SnapHelper

Kotlin |Java

abstract class SnapHelper : RecyclerView.OnFlingListener

kotlin.Any
↳	androidx.recyclerview.widget.RecyclerView.OnFlingListener
	↳	androidx.recyclerview.widget.SnapHelper

Known Direct Subclasses

LinearSnapHelper, PagerSnapHelper

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(int, int) and handle fling manually.

Summary

Public constructors
`<init>()` Class intended to support snapping for a `RecyclerView`.

Public methods
open Unit	`attachToRecyclerView(@Nullable recyclerView: RecyclerView?)` Attaches the `SnapHelper` to the provided RecyclerView, by calling `RecyclerView#setOnFlingListener(RecyclerView.OnFlingListener)`.
abstract IntArray?	`calculateDistanceToFinalSnap(@NonNull layoutManager: RecyclerView.LayoutManager, @NonNull targetView: View)` Override this method to snap to a particular point within the target view or the container view on any axis.
open IntArray!	`calculateScrollDistance(velocityX: Int, velocityY: Int)` Calculated the estimated scroll distance in each direction given velocities on both axes.
abstract View?	`findSnapView(layoutManager: RecyclerView.LayoutManager!)` Override this method to provide a particular target view for snapping.
abstract Int	`findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager!, velocityX: Int, velocityY: Int)` Override to provide a particular adapter target position for snapping.
open Boolean	`onFling(velocityX: Int, velocityY: Int)`

Protected methods
open RecyclerView.SmoothScroller?	`createScroller(@NonNull layoutManager: RecyclerView.LayoutManager)` Creates a scroller to be used in the snapping implementation.
open LinearSmoothScroller?	`createSnapScroller(@NonNull layoutManager: RecyclerView.LayoutManager)` Creates a scroller to be used in the snapping implementation.

Public constructors

<init>

SnapHelper()

Class intended to support snapping for a RecyclerView.

Public methods

attachToRecyclerView

open fun attachToRecyclerView(@Nullable recyclerView: RecyclerView?): Unit

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

Parameters
`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.

Exceptions
`IllegalArgumentException`	if there is already a `RecyclerView.OnFlingListener` attached to the provided `RecyclerView`.

calculateDistanceToFinalSnap

@Nullable abstract fun calculateDistanceToFinalSnap(
    @NonNull layoutManager: RecyclerView.LayoutManager, 
    @NonNull targetView: View
): IntArray?

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
`layoutManager`	RecyclerView.LayoutManager: the `RecyclerView.LayoutManager` associated with the attached `RecyclerView`
`targetView`	View: the target view that is chosen as the view to snap

Return
`IntArray?`	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

open fun calculateScrollDistance(
    velocityX: Int, 
    velocityY: Int
): IntArray!

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

Parameters
`velocityX`	Int: Fling velocity on the horizontal axis.
`velocityY`	Int: Fling velocity on the vertical axis.

Return
`IntArray!`	array holding the calculated distances in x and y directions respectively.

findSnapView

@Nullable abstract fun findSnapView(layoutManager: RecyclerView.LayoutManager!): View?

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
`layoutManager`	RecyclerView.LayoutManager!: the `RecyclerView.LayoutManager` associated with the attached `RecyclerView`

Return
`View?`	the target view to which to snap on fling or end of scroll

findTargetSnapPosition

abstract fun findTargetSnapPosition(
    layoutManager: RecyclerView.LayoutManager!, 
    velocityX: Int, 
    velocityY: Int
): Int

Override to provide a particular adapter target position for snapping.

Parameters
`layoutManager`	RecyclerView.LayoutManager!: the `RecyclerView.LayoutManager` associated with the attached `RecyclerView`
`velocityX`	Int: fling velocity on the horizontal axis
`velocityY`	Int: fling velocity on the vertical axis

Return
`Int`	the target adapter position to you want to snap or `RecyclerView#NO_POSITION` if no snapping should happen

onFling

open fun onFling(
    velocityX: Int, 
    velocityY: Int
): Boolean

Protected methods

createScroller

@Nullable protected open fun createScroller(@NonNull layoutManager: RecyclerView.LayoutManager): RecyclerView.SmoothScroller?

Creates a scroller to be used in the snapping implementation.

Parameters
`layoutManager`	RecyclerView.LayoutManager: The `RecyclerView.LayoutManager` associated with the attached `RecyclerView`.

Return
`RecyclerView.SmoothScroller?`	a `RecyclerView.SmoothScroller` which will handle the scrolling.

createSnapScroller

@Nullable protected open fun createSnapScroller(@NonNull layoutManager: RecyclerView.LayoutManager): LinearSmoothScroller?

Deprecated: use createScroller(RecyclerView.LayoutManager) instead.

Creates a scroller to be used in the snapping implementation.

Parameters
`layoutManager`	RecyclerView.LayoutManager: The `RecyclerView.LayoutManager` associated with the attached `RecyclerView`.

Return
`LinearSmoothScroller?`	a `LinearSmoothScroller` which will handle the scrolling.