Understanding Android NestedScrolling Mechanism

Android added the NestedScrolling mechanism in version 5.0, providing a more convenient way to coordinate scrolling between child and parent views.

The core interfaces are NestedScrollingChild , NestedScrollingParent , NestedScrollingChildHelper , and NestedScrollingParentHelper . A sample demo mimics a travel‑ticket homepage using a custom CollapsingLayout as the parent and a NestedScrollView as the child.

The interaction follows four main steps:

Child view receives ACTION_DOWN and calls startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL) , which triggers the parent’s onStartNestedScroll and onNestedScrollAccepted callbacks.

During ACTION_MOVE , the child invokes dispatchNestedPreScroll ; the parent’s onNestedPreScroll consumes part of the scroll distance before the child processes the remainder.

After the child finishes its own scrolling, it calls dispatchNestedScroll , allowing the parent’s onNestedScroll to handle any unconsumed distance.

When the touch sequence ends with ACTION_UP , the child calls stopNestedScroll() , prompting the parent’s onStopNestedScroll to perform final cleanup.

The article also provides a table mapping child methods to corresponding parent callbacks and includes several code excerpts illustrating the implementation of these callbacks.

Understanding this flow helps developers avoid the traditional problem where a child view consumes touch events entirely, making it difficult for the parent to participate in scrolling.

Further tutorials will show how to combine CoordinatorLayout , AppBarLayout , and CollapsingToolbarLayout to recreate the travel‑ticket homepage interaction.