I have a ListView
which might contains a lot of items, so it is virtualized
and recycling items. It does not use sort. I need to refresh some value display, but when there are too many items, it is too slow to update everything, so I would like to refresh only the visible items.
How could I get a list of all currently displayed items ? I tried to look into the ListView
or in the ScrollViewer
, but I still have no idea how to achieve this. The solution must NOT go through all items to test if they can be seen, because this would be too slow.
I'm not sure code or xaml would be useful, it is just a Virtualized
/Recycling ListView
with its ItemSource
bound to an Array
.
Edit :
Answer :
thanks to akjoshi, I found the way :
get the ScrollViewer
of the ListView
(with a FindDescendant
method, that you can do yourself with the VisualTreeHelper
).
read its ScrollViewer.VerticalOffset
: it is the number of the first item shown
ScrollViewer.ViewportHeight
: it is the count of items shown.CanContentScroll
must be true.Have a look at this question on MSDN showing a technique to find out the visible ListView
items -
How to find the rows (ListViewItem(s)) in a ListView that are actually visible?
Here's the relevant code from that post -
listView.ItemsSource = from i in Enumerable.Range(0, 100) select "Item" + i.ToString();
listView.Loaded += (sender, e) =>
{
ScrollViewer scrollViewer = listView.GetVisualChild<ScrollViewer>(); //Extension method
if (scrollViewer != null)
{
ScrollBar scrollBar = scrollViewer.Template.FindName("PART_VerticalScrollBar", scrollViewer) as ScrollBar;
if (scrollBar != null)
{
scrollBar.ValueChanged += delegate
{
//VerticalOffset and ViweportHeight is actually what you want if UI virtualization is turned on.
Console.WriteLine("Visible Item Start Index:{0}", scrollViewer.VerticalOffset);
Console.WriteLine("Visible Item Count:{0}", scrollViewer.ViewportHeight);
};
}
}
};
Another thing you should do is to use ObservableCollection
as your ItemSource
instead of an Array
; that will definitely improve the performance.
Update:
Ya that might be true(array
vs. ObservableCollection
) but I would like to see some statistics related to this;
The real benefit of ObservableCollection
is if you have a requirement to add/remove items from your ListView
at run-time, in case of an Array
you will have to reassign the ItemSource
of ListView
and the ListView
first throws away its previous items and regenerates its entire list.
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