What are best practices for implementing Back to Top functionality on a (potentially) long activity feed, on a mobile app? At this point it's not clear whether the feed will be infinitely scrollable or not, but in any case it'll display lots of feed items, and users should have an easy way to go back to top.
The common practice seems to be when scrolling down the feed nothing appears, but as soon as the user starts scrolling back up have a "Back to top" button appear at the top of the feed. It is that way for obvious reasons, when the user is scrolling down they don't want to go up, if they want to go all the way up they'll start scrolling up.
Make sure to keep the button small, if they only want to scroll up a few you don't want a giant button blocking half the feed to where the user can't see or may accidentally click. I would also make sure it's labeled, a simple arrow pointing up may be ambiguous that it will take you all the way up. Something like this will do.