Add nested pre-fling for nested scrolling

Nested pre-fling allows a nested scrolling parent to consume a fling
operation before the child view does. If a parent has been consuming
nested pre-scroll events in a particular direction, this is generally
a good indication that it should also consume the resulting fling at
the end.

Bug 15538504

Change-Id: I88e8753a96c9b41815c3be530cafab8345164e64

1 files changed, 23 insertions, 1 deletions

diff --git a/core/java/android/view/ViewParent.java b/core/java/android/view/ViewParent.java
index 588b9cd7ce8f..87a37f4739be 100644
--- a/core/java/android/view/ViewParent.java
+++ b/core/java/android/view/ViewParent.java
@@ -523,10 +523,32 @@ public interface ViewParent {
      * parent instead. The parent may optionally consume the fling or observe a child fling.</p>
      *
      * @param target View that initiated the nested scroll
-     * @param velocityX Horizontal velocity in pixels per second.
+     * @param velocityX Horizontal velocity in pixels per second
      * @param velocityY Vertical velocity in pixels per second
      * @param consumed true if the child consumed the fling, false otherwise
      * @return true if this parent consumed or otherwise reacted to the fling
      */
     public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed);
+
+    /**
+     * React to a nested fling before the target view consumes it.
+     *
+     * <p>This method siginfies that a nested scrolling child has detected a fling with the given
+     * velocity along each axis. Generally this means that a touch scroll has ended with a
+     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
+     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
+     * along a scrollable axis.</p>
+     *
+     * <p>If a nested scrolling parent is consuming motion as part of a
+     * {@link #onNestedPreScroll(View, int, int, int[]) pre-scroll}, it may be appropriate for
+     * it to also consume the pre-fling to complete that same motion. By returning
+     * <code>true</code> from this method, the parent indicates that the child should not
+     * fling its own internal content as well.</p>
+     *
+     * @param target View that initiated the nested scroll
+     * @param velocityX Horizontal velocity in pixels per second
+     * @param velocityY Vertical velocity in pixels per second
+     * @return true if this parent consumed the fling ahead of the target view
+     */
+    public boolean onNestedPreFling(View target, float velocityX, float velocityY);
 }