PagedList
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android
abstract class PagedList<T : Any> : AbstractList
Lazy loading list that pages in immutable content from a PagingSource.
A PagedList is a List which loads its data in chunks (pages) from a PagingSource. Items can be accessed with get, and further loading can be triggered with loadAround. To display a PagedList, see androidx.paging.PagedListAdapter, which enables the binding of a PagedList to a androidx.recyclerview.widget.RecyclerView.
Loading Data
All data in a PagedList is loaded from its PagingSource. Creating a PagedList loads the first chunk of data from the PagingSource immediately, and should for this reason be done on a background thread. The constructed PagedList may then be passed to and used on the UI thread. This is done to prevent passing a list with no loaded content to the UI thread, which should generally not be presented to the user.
A PagedList initially presents this first partial load as its content, and expands over time as content is loaded in. When loadAround is called, items will be loaded in near the passed list index. If placeholder nulls are present in the list, they will be replaced as content is loaded. If not, newly loaded items will be inserted at the beginning or end of the list.
PagedList can present data for an unbounded, infinite scrolling list, or a very large but countable list. Use PagedList.Config to control how many items a PagedList loads, and when.
If you use androidx.paging.LivePagedListBuilder to get a androidx.lifecycle.LiveData, it will initialize PagedLists on a background thread for you.
Placeholders
There are two ways that PagedList can represent its not-yet-loaded data - with or without null placeholders.
With placeholders, the PagedList is always the full size of the data set. get(N) returns the Nth item in the data set, or null if it's not yet loaded.
Without null placeholders, the PagedList is the sublist of data that has already been loaded. The size of the PagedList is the number of currently loaded items, and get(N) returns the Nth loaded item. This is not necessarily the Nth item in the data set.
Placeholders have several benefits:
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They express the full sized list to the presentation layer (often a
androidx.paging.PagedListAdapter), and so can support scrollbars (without jumping as pages are loaded or dropped) and fast-scrolling to any position, loaded or not. -
They avoid the need for a loading spinner at the end of the loaded list, since the list is always full sized.
They also have drawbacks:
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Your Adapter needs to account for
nullitems. This often means providing default values in data you bind to aandroidx.recyclerview.widget.RecyclerView.ViewHolder. -
They don't work well if your item views are of different sizes, as this will prevent loading items from cross-fading nicely.
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They require you to count your data set, which can be expensive or impossible, depending on your
PagingSource.
Placeholders are enabled by default, but can be disabled in two ways. They are disabled if the PagingSource does not count its data set in its initial load, or if false is passed to PagedList.Config.Builder.setEnablePlaceholders when building a PagedList.Config.
Mutability and Snapshots
A PagedList is mutable while loading, or ready to load from its PagingSource. As loads succeed, a mutable PagedList will be updated via Runnables on the main thread. You can listen to these updates with a PagedList.Callback. (Note that androidx.paging .PagedListAdapter will listen to these to signal RecyclerView about the updates/changes).
If a PagedList attempts to load from an invalid PagingSource, it will detach from the PagingSource, meaning that it will no longer attempt to load data. It will return true from isImmutable, and a new PagingSource / PagedList pair must be created to load further data.
See PagingSource and androidx.paging.LivePagedListBuilder for how new PagedLists are created to represent changed data.
A PagedList snapshot is simply an immutable shallow copy of the current state of the PagedList as a List. It will reference the same inner items, and contain the same null placeholders, if present.
| Parameters | |
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<T : Any> |
The type of the entries in the list. |
Summary
Nested types |
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@MainThreadThis class is deprecated. PagedList is deprecated and has been replaced by PagingData |
class PagedList.Builder<Key : Any, Value : Any>This class is deprecated. PagedList is deprecated and has been replaced by PagingData, which no longer supports constructing snapshots of loaded data manually. |
abstract class PagedList.CallbackThis class is deprecated. PagedList is deprecated and has been replaced by PagingData |
class PagedList.ConfigThis class is deprecated. PagedList is deprecated and has been replaced by PagingData |
class PagedList.Config.BuilderThis class is deprecated. PagedList is deprecated and has been replaced by PagingData |
Public functions |
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Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Unit |
This function is deprecated. Dispatching a diff since snapshot created is behavior that can be instead tracked by attaching a Callback to the PagedList that is mutating, and tracking changes since calling PagedList.snapshot(). |
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Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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abstract Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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open operator T? |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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open Unit |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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List<T> |
This function is deprecated. PagedList is deprecated and has been replaced by PagingData |
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PagedList.Config |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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DataSource<*, T> |
This property is deprecated. DataSource is deprecated and has been replaced by PagingSource. |
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abstract Boolean |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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open Boolean |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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abstract Any? |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Int |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Int |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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open Int |
This property is deprecated. PagedList is deprecated and has been replaced by PagingData |
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Inherited properties |
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Public functions
addWeakCallback
funaddWeakCallback(callback: PagedList.Callback): Unit
Adds a callback.
