Adapter
abstract class Adapter<VH : RecyclerView.ViewHolder!>
kotlin.Any | |
↳ | androidx.recyclerview.widget.RecyclerView.Adapter |
Base class for an Adapter
Adapters provide a binding from an app-specific data set to views that are displayed within a RecyclerView
.
Summary
Public constructors |
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<init>() Base class for an Adapter |
Public methods |
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Unit |
bindViewHolder(@NonNull holder: VH, position: Int) This method internally calls |
VH |
createViewHolder(@NonNull parent: ViewGroup, viewType: Int) This method calls |
abstract Int |
Returns the total number of items in the data set held by the adapter. |
open Long |
Return the stable ID for the item at |
open Int |
getItemViewType(position: Int) Return the view type of the item at |
Boolean |
Returns true if one or more observers are attached to this adapter. |
Boolean |
Returns true if this adapter publishes a unique |
Unit |
Notify any registered observers that the data set has changed. |
Unit |
notifyItemChanged(position: Int) Notify any registered observers that the item at |
Unit |
notifyItemChanged(position: Int, @Nullable payload: Any?) Notify any registered observers that the item at |
Unit |
notifyItemInserted(position: Int) Notify any registered observers that the item reflected at |
Unit |
notifyItemMoved(fromPosition: Int, toPosition: Int) Notify any registered observers that the item reflected at |
Unit |
notifyItemRangeChanged(positionStart: Int, itemCount: Int) Notify any registered observers that the |
Unit |
notifyItemRangeChanged(positionStart: Int, itemCount: Int, @Nullable payload: Any?) Notify any registered observers that the |
Unit |
notifyItemRangeInserted(positionStart: Int, itemCount: Int) Notify any registered observers that the currently reflected |
Unit |
notifyItemRangeRemoved(positionStart: Int, itemCount: Int) Notify any registered observers that the |
Unit |
notifyItemRemoved(position: Int) Notify any registered observers that the item previously located at |
open Unit |
onAttachedToRecyclerView(@NonNull recyclerView: RecyclerView) Called by RecyclerView when it starts observing this Adapter. |
abstract Unit |
onBindViewHolder(@NonNull holder: VH, position: Int) Called by RecyclerView to display the data at the specified position. |
open Unit |
onBindViewHolder(@NonNull holder: VH, position: Int, @NonNull payloads: MutableList<Any!>) Called by RecyclerView to display the data at the specified position. |
abstract VH |
onCreateViewHolder(@NonNull parent: ViewGroup, viewType: Int) Called when RecyclerView needs a new |
open Unit |
onDetachedFromRecyclerView(@NonNull recyclerView: RecyclerView) Called by RecyclerView when it stops observing this Adapter. |
open Boolean |
onFailedToRecycleView(@NonNull holder: VH) Called by the RecyclerView if a ViewHolder created by this Adapter cannot be recycled due to its transient state. |
open Unit |
onViewAttachedToWindow(@NonNull holder: VH) Called when a view created by this adapter has been attached to a window. |
open Unit |
onViewDetachedFromWindow(@NonNull holder: VH) Called when a view created by this adapter has been detached from its window. |
open Unit |
onViewRecycled(@NonNull holder: VH) Called when a view created by this adapter has been recycled. |
open Unit |
registerAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver) Register a new observer to listen for data changes. |
open Unit |
setHasStableIds(hasStableIds: Boolean) Indicates whether each item in the data set can be represented with a unique identifier of type |
open Unit |
unregisterAdapterDataObserver(@NonNull observer: RecyclerView.AdapterDataObserver) Unregister an observer currently listening for data changes. |
Public constructors
<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
.
Public methods
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.
See Also
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.
See Also
getItemCount
abstract fun getItemCount(): Int
Returns the total number of items in the data set held by the adapter.
Return | |
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Int: The total number of items in this 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
.
