ViewportFiller
Interface that is responsible for filling the viewport of the list with initial layouts according to the LayoutManager. The goal here is to have the layouts that the RecyclerView will ask for when it comes onto the screen already computed, e.g. in the background, so that we don't drop frames on the main thread. NB: This class should try to respect the layout of views as they will appear in the RecyclerView.
Inheritors
Functions
This will be called to inform implementations that the next layout has been computed. Implementations should use the width/height to determine whether they still need more views to fill their initial viewport (which should be reflected in the next call to wantsMore
Return the fill along the main axis (i.e. height for VERTICAL and width for HORIZONTAL), this method is available after ViewportFiller.add is called.