Equivalent of ItemAppearing in xamarin UWP - xamarin.forms

Question

I have a ListView bind to list of BitmapImage.
I want to get the Index of current image in focus when I scroll thru this list.
But, I notice that ItemAppearing property is not there in UWP but it is there in Xamarin Forms.
How can I get the index of the current item in view?
Thanks!
<ScrollViewer Grid.Row="0" ZoomMode="{x:Bind ZoomMode, Mode=OneWay}" HorizontalScrollMode="Enabled" HorizontalScrollBarVisibility="Auto">
<ListView HorizontalAlignment="Center" ItemsSource="{x:Bind ImagePages, Mode=OneWay}">
<ItemsControl.ItemTemplate>
<DataTemplate x:DataType="BitmapImage">
<Image Source="{x:Bind }" Margin="0 2" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ListView>
</ScrollViewer>

Answer 1

For starters, the ItemAppearing property is not the behavior you are looking for. The ItemAppearing event for the ListView in Xamarin Forms is fired when the list item is rendered. For a small list this event will be fired for all items immediately. The equivalent event in UWP is ListView.ChoosingItemContainer event which like the ItemAppearing event, unless the ListView is virtualized is fired for all items in the list. Even for a large virtualized list, it is fired for several pages of items.
This is not what you want. As I understand it, you want to know the image that is visible at the top of the list view when the list is scrolled. Here is how to do that.
First of all. Get rid of the ScrollViewer. The ListView already has a ScrollViewer inside of it.
<ListView x:Name="listViewImage" Grid.Row="0" HorizontalAlignment="Center" ItemsSource="{x:Bind ImagePages, Mode=OneWay}"
Loaded="listViewImage_Loaded">
<ItemsControl.ItemTemplate>
<DataTemplate x:DataType="BitmapImage">
<Image Source="{x:Bind }" Margin="0 2"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ListView>
Note that I have named the ListView and I have added a Loaded event handler. In this handler, find the ScrollViewer inside the ListView and attach a handler to the ViewChanged event.
private void listViewImage_Loaded(object sender, RoutedEventArgs e)
{
Border b = VisualTreeHelper.GetChild(listViewImage, 0) as Border;
ScrollViewer sv = b.Child as ScrollViewer;
sv.ViewChanged += Sv_ViewChanged;
}
In the view changed handler, find the first visible ListViewItem and get its index in the collection. This is what you want.
private void Sv_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
ScrollViewer sv = sender as ScrollViewer;
GeneralTransform gt = sv.TransformToVisual(this);
Point p = gt.TransformPoint(new Point(0, 0));
List<UIElement> list = new List<UIElement>(VisualTreeHelper.FindElementsInHostCoordinates(p, sv));
ListViewItem item = list.OfType<ListViewItem>().FirstOrDefault();
if(item != null)
{
int index = listViewImage.IndexFromContainer(item);
Debug.WriteLine("Visible item at top of list is " + index);
}
}