Reacting to user scrolling a ListView

[Zerachiel01]

Hello everyone!

In my android app I have a page with essentially two elements: An Expander at the top and a ListView for the rest of the screen. The Expander contains some filter settings for the resulting elements in the ListView.

Now I want the following behaviour: The expander should collapse as soon as the user scrolls the ListView up or down.

I saw the ScrollChange event on ListView, but I did not use it, because it is only available for Android and I want to be able to use iOS later as well, so I do not want to use anything that only works for Android.

I also tried to get the ScrollViewer element of the ListView through the VisualTreeHelper and react to the ViewChanged event. Problem here is, that the ViewChanged event also occurs when the Expander expands, which results in my Expander to immediately collpapse again if I try to expand it manually.
I thought about setting a flag when the Expander expands to prevent my "collapse Expander on scroll" logic from happening, but the Expander does not seem to have a "Expanded" event for me to reset the flag after my Expander is fully expanded.

Am I missing the obvious solution here? Or does anyone have another idea?

[Xiaoy312]

Indeed, it seems there is a particular problem on native-android, where changing the size of the ListView would cause it to scroll up a bit each time. And, that seem to cause this problem for you. I will create an issue to track this: #21039

That said, there is a simple workaround for you, you can use the ScrollViewerViewChangedEventArgs.IsIntermediate from the ScrollViewer::ViewChanged event to isolate manually scrolling vs scrolling caused by resizing.

In your event handler, use a parameter of type ScrollViewerViewChangedEventArgs for the second parameter of your handler method. ScrollViewerViewChangedEventArgs includes the IsIntermediate property, which reports whether the manipulation that fired the event is completed or still ongoing.

this.Loaded += (s, e) =>
{
	if (TestLV.FindFirstDescendant<ScrollViewer>() is { } sv)
	{
		sv.ViewChanged += (s2, e2) =>
		{
			if (e2.IsIntermediate && TopExpander.IsExpanded)
			{
				TopExpander.IsExpanded = false;
			}
		};
	}
};

This property will be true, for all the ViewChanged events raised during the manually scrolling (including quick flicking), except the last one.

[Zerachiel01]

Works like a charm, thank you very much @Xiaoy312