Adapter

<init>

Adapter()

Base class for an Adapter

Adapters provide a binding from an app-specific data set to views that are displayed within a RecyclerView.

bindViewHolder

fun bindViewHolder(
    @NonNull holder: VH, 
    position: Int
): Unit

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.

createViewHolder

@NonNull fun createViewHolder(
    @NonNull parent: ViewGroup, 
    viewType: Int
): VH

This method calls onCreateViewHolder(ViewGroup, int) to create a new ViewHolder and initializes some private fields to be used by RecyclerView.

findRelativeAdapterPositionIn

open fun findRelativeAdapterPositionIn(
    @NonNull adapter: RecyclerView.Adapter<out RecyclerView.ViewHolder!>, 
    @NonNull viewHolder: RecyclerView.ViewHolder, 
    localPosition: Int
): 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.

getItemCount

abstract fun getItemCount(): Int

Returns the total number of items in the data set held by the adapter.

getItemId

open fun getItemId(position: Int): Long

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.

getItemViewType

open fun getItemViewType(position: Int): 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.

getStateRestorationPolicy

@NonNull fun getStateRestorationPolicy(): RecyclerView.Adapter.StateRestorationPolicy

Returns when this Adapter wants to restore the state.

hasObservers

fun hasObservers(): Boolean

Returns true if one or more observers are attached to this adapter.

hasStableIds

fun hasStableIds(): Boolean

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.

notifyDataSetChanged

fun notifyDataSetChanged(): Unit

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.

notifyItemChanged

fun notifyItemChanged(position: Int): Unit

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.

notifyItemChanged

fun notifyItemChanged(
    position: Int, 
    @Nullable payload: Any?
): Unit

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.

notifyItemInserted

fun notifyItemInserted(position: Int): Unit

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.

notifyItemMoved

fun notifyItemMoved(
    fromPosition: Int, 
    toPosition: Int
): Unit

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.

notifyItemRangeChanged

fun notifyItemRangeChanged(
    positionStart: Int, 
    itemCount: Int
): Unit

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.

notifyItemRangeChanged

fun notifyItemRangeChanged(
    positionStart: Int, 
    itemCount: Int, 
    @Nullable payload: Any?
): Unit

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.

notifyItemRangeInserted

fun notifyItemRangeInserted(
    positionStart: Int, 
    itemCount: Int
): Unit

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.

notifyItemRangeRemoved

fun notifyItemRangeRemoved(
    positionStart: Int, 
    itemCount: Int
): Unit

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.

notifyItemRemoved

fun notifyItemRemoved(position: Int): Unit

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.

onAttachedToRecyclerView

open fun onAttachedToRecyclerView(@NonNull recyclerView: RecyclerView): Unit

Called by RecyclerView when it starts observing this Adapter.

Keep in mind that same adapter may be observed by multiple RecyclerViews.

onBindViewHolder

abstract fun onBindViewHolder(
    @NonNull holder: VH, 
    position: Int
): Unit

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.

onBindViewHolder

open fun onBindViewHolder(
    @NonNull holder: VH, 
    position: Int, 
    @NonNull payloads: MutableList<Any!>
): Unit

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.

onCreateViewHolder

@NonNull abstract fun onCreateViewHolder(
    @NonNull parent: ViewGroup, 
    viewType: Int
): VH

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.

onDetachedFromRecyclerView

open fun onDetachedFromRecyclerView(@NonNull recyclerView: RecyclerView): Unit

Called by RecyclerView when it stops observing this Adapter.

onFailedToRecycleView

open fun onFailedToRecycleView(@NonNull holder: VH): Boolean

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.

onViewAttachedToWindow

open fun onViewAttachedToWindow(@NonNull holder: VH): Unit

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.

onViewDetachedFromWindow

open fun onViewDetachedFromWindow(@NonNull holder: VH): Unit

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.

onViewRecycled

open fun onViewRecycled(@NonNull holder: VH): Unit

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.

registerAdapterDataObserver

open fun registerAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver): Unit

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.

setHasStableIds

open fun setHasStableIds(hasStableIds: Boolean): Unit

Indicates whether each item in the data set can be represented with a unique identifier of type java.lang.Long.

setStateRestorationPolicy

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

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().

unregisterAdapterDataObserver

open fun unregisterAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver): Unit

Unregister an observer currently listening for data changes.

The unregistered observer will no longer receive events about changes to the adapter.