belongs to Maven artifact com.android.support:recyclerview-v7:28.0.0-alpha1
LinearLayoutManager
public
class
LinearLayoutManager
extends RecyclerView.LayoutManager
implements
ItemTouchHelper.ViewDropHandler,
RecyclerView.SmoothScroller.ScrollVectorProvider
java.lang.Object | ||
↳ | android.support.v7.widget.RecyclerView.LayoutManager | |
↳ | android.support.v7.widget.LinearLayoutManager |
Known Direct Subclasses |
A RecyclerView.LayoutManager
implementation which provides
similar functionality to ListView
.
Summary
Nested classes | |
---|---|
class |
LinearLayoutManager.LayoutChunkResult
|
XML attributes | |
---|---|
RecyclerView_android_orientation |
|
RecyclerView_reverseLayout |
|
RecyclerView_stackFromEnd |
Inherited XML attributes | |
---|---|
From
class
android.support.v7.widget.RecyclerView.LayoutManager
|
Constants | |
---|---|
int |
HORIZONTAL
|
int |
INVALID_OFFSET
|
int |
VERTICAL
|
Public constructors | |
---|---|
LinearLayoutManager(Context context)
Creates a vertical LinearLayoutManager |
|
LinearLayoutManager(Context context, int orientation, boolean reverseLayout)
|
|
LinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)
Constructor used when layout manager is set in XML by RecyclerView attribute "layoutManager". |
Public methods | |
---|---|
void
|
assertNotInLayoutOrScroll(String message)
Checks if RecyclerView is in the middle of a layout or scroll and throws an
|
boolean
|
canScrollHorizontally()
Query if horizontal scrolling is currently supported. |
boolean
|
canScrollVertically()
Query if vertical scrolling is currently supported. |
void
|
collectAdjacentPrefetchPositions(int dx, int dy, RecyclerView.State state, RecyclerView.LayoutManager.LayoutPrefetchRegistry layoutPrefetchRegistry)
Gather all positions from the LayoutManager to be prefetched, given specified momentum. |
void
|
collectInitialPrefetchPositions(int adapterItemCount, RecyclerView.LayoutManager.LayoutPrefetchRegistry layoutPrefetchRegistry)
Gather all positions from the LayoutManager to be prefetched in preperation for its RecyclerView to come on screen, due to the movement of another, containing RecyclerView. |
int
|
computeHorizontalScrollExtent(RecyclerView.State state)
Override this method if you want to support scroll bars. |
int
|
computeHorizontalScrollOffset(RecyclerView.State state)
Override this method if you want to support scroll bars. |
int
|
computeHorizontalScrollRange(RecyclerView.State state)
Override this method if you want to support scroll bars. |
PointF
|
computeScrollVectorForPosition(int targetPosition)
Should calculate the vector that points to the direction where the target position can be found. |
int
|
computeVerticalScrollExtent(RecyclerView.State state)
Override this method if you want to support scroll bars. |
int
|
computeVerticalScrollOffset(RecyclerView.State state)
Override this method if you want to support scroll bars. |
int
|
computeVerticalScrollRange(RecyclerView.State state)
Override this method if you want to support scroll bars. |
int
|
findFirstCompletelyVisibleItemPosition()
Returns the adapter position of the first fully visible view. |
int
|
findFirstVisibleItemPosition()
Returns the adapter position of the first visible view. |
int
|
findLastCompletelyVisibleItemPosition()
Returns the adapter position of the last fully visible view. |
int
|
findLastVisibleItemPosition()
Returns the adapter position of the last visible view. |
View
|
findViewByPosition(int position)
Finds the view which represents the given adapter position. |
RecyclerView.LayoutParams
|
generateDefaultLayoutParams()
Create a default |
int
|
getInitialPrefetchItemCount()
Gets the number of items to prefetch in
|
int
|
getOrientation()
Returns the current orientation of the layout. |
boolean
|
getRecycleChildrenOnDetach()
Returns whether LayoutManager will recycle its children when it is detached from RecyclerView. |
boolean
|
getReverseLayout()
Returns if views are laid out from the opposite direction of the layout. |
boolean
|
getStackFromEnd()
|
boolean
|
isAutoMeasureEnabled()
Returns whether the measuring pass of layout should use the AutoMeasure mechanism of
|
boolean
|
isSmoothScrollbarEnabled()
Returns the current state of the smooth scrollbar feature. |
void
|
onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler)
Called when this LayoutManager is detached from its parent RecyclerView or when its parent RecyclerView is detached from its window. |
View
|
onFocusSearchFailed(View focused, int focusDirection, RecyclerView.Recycler recycler, RecyclerView.State state)
Called when searching for a focusable view in the given direction has failed for the current content of the RecyclerView. |
void
|
onInitializeAccessibilityEvent(AccessibilityEvent event)
|
void
|
onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)
