CarouselView.iOSRenderer.cs

void NativeScrolled (object sender, EventArgs e)
{
    var center = _native.ContentOffset.X + (_native.Bounds.Width / 2);
    ((CarouselLayout)Element).SelectedIndex = ((int)center) / ((int)_native.Bounds.Width);
}

void ElementPropertyChanged(object sender, PropertyChangedEventArgs e) {
    if (e.PropertyName == CarouselLayout.SelectedIndexProperty.PropertyName && !Dragging) {
        ScrollToSelection (false);
    }
}

void ScrollToSelection (bool animate)
{
    if (Element == null) return;

    _native.SetContentOffset (new CoreGraphics.CGPoint 
        (_native.Bounds.Width * Math.Max(0, ((CarouselLayout)Element).SelectedIndex), 
            _native.ContentOffset.Y), animate);
}