From iOS 9, UICollectionViewFlowLayout has two very handy
properties to pin the header & footer to the top/bottom of the
collection view bounds
1. sectionHeadersPinToVisibleBounds
When this property is true, section
header views scroll with content until they reach the top of the screen, at
which point they are pinned to the upper bounds of the collection view. Each
new header view that scrolls to the top of the screen pushes the previously
pinned header view offscreen.
2.
sectionFootersPinToVisibleBounds
When this property is true, section footer views scroll with content until they reach the
bottom of the screen, at which point they are pinned to the lower bounds of the
collection view. Each new footer view that scrolls to the bottom of the screen
pushes the previously pinned footer view offscreen.