Lay out all relevant child views from the given adapter. |
void
|
onLayoutCompleted(RecyclerView.State state)
Called after a full layout calculation is finished. |
void
|
onRestoreInstanceState(Parcelable state)
|
Parcelable
|
onSaveInstanceState()
Called when the LayoutManager should save its state. |
int
|
scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler, RecyclerView.State state)
Scroll horizontally by dx pixels in screen coordinates and return the distance traveled. |
void
|
scrollToPosition(int position)
Scroll the RecyclerView to make the position visible. |
void
|
scrollToPositionWithOffset(int position, int offset)
Scroll to the specified adapter position with the given offset from resolved layout start. |
int
|
scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state)
Scroll vertically by dy pixels in screen coordinates and return the distance traveled. |
void
|
setInitialPrefetchItemCount(int itemCount)
Sets the number of items to prefetch in
|
void
|
setOrientation(int orientation)
Sets the orientation of the layout. |
void
|
setRecycleChildrenOnDetach(boolean recycleChildrenOnDetach)
Set whether LayoutManager will recycle its children when it is detached from RecyclerView. |
void
|
setReverseLayout(boolean reverseLayout)
Used to reverse item traversal and layout order. |
void
|
setSmoothScrollbarEnabled(boolean enabled)
When smooth scrollbar is enabled, the position and size of the scrollbar thumb is computed based on the number of visible pixels in the visible items. |
void
|
setStackFromEnd(boolean stackFromEnd)
Compatibility support for |
void
|
smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position)
Smooth scroll to the specified adapter position. |
boolean
|
supportsPredictiveItemAnimations()
Returns whether this LayoutManager supports "predictive item animations". |
Protected methods | |
---|---|
int
|
getExtraLayoutSpace(RecyclerView.State state)
Returns the amount of extra space that should be laid out by LayoutManager. |
boolean
|
isLayoutRTL()
|
Inherited methods | |
---|---|
From
class
android.support.v7.widget.RecyclerView.LayoutManager
| |
From
class
java.lang.Object
| |
From
interface
android.support.v7.widget.helper.ItemTouchHelper.ViewDropHandler
| |
From
interface
android.support.v7.widget.RecyclerView.SmoothScroller.ScrollVectorProvider
|
XML attributes
RecyclerView_android_orientation
Related methods:
RecyclerView_reverseLayout
Related methods:
RecyclerView_stackFromEnd
Related methods:
Constants
Public constructors
LinearLayoutManager
LinearLayoutManager (Context context)
Creates a vertical LinearLayoutManager
Parameters | |
---|---|
context |
Context : Current context, will be used to access resources.
|
LinearLayoutManager
LinearLayoutManager (Context context, int orientation, boolean reverseLayout)
Parameters | |
---|---|
context |
Context : Current context, will be used to access resources. |
orientation |
int : Layout orientation. Should be HORIZONTAL or VERTICAL . |
reverseLayout |
boolean : When set to true, layouts from end to start.
|
LinearLayoutManager
LinearLayoutManager (Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)
Constructor used when layout manager is set in XML by RecyclerView attribute "layoutManager". Defaults to vertical orientation.
Related XML Attributes:
Parameters | |
---|---|
context |
Context |
attrs |
AttributeSet |
defStyleAttr |
int |
defStyleRes |
int |
Public methods
assertNotInLayoutOrScroll
void assertNotInLayoutOrScroll (String message)
Checks if RecyclerView is in the middle of a layout or scroll and throws an
IllegalStateException
if it is.
Parameters | |
---|---|
message |
String : The message for the exception. Can be null. |
canScrollHorizontally
boolean canScrollHorizontally ()
Query if horizontal scrolling is currently supported. The default implementation returns false.
Returns | |
---|---|
boolean |
true if getOrientation() is HORIZONTAL
|
canScrollVertically
boolean canScrollVertically ()
Query if vertical scrolling is currently supported. The default implementation returns false.
Returns | |
---|---|
boolean |
true if getOrientation() is VERTICAL
|
collectAdjacentPrefetchPositions
void collectAdjacentPrefetchPositions (int dx, int dy, RecyclerView.State state, RecyclerView.LayoutManager.LayoutPrefetchRegistry layoutPrefetchRegistry)
Gather all positions from the LayoutManager to be prefetched, given specified momentum.
If item prefetch is enabled, this method is called in between traversals to gather which positions the LayoutManager will soon need, given upcoming movement in subsequent traversals.
The LayoutManager should call addPosition(int, int)
for
each item to be prepared, and these positions will have their ViewHolders created and
bound, if there is sufficient time available, in advance of being needed by a
scroll or layout.