The callback is internally held as weak reference, so PagedList doesn't hold a strong reference to its observer, such as a androidx.paging.PagedListAdapter. If an adapter were held with a strong reference, it would be necessary to clear its PagedList observer before it could be GC'd.
| Parameters | |
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callback: PagedList.Callback |
Callback to dispatch to. |
| See also | |
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removeWeakCallback |
addWeakCallback
funaddWeakCallback(previousSnapshot: List<T>?, callback: PagedList.Callback): Unit
Adds a callback, and issues updates since the previousSnapshot was created.
If previousSnapshot is passed, the callback will also immediately be dispatched any differences between the previous snapshot, and the current state. For example, if the previousSnapshot was of 5 nulls, 10 items, 5 nulls, and the current state was 5 nulls, 12 items, 3 nulls, the callback would immediately receive a call ofonChanged(14, 2).
This allows an observer that's currently presenting a snapshot to catch up to the most recent version, including any changes that may have been made.
The callback is internally held as weak reference, so PagedList doesn't hold a strong reference to its observer, such as a PagedListAdapter. If an adapter were held with a strong reference, it would be necessary to clear its PagedList observer before it could be GC'd.
| Parameters | |
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previousSnapshot: List<T>? |
Snapshot previously captured from this List, or |
callback: PagedList.Callback |
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removeWeakCallback |
addWeakLoadStateListener
funaddWeakLoadStateListener(listener: (LoadType, LoadState) -> Unit): Unit
Add a listener to observe the loading state of the PagedList.
| See also | |
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removeWeakLoadStateListener |
detach
abstract fundetach(): Unit
Detach the PagedList from its PagingSource, and attempt to load no more data.
This is called automatically when a PagingSource is observed to be invalid, which is a signal to stop loading. The PagedList will continue to present existing data, but will not initiate new loads.
get
open operator funget(index: Int): T?
Get the item in the list of loaded items at the provided index.
| Returns | |
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T? |
The item at the passed index, or |
| See also | |
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size |
loadAround
funloadAround(index: Int): Unit
Load adjacent items to passed index.
| Parameters | |
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index: Int |
Index at which to load. |
| Throws | |
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kotlin.IndexOutOfBoundsException |
if index is not within bounds. |
removeWeakCallback
funremoveWeakCallback(callback: PagedList.Callback): Unit
Removes a previously added callback.
| Parameters | |
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callback: PagedList.Callback |
Callback, previously added. |
| See also | |
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addWeakCallback |
removeWeakLoadStateListener
funremoveWeakLoadStateListener(listener: (LoadType, LoadState) -> Unit): Unit
Remove a previously registered load state listener.
| See also | |
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addWeakLoadStateListener |
retry
open funretry(): Unit
Retry any errors associated with this PagedList.
If for example a network PagingSource append timed out, calling this method will retry the failed append load.
You can observe loading state via addWeakLoadStateListener, though generally this is done through the PagedListAdapter or AsyncPagedListDiffer.
snapshot
funsnapshot(): List<T>
Returns an immutable snapshot of the PagedList in its current state.
If this is immutableisImmutable due to its PagingSource being invalid, it will be returned.
Public properties
config
val config: PagedList.Config
Return the Config used to construct this PagedList.
| Returns | |
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PagedList.Config |
the Config of this PagedList |
dataSource
val dataSource: DataSource<*, T>
| Throws | |
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kotlin.IllegalStateException |
if this |
isDetached
abstract val isDetached: Boolean
True if the PagedList has detached the PagingSource it was loading from, and will no longer load new data.
A detached list is immutable.
| Returns | |
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Boolean |
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isImmutable
open val isImmutable: Boolean
Returns whether the list is immutable.
Immutable lists may not become mutable again, and may safely be accessed from any thread.
In the future, this method may return true when a PagedList has completed loading from its PagingSource. Currently, it is equivalent to isDetached.
lastKey
abstract val lastKey: Any?
Return the key for the position passed most recently to loadAround.
When a PagedList is invalidated, you can pass the key returned by this function to initialize the next PagedList. This ensures (depending on load times) that the next PagedList that arrives will have data that overlaps. If you use androidx.paging.LivePagedListBuilder, it will do this for you.
| Returns | |
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Any? |
Key of position most recently passed to |
loadedCount
val loadedCount: Int
Returns the number of items loaded in the PagedList.
Unlike size this counts only loaded items, not placeholders.
If placeholders are disabled, this method is equivalent to size.
| Returns | |
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Int |
Number of items currently loaded, not counting placeholders. |
| See also | |
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size |
positionOffset
val positionOffset: Int
Position offset of the data in the list.
If the PagingSource backing this PagedList is counted, the item returned from get(i) has a position in the original data set of i + getPositionOffset().
If placeholders are enabled, this value is always 0, since get(i) will return either the data in its original index, or null if it is not loaded.
size
open val size: Int
Size of the list, including any placeholders (not-yet-loaded null padding).
To get the number of loaded items, not counting placeholders, use loadedCount.
| See also | |
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loadedCount |