NestedScrollingChild

public interface NestedScrollingChild

androidx.core.view.NestedScrollingChild


This interface should be implemented by View subclasses that wish to support dispatching nested scrolling operations to a cooperating parent ViewGroup.

Classes implementing this interface should create a final instance of a NestedScrollingChildHelper as a field and delegate any View methods to the NestedScrollingChildHelper methods of the same signature.

Views invoking nested scrolling functionality should always do so from the relevant ViewCompat, ViewGroupCompat or ViewParentCompat compatibility shim static methods. This ensures interoperability with nested scrolling views on Android 5.0 Lollipop and newer.

Summary

Public methods

abstract boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed)

Dispatch a fling to a nested scrolling parent.

abstract boolean dispatchNestedPreFling(float velocityX, float velocityY)

Dispatch a fling to a nested scrolling parent before it is processed by this view.

abstract boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow)

Dispatch one step of a nested scroll in progress before this view consumes any portion of it.

abstract boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow)

Dispatch one step of a nested scroll in progress.

abstract boolean hasNestedScrollingParent()

Returns true if this view has a nested scrolling parent.

abstract boolean isNestedScrollingEnabled()

Returns true if nested scrolling is enabled for this view.

abstract void setNestedScrollingEnabled(boolean enabled)

Enable or disable nested scrolling for this view.

abstract boolean startNestedScroll(int axes)

Begin a nestable scroll operation along the given axes.

abstract void stopNestedScroll()

Stop a nested scroll in progress.

Public methods

dispatchNestedFling

public abstract boolean dispatchNestedFling (float velocityX, 
                float velocityY, 
                boolean consumed)

Dispatch a fling to a nested scrolling parent.

This method should be used to indicate that a nested scrolling child has detected suitable conditions for a fling. Generally this means that a touch scroll has ended with a velocity in the direction of scrolling that meets or exceeds the minimum fling velocity along a scrollable axis.

If a nested scrolling child view would normally fling but it is at the edge of its own content, it can use this method to delegate the fling to its nested scrolling parent instead. The parent may optionally consume the fling or observe a child fling.

Parameters
velocityX float: Horizontal fling velocity in pixels per second

velocityY float: Vertical fling velocity in pixels per second

consumed boolean: true if the child consumed the fling, false otherwise

Returns
boolean true if the nested scrolling parent consumed or otherwise reacted to the fling

dispatchNestedPreFling

public abstract boolean dispatchNestedPreFling (float velocityX, 
                float velocityY)

Dispatch a fling to a nested scrolling parent before it is processed by this view.

Nested pre-fling events are to nested fling events what touch intercept is to touch and what nested pre-scroll is to nested scroll. dispatchNestedPreFling offsets an opportunity for the parent view in a nested fling to fully consume the fling before the child view consumes it. If this method returns true, a nested parent view consumed the fling and this view should not scroll as a result.

For a better user experience, only one view in a nested scrolling chain should consume the fling at a time. If a parent view consumed the fling this method will return false. Custom view implementations should account for this in two ways:

  • If a custom view is paged and needs to settle to a fixed page-point, do not call dispatchNestedPreFling; consume the fling and settle to a valid position regardless.
  • If a nested parent does consume the fling, this view should not scroll at all, even to settle back to a valid idle position.

Views should also not offer fling velocities to nested parent views along an axis where scrolling is not currently supported; a ScrollView should not offer a horizontal fling velocity to its parents since scrolling along that axis is not permitted and carrying velocity along that motion does not make sense.

Parameters
velocityX float: Horizontal fling velocity in pixels per second

velocityY float: Vertical fling velocity in pixels per second

Returns
boolean true if a nested scrolling parent consumed the fling

dispatchNestedPreScroll

public abstract boolean dispatchNestedPreScroll (int dx, 
                int dy, 
                int[] consumed, 
                int[] offsetInWindow)

Dispatch one step of a nested scroll in progress before this view consumes any portion of it.

Nested pre-scroll events are to nested scroll events what touch intercept is to touch. dispatchNestedPreScroll offers an opportunity for the parent view in a nested scrolling operation to consume some or all of the scroll operation before the child view consumes it.

Parameters
dx int: Horizontal scroll distance in pixels

dy int: Vertical scroll distance in pixels

consumed int: Output. If not null, consumed[0] will contain the consumed component of dx and consumed[1] the consumed dy.

offsetInWindow int: Optional. If not null, on return this will contain the offset in local view coordinates of this view from before this operation to after it completes. View implementations may use this to adjust expected input coordinate tracking.

Returns
boolean true if the parent consumed some or all of the scroll delta