Parameters | |
---|---|
dx |
int : X movement component. |
dy |
int : Y movement component. |
state |
RecyclerView.State : State of RecyclerView |
layoutPrefetchRegistry |
RecyclerView.LayoutManager.LayoutPrefetchRegistry : PrefetchRegistry to add prefetch entries into. |
collectInitialPrefetchPositions
void collectInitialPrefetchPositions (int adapterItemCount, RecyclerView.LayoutManager.LayoutPrefetchRegistry layoutPrefetchRegistry)
Gather all positions from the LayoutManager to be prefetched in preperation for its RecyclerView to come on screen, due to the movement of another, containing RecyclerView.
This method is only called when a RecyclerView is nested in another RecyclerView.
If item prefetch is enabled for this LayoutManager, as well in another containing LayoutManager, this method is called in between draw traversals to gather which positions this LayoutManager will first need, once it appears on the screen.
For example, if this LayoutManager represents a horizontally scrolling list within a vertically scrolling LayoutManager, this method would be called when the horizontal list is about to come onscreen.
The LayoutManager should call addPosition(int, int)
for
each item to be prepared, and these positions will have their ViewHolders created and
bound, if there is sufficient time available, in advance of being needed by a
scroll or layout.
Parameters | |
---|---|
adapterItemCount |
int : number of items in the associated adapter. |
layoutPrefetchRegistry |
RecyclerView.LayoutManager.LayoutPrefetchRegistry : PrefetchRegistry to add prefetch entries into. |
computeHorizontalScrollExtent
int computeHorizontalScrollExtent (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeHorizontalScrollExtent()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current state of RecyclerView |
Returns | |
---|---|
int |
The horizontal extent of the scrollbar's thumb |
computeHorizontalScrollOffset
int computeHorizontalScrollOffset (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeHorizontalScrollOffset()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current State of RecyclerView where you can find total item count |
Returns | |
---|---|
int |
The horizontal offset of the scrollbar's thumb |
computeHorizontalScrollRange
int computeHorizontalScrollRange (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeHorizontalScrollRange()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current State of RecyclerView where you can find total item count |
Returns | |
---|---|
int |
The total horizontal range represented by the vertical scrollbar |
computeScrollVectorForPosition
PointF computeScrollVectorForPosition (int targetPosition)
Should calculate the vector that points to the direction where the target position can be found.
This method is used by the LinearSmoothScroller
to initiate a scroll towards
the target position.
The magnitude of the vector is not important. It is always normalized before being
used by the LinearSmoothScroller
.
LayoutManager should not check whether the position exists in the adapter or not.
Parameters | |
---|---|
targetPosition |
int : the target position to which the returned vector should point |
Returns | |
---|---|
PointF |
the scroll vector for a given position. |
computeVerticalScrollExtent
int computeVerticalScrollExtent (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeVerticalScrollExtent()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current state of RecyclerView |
Returns | |
---|---|
int |
The vertical extent of the scrollbar's thumb |
computeVerticalScrollOffset
int computeVerticalScrollOffset (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeVerticalScrollOffset()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current State of RecyclerView where you can find total item count |
Returns | |
---|---|
int |
The vertical offset of the scrollbar's thumb |
computeVerticalScrollRange
int computeVerticalScrollRange (RecyclerView.State state)
Override this method if you want to support scroll bars.
Read computeVerticalScrollRange()
for details.
Default implementation returns 0.
Parameters | |
---|---|
state |
RecyclerView.State : Current State of RecyclerView where you can find total item count |
Returns | |
---|---|
int |
The total vertical range represented by the vertical scrollbar |
findFirstCompletelyVisibleItemPosition
int findFirstCompletelyVisibleItemPosition ()
Returns the adapter position of the first fully visible view. This position does not include adapter changes that were dispatched after the last layout pass.
Note that bounds check is only performed in the current orientation. That means, if LayoutManager is horizontal, it will only check the view's left and right edges.
Returns | |
---|---|
int |
The adapter position of the first fully visible item or
NO_POSITION if there aren't any visible items. |
findFirstVisibleItemPosition
int findFirstVisibleItemPosition ()
Returns the adapter position of the first visible view. This position does not include adapter changes that were dispatched after the last layout pass.
Note that, this value is not affected by layout orientation or item order traversal.
(setReverseLayout(boolean)
). Views are sorted by their positions in the adapter,
not in the layout.
If RecyclerView has item decorators, they will be considered in calculations as well.
LayoutManager may pre-cache some views that are not necessarily visible. Those views are ignored in this method.
Returns | |
---|---|
int |
The adapter position of the first visible item or NO_POSITION if
there aren't any visible items. |
findLastCompletelyVisibleItemPosition
int findLastCompletelyVisibleItemPosition ()
Returns the adapter position of the last fully visible view. This position does not include adapter changes that were dispatched after the last layout pass.
Note that bounds check is only performed in the current orientation. That means, if LayoutManager is horizontal, it will only check the view's left and right edges.
Returns | |
---|---|
int |
The adapter position of the last fully visible view or
NO_POSITION if there aren't any visible items. |
findLastVisibleItemPosition
int findLastVisibleItemPosition ()
Returns the adapter position of the last visible view. This position does not include adapter changes that were dispatched after the last layout pass.
