This is a Xamarin.Android implementation of the swipe-down-to-refresh pattern that Google introduced with their 2013 Gmail app update.
The idea of the pattern is that when a ListView is positioned at the beginning, starting an overscroll will, in addition to the normal edge effect, change the ActionBar to display an action message and an horizontal centered progress bar defining when the movement results in a refresh of the content.
This code is available under the Apache 2.0 license. Reuse as much as you want.