on (e.g. in a click listener), use `ViewHolder#getBindingAdapterPosition()` which will have the updated adapter position. Partial bind vs full bind: The payloads parameter is a merge list from `notifyItemChanged(int, Object)` or `notifyItemRangeChanged(int, int, Object)`. If the payloads list is not empty, the ViewHolder is currently bound to old data and Adapter may run an efficient partial update using the payload info. If the payload is empty, Adapter must run a full bind. Adapter should not assume that the payload passed in notify methods will be received by onBindViewHolder(). For example when the view is not attached to the screen, the payload in notifyItemChange() will be simply dropped.
`VH`	`onCreateViewHolder(@NonNull parent: ViewGroup, viewType: Int)` Called when RecyclerView needs a new `ViewHolder` of the given type to represent an item. This new ViewHolder should be constructed with a new View that can represent the items of the given type. You can either create a new View manually or inflate it from an XML layout file. The new ViewHolder will be used to display items of the adapter using `onBindViewHolder(ViewHolder, int, List)`. Since it will be re-used to display different items in the data set, it is a good idea to cache references to sub views of the View to avoid unnecessary `View#findViewById(int)` calls.
`Unit`	`onDetachedFromRecyclerView(@NonNull recyclerView: RecyclerView)` Called by RecyclerView when it stops observing this Adapter.
`Boolean`	`onFailedToRecycleView(@NonNull holder: VH)` Called by the RecyclerView if a ViewHolder created by this Adapter cannot be recycled due to its transient state. Upon receiving this callback, Adapter can clear the animation(s) that effect the View's transient state and return `true` so that the View can be recycled. Keep in mind that the View in question is already removed from the RecyclerView. In some cases, it is acceptable to recycle a View although it has transient state. Most of the time, this is a case where the transient state will be cleared in `onBindViewHolder(ViewHolder, int)` call when View is rebound to a new position. For this reason, RecyclerView leaves the decision to the Adapter and uses the return value of this method to decide whether the View should be recycled or not. Note that when all animations are created by `RecyclerView.ItemAnimator`, you should never receive this callback because RecyclerView keeps those Views as children until their animations are complete. This callback is useful when children of the item views create animations which may not be easy to implement using an `ItemAnimator`. You should never fix this issue by calling `holder.itemView.setHasTransientState(false);` unless you've previously called `holder.itemView.setHasTransientState(true);`. Each `View.setHasTransientState(true)` call must be matched by a `View.setHasTransientState(false)` call, otherwise, the state of the View may become inconsistent. You should always prefer to end or cancel animations that are triggering the transient state instead of handling it manually.
`Unit`	`onViewAttachedToWindow(@NonNull holder: VH)` Called when a view created by this adapter has been attached to a window. This can be used as a reasonable signal that the view is about to be seen by the user. If the adapter previously freed any resources in `onViewDetachedFromWindow` those resources should be restored here.
`Unit`	`onViewDetachedFromWindow(@NonNull holder: VH)` Called when a view created by this adapter has been detached from its window. Becoming detached from the window is not necessarily a permanent condition; the consumer of an Adapter's views may choose to cache views offscreen while they are not visible, attaching and detaching them as appropriate.
`Unit`	`onViewRecycled(@NonNull holder: VH)` Called when a view created by this adapter has been recycled. A view is recycled when a `LayoutManager` decides that it no longer needs to be attached to its parent `RecyclerView`. This can be because it has fallen out of visibility or a set of cached views represented by views still attached to the parent RecyclerView. If an item view has large or expensive data bound to it such as large bitmaps, this may be a good place to release those resources. RecyclerView calls this method right before clearing ViewHolder's internal data and sending it to RecycledViewPool. This way, if ViewHolder was holding valid information before being recycled, you can call `ViewHolder#getBindingAdapterPosition()` to get its adapter position.
`Unit`	`registerAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver)` Register a new observer to listen for data changes. The adapter may publish a variety of events describing specific changes. Not all adapters may support all change types and some may fall back to a generic `"something changed"` event if more specific data is not available. Components registering observers with an adapter are responsible for `unregistering` those observers when finished.
`Unit`	`setHasStableIds(hasStableIds: Boolean)` Indicates whether each item in the data set can be represented with a unique identifier of type `java.lang.Long`.
`Unit`	`setStateRestorationPolicy(@NonNull strategy: RecyclerView.Adapter.StateRestorationPolicy)` Sets the state restoration strategy for the Adapter. By default, it is set to `StateRestorationPolicy#ALLOW` which means RecyclerView expects any set Adapter to be immediately capable of restoring the RecyclerView's saved scroll position. This behaviour might be undesired if the Adapter's data is loaded asynchronously, and thus unavailable during initial layout (e.g. after Activity rotation). To avoid losing scroll position, you can change this to be either `StateRestorationPolicy#PREVENT_WHEN_EMPTY` or `StateRestorationPolicy#PREVENT`. Note that the former means your RecyclerView will restore state as soon as Adapter has 1 or more items while the latter requires you to call `setStateRestorationPolicy(StateRestorationPolicy)` with either `StateRestorationPolicy#ALLOW` or `StateRestorationPolicy#PREVENT_WHEN_EMPTY` again when the Adapter is ready to restore its state. RecyclerView will still layout even when State restoration is disabled. The behavior of how State is restored is up to the `LayoutManager`. All default LayoutManagers will override current state with restored state when state restoration happens (unless an explicit call to `LayoutManager#scrollToPosition(int)` is made). Calling this method after state is restored will not have any effect other than changing the return value of `getStateRestorationPolicy()`.
`Unit`	`unregisterAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver)` Unregister an observer currently listening for data changes. The unregistered observer will no longer receive events about changes to the adapter.