Note that, this value is not affected by layout orientation or item order traversal.
(setReverseLayout(boolean)
). Views are sorted by their positions in the adapter,
not in the layout.
If RecyclerView has item decorators, they will be considered in calculations as well.
LayoutManager may pre-cache some views that are not necessarily visible. Those views are ignored in this method.
Returns | |
---|---|
int |
The adapter position of the last visible view or NO_POSITION if
there aren't any visible items. |
findViewByPosition
View findViewByPosition (int position)
Finds the view which represents the given adapter position.
This method traverses each child since it has no information about child order. Override this method to improve performance if your LayoutManager keeps data about child views.
If a view is ignored via ignoreView(View)
, it is also ignored by this method.
Parameters | |
---|---|
position |
int : Position of the item in adapter |
Returns | |
---|---|
View |
The child view that represents the given position or null if the position is not laid out |
generateDefaultLayoutParams
RecyclerView.LayoutParams generateDefaultLayoutParams ()
Create a default LayoutParams
object for a child of the RecyclerView.
LayoutManagers will often want to use a custom LayoutParams
type
to store extra information specific to the layout. Client code should subclass
RecyclerView.LayoutParams
for this purpose.
Important: if you use your own custom LayoutParams
type
you must also override
checkLayoutParams(LayoutParams)
,
generateLayoutParams(android.view.ViewGroup.LayoutParams)
and
generateLayoutParams(android.content.Context, android.util.AttributeSet)
.
Returns | |
---|---|
RecyclerView.LayoutParams |
A new LayoutParams for a child view |
getInitialPrefetchItemCount
int getInitialPrefetchItemCount ()
Gets the number of items to prefetch in
collectInitialPrefetchPositions(int, LayoutPrefetchRegistry)
, which defines
how many inner items should be prefetched when this LayoutManager's RecyclerView
is nested inside another RecyclerView.
Returns | |
---|---|
int |
number of items to prefetch. |
getOrientation
int getOrientation ()
Returns the current orientation of the layout.
Returns | |
---|---|
int |
Current orientation, either HORIZONTAL or VERTICAL |
See also:
getRecycleChildrenOnDetach
boolean getRecycleChildrenOnDetach ()
Returns whether LayoutManager will recycle its children when it is detached from RecyclerView.
Returns | |
---|---|
boolean |
true if LayoutManager will recycle its children when it is detached from RecyclerView. |
getReverseLayout
boolean getReverseLayout ()
Returns if views are laid out from the opposite direction of the layout.
Returns | |
---|---|
boolean |
If layout is reversed or not. |
See also:
isAutoMeasureEnabled
boolean isAutoMeasureEnabled ()
Returns whether the measuring pass of layout should use the AutoMeasure mechanism of
RecyclerView
or if it should be done by the LayoutManager's implementation of
onMeasure(Recycler, State, int, int)
.
This method returns false by default (it actually returns the value passed to the
deprecated setAutoMeasureEnabled(boolean)
) and should be overridden to return
true if a LayoutManager wants to be auto measured by the RecyclerView.
If this method is overridden to return true,
onMeasure(Recycler, State, int, int)
should not be overridden.
AutoMeasure is a RecyclerView mechanism that handles the measuring pass of layout in a
simple and contract satisfying way, including the wrapping of children laid out by
LayoutManager. Simply put, it handles wrapping children by calling
onLayoutChildren(Recycler, State)
during a call to
onMeasure(int, int)
, and then calculating desired dimensions based
on children's dimensions and positions. It does this while supporting all existing
animation capabilities of the RecyclerView.
More specifically:
- When
onMeasure(int, int)
is called, if the provided measure specs both have a mode ofEXACTLY
, RecyclerView will set its measured dimensions accordingly and return, allowing layout to continue as normal (Actually, RecyclerView will callonMeasure(Recycler, State, int, int)
for backwards compatibility reasons but it should not be overridden if AutoMeasure is being used). - If one of the layout specs is not
EXACT
, the RecyclerView will start the layout process. It will first process all pending Adapter updates and then decide whether to run a predictive layout. If it decides to do so, it will first callonLayoutChildren(Recycler, State)
withisPreLayout()
set totrue
. At this stage,getWidth()
andgetHeight()
will still return the width and height of the RecyclerView as of the last layout calculation.After handling the predictive case, RecyclerView will call
onLayoutChildren(Recycler, State)
withisMeasuring()
set totrue
andisPreLayout()
set tofalse
. The LayoutManager can access the measurement specs viagetHeight()
,getHeightMode()
,getWidth()
andgetWidthMode()
. - After the layout calculation, RecyclerView sets the measured width & height by
calculating the bounding box for the children (+ RecyclerView's padding). The
LayoutManagers can override
setMeasuredDimension(Rect, int, int)
to choose different values. For instance, GridLayoutManager overrides this value to handle the case where if it is vertical and has 3 columns but only 2 items, it should still measure its width to fit 3 items, not 2. - Any following calls to
onMeasure(int, int)
will runonLayoutChildren(Recycler, State)
withisMeasuring()
set totrue
andisPreLayout()
set tofalse
. RecyclerView will take care of which views are actually added / removed / moved / changed for animations so that the LayoutManager should not worry about them and handle eachonLayoutChildren(Recycler, State)
call as if it is the last one. - When measure is complete and RecyclerView's
onLayout(boolean, int, int, int, int)
method is called, RecyclerView checks whether it already did layout calculations during the measure pass and if so, it re-uses that information. It may still decide to callonLayoutChildren(Recycler, State)
if the last measure spec was different from the final dimensions or adapter contents have changed between the measure call and the layout call. - Finally, animations are calculated and run as usual.