Parameters | |
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position |
Int: Adapter position to query |
Return | |
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Long: the stable ID of the item at position |
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.
Parameters | |
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position |
Int: position to query |
Return | |
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Int: integer value identifying the type of the view needed to represent the item at position . Type codes need not be contiguous. |
hasObservers
fun hasObservers(): Boolean
Returns true if one or more observers are attached to this adapter.
Return | |
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Boolean: true if this adapter has observers |
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.
Return | |
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Boolean: true if this adapter's items have stable IDs |
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.
Parameters | |
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position |
Int: Position of the item that has changed |
See Also
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.
Parameters | |
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position |
Int: Position of the item that has changed |
payload |
Int: Optional parameter, use null to identify a "full" update |
See Also
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.
Parameters | |
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position |
Int: Position of the newly inserted item in the data set |
See Also
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.
Parameters | |
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fromPosition |
Int: Previous position of the item. |
toPosition |
Int: New position of the item. |
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.
Parameters | |
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positionStart |
Int: Position of the first item that has changed |
itemCount |
Int: Number of items that have changed |
See Also
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.
Parameters | |
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positionStart |
Int: Position of the first item that has changed |
itemCount |
Int: Number of items that have changed |
payload |
Int: Optional parameter, use null to identify a "full" update |
See Also
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.
Parameters | |
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positionStart |
Int: Position of the first item that was inserted |
itemCount |
Int: Number of items inserted |
See Also
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.
Parameters | |
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positionStart |
Int: Previous position of the first item that was removed |
itemCount |
Int: Number of items removed from the data set |
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.
Parameters | |
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position |
Int: Position of the item that has now been removed |
See Also
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.
Parameters | |
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recyclerView |
RecyclerView: The RecyclerView instance which started observing this adapter. |
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#getAdapterPosition()
which will have the updated adapter position. Override onBindViewHolder(ViewHolder, int, List)
instead if Adapter can handle efficient partial bind.
Parameters | |
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holder |
VH: The ViewHolder which should be updated to represent the contents of the item at the given position in the data set. |
position |
VH: The position of the item within the adapter's data set. |
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#getAdapterPosition()
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.
Parameters | |
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holder |
VH: The ViewHolder which should be updated to represent the contents of the item at the given position in the data set. |
position |
VH: The position of the item within the adapter's data set. |
payloads |
VH: A non-null list of merged payloads. Can be empty list if requires full update. |
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.
Parameters | |
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parent |
ViewGroup: The ViewGroup into which the new View will be added after it is bound to an adapter position. |
viewType |
ViewGroup: The view type of the new View. |
Return | |
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VH: A new ViewHolder that holds a View of the given view type. |
onDetachedFromRecyclerView
open fun onDetachedFromRecyclerView(@NonNull recyclerView: RecyclerView): Unit
Called by RecyclerView when it stops observing this Adapter.
Parameters | |
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recyclerView |
RecyclerView: The RecyclerView instance which stopped 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.
Parameters | |
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holder |
VH: The ViewHolder containing the View that could not be recycled due to its transient state. |
Return | |
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Boolean: True if the View should be recycled, false otherwise. Note that if this method returns true , RecyclerView will ignore the transient state of the View and recycle it regardless. If this method returns false , RecyclerView will check the View's transient state again before giving a final decision. Default implementation returns false. |
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.
Parameters | |
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holder |
VH: Holder of the view being attached |
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.
Parameters | |
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holder |
VH: Holder of the view being detached |
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#getAdapterPosition()
to get its adapter position.
Parameters | |
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holder |
VH: The ViewHolder for the view being recycled |
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.
Parameters | |
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observer |
RecyclerView.AdapterDataObserver: Observer to register |
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
.
Parameters | |
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hasStableIds |
Boolean: Whether items in data set have unique identifiers or not. |
See Also
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.
Parameters | |
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observer |
RecyclerView.AdapterDataObserver: Observer to unregister |