Properties
open PagedList<T>?	`currentList` Returns the PagedList currently being displayed by the PagedListAdapter.

Protected constructors

<init>

protected PagedListAdapter(diffCallback: DiffUtil.ItemCallback<T>)

Creates a PagedListAdapter with default threading and androidx.recyclerview.widget.ListUpdateCallback.

Convenience for PagedListAdapter, which uses default threading behavior.

Parameters
diffCallback: DiffUtil.ItemCallback<T>	The DiffUtil.ItemCallback instance to compare items in the list.

<init>

protected PagedListAdapter(config: AsyncDifferConfig<T>)

Public methods

addLoadStateListener

open fun addLoadStateListener(listener: (LoadType, LoadState) -> Unit): Unit

Add a LoadState listener to observe the loading state of the current PagedList.

As new PagedLists are submitted and displayed, the listener will be notified to reflect current LoadType.REFRESH, LoadType.PREPEND, and LoadType.APPEND states.

Parameters
listener: (LoadType, LoadState) -> Unit	Listener to receive LoadState updates.

See Also

removeLoadStateListener

getItemCount

open fun getItemCount(): Int

onCurrentListChanged

open fun onCurrentListChanged(currentList: PagedList<T>?): Unit

Deprecated.

Called when the current PagedList is updated.

This may be dispatched as part of .submitList if a background diff isn't needed (such as when the first list is passed, or the list is cleared). In either case, PagedListAdapter will simply call notifyItemRangeInserted/Removed(0, mPreviousSize).

This method will notbe called when the Adapter switches from presenting a PagedList to a snapshot version of the PagedList during a diff. This means you cannot observe each PagedList via this method.

Parameters
currentList: PagedList<T>?	new PagedList being displayed, may be null.

See Also

currentList

onCurrentListChanged

open fun onCurrentListChanged(
    previousList: PagedList<T>?, 
    currentList: PagedList<T>?
): Unit

Called when the current PagedList is updated.

Parameters
previousList: PagedList<T>?	PagedList that was previously displayed, may be null.
currentList: PagedList<T>?	new PagedList being displayed, may be null.

See Also

currentList

removeLoadStateListener

open fun removeLoadStateListener(listener: (LoadType, LoadState) -> Unit): Unit

Remove a previously registered LoadState listener.

Parameters
listener: (LoadType, LoadState) -> Unit	Previously registered listener.

See Also

addLoadStateListener

submitList

open fun submitList(pagedList: PagedList<T>?): Unit

Set the new list to be displayed.

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.

Parameters
pagedList: PagedList<T>?	The new list to be displayed.

submitList

open fun submitList(
    pagedList: PagedList<T>?, 
    commitCallback: Runnable?
): Unit

Set the new list to be displayed.

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.

The commit callback can be used to know when the PagedList is committed, but note that it may not be executed. If PagedList B is submitted immediately after PagedList A, and is committed directly, the callback associated with PagedList A will not be run.

Parameters
pagedList: PagedList<T>?	The new list to be displayed.
commitCallback: Runnable?	Optional runnable that is executed when the PagedList is committed, if it is committed.

withLoadStateFooter

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

Create a ConcatAdapter with the provided LoadStateAdapters displaying the LoadType.PREPEND 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.APPEND 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 methods

getItem

protected open fun getItem(position: Int): T?

Properties

currentList

open val currentList: PagedList<T>?

Returns the PagedList currently being displayed by the PagedListAdapter.

This is not necessarily the most recent list passed to submitList, because a diff is computed asynchronously between the new list and the current list before updating the currentList value. May be null if no PagedList is being presented.

Return
The list currently being displayed.

See Also

onCurrentListChanged