Returns | |
---|---|
boolean |
True if the measuring pass of layout should use the AutoMeasure
mechanism of RecyclerView or False if it should be done by the
LayoutManager's implementation of
onMeasure(Recycler, State, int, int) . |
isSmoothScrollbarEnabled
boolean isSmoothScrollbarEnabled ()
Returns the current state of the smooth scrollbar feature. It is enabled by default.
Returns | |
---|---|
boolean |
True if smooth scrollbar is enabled, false otherwise. |
See also:
onDetachedFromWindow
void onDetachedFromWindow (RecyclerView view, RecyclerView.Recycler recycler)
Called when this LayoutManager is detached from its parent RecyclerView or when its parent RecyclerView is detached from its window.
LayoutManager should clear all of its View references as another LayoutManager might be assigned to the RecyclerView.
If the RecyclerView is re-attached with the same LayoutManager and Adapter, it may not
call onLayoutChildren(Recycler, State)
if nothing has changed and a layout was
not requested on the RecyclerView while it was detached.
If your LayoutManager has View references that it cleans in on-detach, it should also
call requestLayout()
to ensure that it is re-laid out when
RecyclerView is re-attached.
Subclass implementations should always call through to the superclass implementation.
Parameters | |
---|---|
view |
RecyclerView : The RecyclerView this LayoutManager is bound to |
recycler |
RecyclerView.Recycler : The recycler to use if you prefer to recycle your children instead of
keeping them around. |
onFocusSearchFailed
View onFocusSearchFailed (View focused, int focusDirection, RecyclerView.Recycler recycler, RecyclerView.State state)
Called when searching for a focusable view in the given direction has failed for the current content of the RecyclerView.
This is the LayoutManager's opportunity to populate views in the given direction to fulfill the request if it can. The LayoutManager should attach and return the view to be focused, if a focusable view in the given direction is found. Otherwise, if all the existing (or the newly populated views) are unfocusable, it returns the next unfocusable view to become visible on the screen. This unfocusable view is typically the first view that's either partially or fully out of RV's padded bounded area in the given direction. The default implementation returns null.
Parameters | |
---|---|
focused |
View : The currently focused view |
focusDirection |
int : One of FOCUS_UP , FOCUS_DOWN ,
FOCUS_LEFT , FOCUS_RIGHT ,
FOCUS_BACKWARD , FOCUS_FORWARD
or 0 for not applicable |
recycler |
RecyclerView.Recycler : The recycler to use for obtaining views for currently offscreen items |
state |
RecyclerView.State : Transient state of RecyclerView |
Returns | |
---|---|
View |
The chosen view to be focused if a focusable view is found, otherwise an unfocusable view to become visible onto the screen, else null. |
onInitializeAccessibilityEvent
void onInitializeAccessibilityEvent (AccessibilityEvent event)
Parameters | |
---|---|
event |
AccessibilityEvent |
onLayoutChildren
void onLayoutChildren (RecyclerView.Recycler recycler, RecyclerView.State state)
Lay out all relevant child views from the given adapter.
The LayoutManager is in charge of the behavior of item animations. By default,
RecyclerView has a non-null ItemAnimator
, and simple
item animations are enabled. This means that add/remove operations on the
adapter will result in animations to add new or appearing items, removed or
disappearing items, and moved items. If a LayoutManager returns false from
supportsPredictiveItemAnimations()
, which is the default, and runs a
normal layout operation during onLayoutChildren(Recycler, State)
, the
RecyclerView will have enough information to run those animations in a simple
way. For example, the default ItemAnimator, DefaultItemAnimator
, will
simply fade views in and out, whether they are actually added/removed or whether
they are moved on or off the screen due to other add/remove operations.
