SnapHelper

public abstract class SnapHelper extends RecyclerView.OnFlingListener

java.lang.Object
   ↳ androidx.recyclerview.widget.RecyclerView.OnFlingListener
     ↳ androidx.recyclerview.widget.SnapHelper
Known direct subclasses
LinearSnapHelper

Implementation of the SnapHelper supporting snapping in either vertical or horizontal orientation.
PagerSnapHelper

Implementation of the SnapHelper supporting pager style snapping in either vertical or horizontal orientation.

Class intended to support snapping for a RecyclerView.

SnapHelper tries to handle fling as well but for this to work properly, the RecyclerView.LayoutManager must implement the RecyclerView.SmoothScroller.ScrollVectorProvider interface or you should override onFling and handle fling manually.

Summary

Public constructors

Public methods
void

Attaches the SnapHelper to the provided RecyclerView, by calling setOnFlingListener.
abstract @Nullable int[]
calculateDistanceToFinalSnap(
    @NonNull RecyclerView.LayoutManager layoutManager,
    @NonNull View targetView
)

Override this method to snap to a particular point within the target view or the container view on any axis.
int[]
calculateScrollDistance(int velocityX, int velocityY)

Calculated the estimated scroll distance in each direction given velocities on both axes.
abstract @Nullable View

Override this method to provide a particular target view for snapping.
abstract int
findTargetSnapPosition(
    RecyclerView.LayoutManager layoutManager,
    int velocityX,
    int velocityY
)

Override to provide a particular adapter target position for snapping.
boolean
onFling(int velocityX, int velocityY)

Override this to handle a fling given the velocities in both x and y directions.

Protected methods
@Nullable RecyclerView.SmoothScroller

Creates a scroller to be used in the snapping implementation.
@Nullable LinearSmoothScroller

This method is deprecated.

use createScroller instead.

Public constructors

SnapHelper

Added in 1.0.0
public SnapHelper()

Public methods

attachToRecyclerView

Added in 1.0.0
public void attachToRecyclerView(@Nullable RecyclerView recyclerView)

Attaches the SnapHelper to the provided RecyclerView, by calling setOnFlingListener. You can call this method with null to detach it from the current RecyclerView.

Parameters
@Nullable RecyclerView recyclerView

The RecyclerView instance to which you want to add this helper or null if you want to remove SnapHelper from the current RecyclerView.
Throws
java.lang.IllegalArgumentException

if there is already a RecyclerView.OnFlingListener attached to the provided RecyclerView.
java.lang.IllegalStateException

calculateDistanceToFinalSnap

Added in 1.0.0
public abstract @Nullable int[] calculateDistanceToFinalSnap(
    @NonNull RecyclerView.LayoutManager layoutManager,
    @NonNull View targetView
)

Override this method to snap to a particular point within the target view or the container view on any axis.

This method is called when the SnapHelper has intercepted a fling and it needs to know the exact distance required to scroll by in order to snap to the target view.

Parameters
@NonNull RecyclerView.LayoutManager layoutManager

the RecyclerView.LayoutManager associated with the attached RecyclerView
@NonNull View targetView

the target view that is chosen as the view to snap
Returns
@Nullable int[]

the output coordinates the put the result into. out[0] is the distance on horizontal axis and out[1] is the distance on vertical axis.

calculateScrollDistance

Added in 1.0.0
public int[] calculateScrollDistance(int velocityX, int velocityY)

Calculated the estimated scroll distance in each direction given velocities on both axes.

Parameters
int velocityX

Fling velocity on the horizontal axis.
int velocityY

Fling velocity on the vertical axis.
Returns
int[]

array holding the calculated distances in x and y directions respectively.

findSnapView

Added in 1.0.0
public abstract @Nullable View findSnapView(RecyclerView.LayoutManager layoutManager)

Override this method to provide a particular target view for snapping.

This method is called when the SnapHelper is ready to start snapping and requires a target view to snap to. It will be explicitly called when the scroll state becomes idle after a scroll. It will also be called when the SnapHelper is preparing to snap after a fling and requires a reference view from the current set of child views.

If this method returns null, SnapHelper will not snap to any view.

Parameters
RecyclerView.LayoutManager layoutManager

the RecyclerView.LayoutManager associated with the attached RecyclerView
Returns
@Nullable View

the target view to which to snap on fling or end of scroll

findTargetSnapPosition

Added in 1.0.0
public abstract int findTargetSnapPosition(
    RecyclerView.LayoutManager layoutManager,
    int velocityX,
    int velocityY
)

Override to provide a particular adapter target position for snapping.

Parameters
RecyclerView.LayoutManager layoutManager

the RecyclerView.LayoutManager associated with the attached RecyclerView
int velocityX

fling velocity on the horizontal axis
int velocityY

fling velocity on the vertical axis
Returns
int

the target adapter position to you want to snap or NO_POSITION if no snapping should happen

onFling

Added in 1.4.0-rc01
public boolean onFling(int velocityX, int velocityY)

Override this to handle a fling given the velocities in both x and y directions. Note that this method will only be called if the associated LayoutManager supports scrolling and the fling is not handled by nested scrolls first.

Parameters
int velocityX

the fling velocity on the X axis
int velocityY

the fling velocity on the Y axis
Returns
boolean

true if the fling was handled, false otherwise.

Protected methods

createScroller

Added in 1.0.0
protected @Nullable RecyclerView.SmoothScroller createScroller(@NonNull RecyclerView.LayoutManager layoutManager)

Creates a scroller to be used in the snapping implementation.

Parameters
@NonNull RecyclerView.LayoutManager layoutManager

The RecyclerView.LayoutManager associated with the attached RecyclerView.
Returns
@Nullable RecyclerView.SmoothScroller

a RecyclerView.SmoothScroller which will handle the scrolling.

createSnapScroller

Added in 1.0.0
Deprecated in 1.0.0
protected @Nullable LinearSmoothScroller createSnapScroller(@NonNull RecyclerView.LayoutManager layoutManager)

Creates a scroller to be used in the snapping implementation.

Parameters
@NonNull RecyclerView.LayoutManager layoutManager

The RecyclerView.LayoutManager associated with the attached RecyclerView.
Returns
@Nullable LinearSmoothScroller

a LinearSmoothScroller which will handle the scrolling.