CoordinatorLayout behaviour for a parallaxing header above a RecyclerView

ParallaxScrollBehaviour.java

//
// ParallaxScrollBehaviour
// Created by Ben De La Haye on 17/10/2016
//
public class ParallaxScrollBehavior<V extends View> extends CoordinatorLayout.Behavior<V> {

    private static final float PARALLAX_RATIO = 0.8f;

    private RecyclerView _recyclerView;

    private V _child;

    private RecyclerView.OnScrollListener _scrollListener = new RecyclerView.OnScrollListener() {

        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {}

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            // We can only trust the position of the top view for an accurate scroll position
            if (_recyclerView.getLayoutManager() != null && _recyclerView.getLayoutManager().getChildAt(0) != null) {

                int decoratedTop = _recyclerView.getLayoutManager().getDecoratedTop(_recyclerView.getLayoutManager().getChildAt(0));
                _child.setTranslationY((decoratedTop - _child.getHeight()) * PARALLAX_RATIO);
            }
        }
    };

    public ParallaxScrollBehavior() {}

    public ParallaxScrollBehavior(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean layoutDependsOn(CoordinatorLayout parent, V child, View dependency) {

        // This is the (header) View this behaviour is attached to
        _child = child;

        // We want to get a reference to the first RecyclerView child of the coordinator layout
        if (dependency instanceof RecyclerView) {

            _recyclerView = ((RecyclerView) dependency);

            // This method may be called multiple times, and we only want one _listener
            _recyclerView.removeOnScrollListener(_scrollListener);

            // Add a _listener so we can get updates for scroll events
            _recyclerView.addOnScrollListener(_scrollListener);

            return true;
        }

        // We don't care about getting updates to onDependantViewChanged()
        return false;
    }

    @Override
    public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {

        // We can only trust the position of the top view for an accurate scroll position
        if (_recyclerView != null && _recyclerView.getLayoutManager() != null && _recyclerView.getLayoutManager().getChildAt(0) != null) {

            int decoratedTop = _recyclerView.getLayoutManager().getDecoratedTop(_recyclerView.getLayoutManager().getChildAt(0));
            _child.setTranslationY((decoratedTop - _child.getHeight()) * PARALLAX_RATIO);
        }

        return super.onLayoutChild(parent, child, layoutDirection);
    }

    @Override
    public boolean onDependentViewChanged(CoordinatorLayout parent, V child, View dependency) {

        _child.post(() -> {
            if (_recyclerView.getPaddingTop() < _child.getHeight()) {

                // Add enough padding to the RecyclerView to show our header above it
                _recyclerView.setPadding(_recyclerView.getPaddingLeft(), _child.getHeight(), _recyclerView.getPaddingRight(), _recyclerView.getPaddingBottom());
            }
        });

        return true;
    }
}