A LayoutManager wanting a better item animation experience, where items can be
animated onto and off of the screen according to where the items exist when they
are not on screen, then the LayoutManager should return true from
supportsPredictiveItemAnimations()
and add additional logic to
onLayoutChildren(Recycler, State)
. Supporting predictive animations
means that onLayoutChildren(Recycler, State)
will be called twice;
once as a "pre" layout step to determine where items would have been prior to
a real layout, and again to do the "real" layout. In the pre-layout phase,
items will remember their pre-layout positions to allow them to be laid out
appropriately. Also, removed
items will
be returned from the scrap to help determine correct placement of other items.
These removed items should not be added to the child list, but should be used
to help calculate correct positioning of other views, including views that
were not previously onscreen (referred to as APPEARING views), but whose
pre-layout offscreen position can be determined given the extra
information about the pre-layout removed views.
The second layout pass is the real layout in which only non-removed views
will be used. The only additional requirement during this pass is, if
supportsPredictiveItemAnimations()
returns true, to note which
views exist in the child list prior to layout and which are not there after
layout (referred to as DISAPPEARING views), and to position/layout those views
appropriately, without regard to the actual bounds of the RecyclerView. This allows
the animation system to know the location to which to animate these disappearing
views.
The default LayoutManager implementations for RecyclerView handle all of these requirements for animations already. Clients of RecyclerView can either use one of these layout managers directly or look at their implementations of onLayoutChildren() to see how they account for the APPEARING and DISAPPEARING views.
Parameters | |
---|---|
recycler |
RecyclerView.Recycler : Recycler to use for fetching potentially cached views for a
position |
state |
RecyclerView.State : Transient state of RecyclerView
|
onLayoutCompleted
void onLayoutCompleted (RecyclerView.State state)
Called after a full layout calculation is finished. The layout calculation may include
multiple onLayoutChildren(Recycler, State)
calls due to animations or
layout measurement but it will include only one onLayoutCompleted(State)
call.
This method will be called at the end of layout(int, int, int, int)
call.
This is a good place for the LayoutManager to do some cleanup like pending scroll position, saved state etc.
Parameters | |
---|---|
state |
RecyclerView.State : Transient state of RecyclerView
|
onRestoreInstanceState
void onRestoreInstanceState (Parcelable state)
Parameters | |
---|---|
state |
Parcelable |
onSaveInstanceState
Parcelable onSaveInstanceState ()
Called when the LayoutManager should save its state. This is a good time to save your scroll position, configuration and anything else that may be required to restore the same layout state if the LayoutManager is recreated.
RecyclerView does NOT verify if the LayoutManager has changed between state save and restore. This will let you share information between your LayoutManagers but it is also your responsibility to make sure they use the same parcelable class.
Returns | |
---|---|
Parcelable |
Necessary information for LayoutManager to be able to restore its state |
scrollHorizontallyBy
int scrollHorizontallyBy (int dx, RecyclerView.Recycler recycler, RecyclerView.State state)
Scroll horizontally by dx pixels in screen coordinates and return the distance traveled. The default implementation does nothing and returns 0.
Parameters | |
---|---|
dx |
int : distance to scroll by in pixels. X increases as scroll position
approaches the right. |
recycler |
RecyclerView.Recycler : Recycler to use for fetching potentially cached views for a
position |
state |
RecyclerView.State : Transient state of RecyclerView |
Returns | |
---|---|
int |
The actual distance scrolled. The return value will be negative if dx was
negative and scrolling proceeeded in that direction.
Math.abs(result) may be less than dx if a boundary was reached.
|
scrollToPosition
void scrollToPosition (int position)
Scroll the RecyclerView to make the position visible.
RecyclerView will scroll the minimum amount that is necessary to make the
target position visible. If you are looking for a similar behavior to
setSelection(int)
or
setSelectionFromTop(int, int)
, use
scrollToPositionWithOffset(int, int)
.
Note that scroll position change will not be reflected until the next layout call.
Parameters | |
---|---|
position |
int : Scroll to this adapter position |
See also:
scrollToPositionWithOffset
void scrollToPositionWithOffset (int position, int offset)
Scroll to the specified adapter position with the given offset from resolved layout
start. Resolved layout start depends on getReverseLayout()
,
getLayoutDirection(android.view.View)
and getStackFromEnd()
.
For example, if layout is VERTICAL
and getStackFromEnd()
is true, calling
scrollToPositionWithOffset(10, 20)
will layout such that
item[10]
's bottom is 20 pixels above the RecyclerView's bottom.
Note that scroll position change will not be reflected until the next layout call.
If you are just trying to make a position visible, use scrollToPosition(int)
.
Parameters | |
---|---|
position |
int : Index (starting at 0) of the reference item. |
offset |
int : The distance (in pixels) between the start edge of the item view and
start edge of the RecyclerView. |
scrollVerticallyBy
int scrollVerticallyBy (int dy, RecyclerView.Recycler recycler, RecyclerView.State state)
Scroll vertically by dy pixels in screen coordinates and return the distance traveled. The default implementation does nothing and returns 0.
