An enhanced version of Android's ViewPager.PageScrolled event using Reactive Extensions.

PageScrolledRx.cs

Observable.FromEventPattern<ViewPager.PageScrolledEventArgs> (
	x => _viewPager.PageScrolled += x,
	x => _viewPager.PageScrolled -= x)
  .Select (args => args.EventArgs)
  .StartWith(new ViewPager.PageScrolledEventArgs(0, 0, 0))
  .PairWithPrevious()
  .Skip(1)
  .Select(args => {
	var scrollDirection =
		args.Item1.Position != args.Item2.Position
			? ScrollDirection.Center
			: args.Item1.PositionOffset > args.Item2.PositionOffset
				? ScrollDirection.Left
				: ScrollDirection.Right;
	return new {
		Current =  _pagerAdapter.GetItem(args.Item2.Position) as Walkthrough, 
		Next = _pagerAdapter.GetItem(args.Item2.Position+1) as Walkthrough, 
		ScrollDirection = scrollDirection,
		args.Item2.PositionOffset,
	};
    })
    .Where(args => args.ScrollDirection == ScrollDirection.Center || args.Current != null && args.Next != null)
    .Subscribe (args => {
	if (args.ScrollDirection == ScrollDirection.Center) {
		args.Current.View?.SetBackgroundColor (args.Current.ViewModel.BackgroundColor.ToNative());
	} else if (args.ScrollDirection == ScrollDirection.Left || args.ScrollDirection == ScrollDirection.Right) {
		var color = args.Current.ViewModel.BackgroundColor.Lerp (args.Next.ViewModel.BackgroundColor, args.PositionOffset).ToNative ();
		args.Current.View?.SetBackgroundColor (color);
		args.Next.View?.SetBackgroundColor (color);
	}	
    }).DisposeWith(ControlBindings);