Parameters | |
---|---|
dy |
int : distance to scroll in pixels. Y increases as scroll position
approaches the bottom. |
recycler |
RecyclerView.Recycler : Recycler to use for fetching potentially cached views for a
position |
state |
RecyclerView.State : Transient state of RecyclerView |
Returns | |
---|---|
int |
The actual distance scrolled. The return value will be negative if dy was
negative and scrolling proceeeded in that direction.
Math.abs(result) may be less than dy if a boundary was reached.
|
setInitialPrefetchItemCount
void setInitialPrefetchItemCount (int itemCount)
Sets the number of items to prefetch in
collectInitialPrefetchPositions(int, LayoutPrefetchRegistry)
, which defines
how many inner items should be prefetched when this LayoutManager's RecyclerView
is nested inside another RecyclerView.
Set this value to the number of items this inner LayoutManager will display when it is first scrolled into the viewport. RecyclerView will attempt to prefetch that number of items so they are ready, avoiding jank as the inner RecyclerView is scrolled into the viewport.
For example, take a vertically scrolling RecyclerView with horizontally scrolling inner
RecyclerViews. The rows always have 4 items visible in them (or 5 if not aligned). Passing
4
to this method for each inner RecyclerView's LinearLayoutManager will enable
RecyclerView's prefetching feature to do create/bind work for 4 views within a row early,
before it is scrolled on screen, instead of just the default 2.
Calling this method does nothing unless the LayoutManager is in a RecyclerView nested in another RecyclerView.
Note: Setting this value to be larger than the number of views that will be visible in this view can incur unnecessary bind work, and an increase to the number of Views created and in active use.
Parameters | |
---|---|
itemCount |
int : Number of items to prefetch |
setOrientation
void setOrientation (int orientation)
Sets the orientation of the layout. LinearLayoutManager
will do its best to keep scroll position.
Parameters | |
---|---|
orientation |
int : HORIZONTAL or VERTICAL
|
setRecycleChildrenOnDetach
void setRecycleChildrenOnDetach (boolean recycleChildrenOnDetach)
Set whether LayoutManager will recycle its children when it is detached from RecyclerView.
If you are using a RecyclerView.RecycledViewPool
, it might be a good idea to set
this flag to true
so that views will be available to other RecyclerViews
immediately.
Note that, setting this flag will result in a performance drop if RecyclerView is restored.
Parameters | |
---|---|
recycleChildrenOnDetach |
boolean : Whether children should be recycled in detach or not.
|
setReverseLayout
void setReverseLayout (boolean reverseLayout)
Used to reverse item traversal and layout order.
This behaves similar to the layout change for RTL views. When set to true, first item is
laid out at the end of the UI, second item is laid out before it etc.
For horizontal layouts, it depends on the layout direction.
When set to true, If RecyclerView
is LTR, than it will
layout from RTL, if RecyclerView
} is RTL, it will layout
from LTR.
If you are looking for the exact same behavior of
setStackFromBottom(boolean)
, use
setStackFromEnd(boolean)
Parameters | |
---|---|
reverseLayout |
boolean |
setSmoothScrollbarEnabled
void setSmoothScrollbarEnabled (boolean enabled)
When smooth scrollbar is enabled, the position and size of the scrollbar thumb is computed based on the number of visible pixels in the visible items. This however assumes that all list items have similar or equal widths or heights (depending on list orientation). If you use a list in which items have different dimensions, the scrollbar will change appearance as the user scrolls through the list. To avoid this issue, you need to disable this property. When smooth scrollbar is disabled, the position and size of the scrollbar thumb is based solely on the number of items in the adapter and the position of the visible items inside the adapter. This provides a stable scrollbar as the user navigates through a list of items with varying widths / heights.
Parameters | |
---|---|
enabled |
boolean : Whether or not to enable smooth scrollbar. |
See also:
setStackFromEnd
void setStackFromEnd (boolean stackFromEnd)
Compatibility support for setStackFromBottom(boolean)
Parameters | |
---|---|
stackFromEnd |
boolean |
smoothScrollToPosition
void smoothScrollToPosition (RecyclerView recyclerView, RecyclerView.State state, int position)
Smooth scroll to the specified adapter position.
To support smooth scrolling, override this method, create your RecyclerView.SmoothScroller
instance and call startSmoothScroll(SmoothScroller)
.
Parameters | |
---|---|
recyclerView |
RecyclerView : The RecyclerView to which this layout manager is attached |
state |
RecyclerView.State : Current State of RecyclerView |
position |
int : Scroll to this adapter position.
|
supportsPredictiveItemAnimations
boolean supportsPredictiveItemAnimations ()
Returns whether this LayoutManager supports "predictive item animations".
"Predictive item animations" are automatically created animations that show where items came from, and where they are going to, as items are added, removed, or moved within a layout.
A LayoutManager wishing to support predictive item animations must override this
method to return true (the default implementation returns false) and must obey certain
behavioral contracts outlined in onLayoutChildren(Recycler, State)
.
Whether item animations actually occur in a RecyclerView is actually determined by both
the return value from this method and the
ItemAnimator
set on the
RecyclerView itself. If the RecyclerView has a non-null ItemAnimator but this
method returns false, then only "simple item animations" will be enabled in the
RecyclerView, in which views whose position are changing are simply faded in/out. If the
RecyclerView has a non-null ItemAnimator and this method returns true, then predictive
item animations will be enabled in the RecyclerView.
Returns | |
---|---|
boolean |
true if this LayoutManager supports predictive item animations, false otherwise. |
Protected methods
getExtraLayoutSpace
int getExtraLayoutSpace (RecyclerView.State state)
Returns the amount of extra space that should be laid out by LayoutManager.
By default, LinearLayoutManager
lays out 1 extra page
of items while smooth scrolling and 0 otherwise. You can override this method to implement
your custom layout pre-cache logic.
Note:Laying out invisible elements generally comes with significant performance cost. It's typically only desirable in places like smooth scrolling to an unknown location, where 1) the extra content helps LinearLayoutManager know in advance when its target is approaching, so it can decelerate early and smoothly and 2) while motion is continuous.
Extending the extra layout space is especially expensive if done while the user may change scrolling direction. Changing direction will cause the extra layout space to swap to the opposite side of the viewport, incurring many rebinds/recycles, unless the cache is large enough to handle it.
Parameters | |
---|---|
state |
RecyclerView.State |
Returns | |
---|---|
int |
The extra space that should be laid out (in pixels). |
Annotations
Interfaces
- ActionMenuView.OnMenuItemClickListener
- PopupMenu.OnDismissListener
- PopupMenu.OnMenuItemClickListener
- RecyclerView.ChildDrawingOrderCallback
- RecyclerView.ItemAnimator.ItemAnimatorFinishedListener
- RecyclerView.LayoutManager.LayoutPrefetchRegistry
- RecyclerView.OnChildAttachStateChangeListener
- RecyclerView.OnItemTouchListener
- RecyclerView.RecyclerListener
- RecyclerView.SmoothScroller.ScrollVectorProvider
- SearchView.OnCloseListener
- SearchView.OnQueryTextListener
- SearchView.OnSuggestionListener
- ShareActionProvider.OnShareTargetSelectedListener
- ThemedSpinnerAdapter
- Toolbar.OnMenuItemClickListener
Classes
- ActionMenuView
- ActionMenuView.LayoutParams
- AppCompatAutoCompleteTextView
- AppCompatButton
- AppCompatCheckBox
- AppCompatCheckedTextView
- AppCompatEditText
- AppCompatImageButton
- AppCompatImageView
- AppCompatMultiAutoCompleteTextView
- AppCompatRadioButton
- AppCompatRatingBar
- AppCompatSeekBar
- AppCompatSpinner
- AppCompatTextView
- CardView
- DefaultItemAnimator
- DividerItemDecoration
- GridLayout
- GridLayout.Alignment
- GridLayout.LayoutParams
- GridLayout.Spec
- GridLayoutManager
- GridLayoutManager.DefaultSpanSizeLookup
- GridLayoutManager.LayoutParams
- GridLayoutManager.SpanSizeLookup
- LinearLayoutCompat
- LinearLayoutCompat.LayoutParams
- LinearLayoutManager
- LinearLayoutManager.LayoutChunkResult
- LinearSmoothScroller
- LinearSnapHelper
- ListPopupWindow
- OrientationHelper
- PagerSnapHelper
- PopupMenu
- RecyclerView
- RecyclerView.Adapter
- RecyclerView.AdapterDataObserver
- RecyclerView.EdgeEffectFactory
- RecyclerView.ItemAnimator
- RecyclerView.ItemAnimator.ItemHolderInfo
- RecyclerView.ItemDecoration
- RecyclerView.LayoutManager
- RecyclerView.LayoutManager.Properties
- RecyclerView.LayoutParams
- RecyclerView.OnFlingListener
- RecyclerView.OnScrollListener
- RecyclerView.RecycledViewPool
- RecyclerView.Recycler
- RecyclerView.SimpleOnItemTouchListener
- RecyclerView.SmoothScroller
- RecyclerView.SmoothScroller.Action
- RecyclerView.State
- RecyclerView.ViewCacheExtension
- RecyclerView.ViewHolder
- RecyclerViewAccessibilityDelegate
- RecyclerViewAccessibilityDelegate.ItemDelegate
- SearchView
- ShareActionProvider
- SimpleItemAnimator
- SnapHelper
- StaggeredGridLayoutManager
- StaggeredGridLayoutManager.LayoutParams
- SwitchCompat
- ThemedSpinnerAdapter.Helper
- Toolbar
- Toolbar.LayoutParams
- Toolbar.SavedState
- TooltipCompat
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Last updated 2024-04-11 UTC.