public static (int firstVisibleItemIndex, int lastVisibleItemIndex, int centerItemIndex) GetVisibleIndexes(ListViewBase listViewBase, ItemsLayoutOrientation itemsLayoutOrientation)
{
int firstVisibleItemIndex = -1;
int lastVisibleItemIndex = -1;
int centerItemIndex = -1;
var scrollViewer = listViewBase.GetFirstDescendant <ScrollViewer>();
var presenters = listViewBase.GetChildren <ListViewItemPresenter>();
if (presenters != null || scrollViewer == null)
{
int count = 0;
foreach (ListViewItemPresenter presenter in presenters)
{
if (CollectionViewExtensions.IsListViewItemVisible(presenter, scrollViewer, itemsLayoutOrientation))
{
if (firstVisibleItemIndex == -1)
{
firstVisibleItemIndex = count;
}
lastVisibleItemIndex = count;
}
count++;
}
centerItemIndex = (lastVisibleItemIndex + firstVisibleItemIndex) / 2;
}
return(firstVisibleItemIndex, lastVisibleItemIndex, centerItemIndex);
}
void OnScrollViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
{
HorizontalOffset = _scrollViewer.HorizontalOffset,
HorizontalDelta = _scrollViewer.HorizontalOffset - _previousHorizontalOffset,
VerticalOffset = _scrollViewer.VerticalOffset,
VerticalDelta = _scrollViewer.VerticalOffset - _previousVerticalOffset,
};
_previousHorizontalOffset = _scrollViewer.HorizontalOffset;
_previousVerticalOffset = _scrollViewer.VerticalOffset;
int firstVisibleItemIndex = -1;
int lastVisibleItemIndex = -1;
int firstVisibleSourceItemIndex = -1;
List <object> sourceItems = GetSourceItems();
var presenters = ListViewBase.GetChildren <ListViewItemPresenter>();
if (presenters != null)
{
int count = 0;
foreach (ListViewItemPresenter presenter in presenters)
{
if (IsListViewItemVisible(presenter, _scrollViewer))
{
if (firstVisibleItemIndex == -1)
{
firstVisibleItemIndex = count;
firstVisibleSourceItemIndex = GetFirstVisibleSourceItemIndex(sourceItems, presenter);
}
lastVisibleItemIndex = count;
}
count++;
}
itemsViewScrolledEventArgs.FirstVisibleItemIndex = firstVisibleItemIndex;
itemsViewScrolledEventArgs.CenterItemIndex = (lastVisibleItemIndex + firstVisibleItemIndex) / 2;
itemsViewScrolledEventArgs.LastVisibleItemIndex = lastVisibleItemIndex;
}
Element.SendScrolled(itemsViewScrolledEventArgs);
int lastVisibleSourceItemIndex = GetLastVisibleSourceItemIndex(firstVisibleSourceItemIndex,
firstVisibleItemIndex, lastVisibleItemIndex);
Element.ResolveRemainingItemsThreshold(sourceItems.Count, lastVisibleSourceItemIndex);
}
public static (int firstVisibleItemIndex, int lastVisibleItemIndex, int centerItemIndex) GetVisibleIndexes(this ListViewBase listViewBase, ItemsLayoutOrientation itemsLayoutOrientation, bool goingNext)
{
int firstVisibleItemIndex = -1;
int lastVisibleItemIndex = -1;
var itemsPanel = (listViewBase.ItemsPanelRoot as ItemsStackPanel);
if (itemsPanel != null)
{
firstVisibleItemIndex = itemsPanel.FirstVisibleIndex;
lastVisibleItemIndex = itemsPanel.LastVisibleIndex;
}
else
{
var scrollViewer = listViewBase.GetFirstDescendant <ScrollViewer>();
var presenters = listViewBase.GetChildren <ListViewItemPresenter>();
if (presenters != null || scrollViewer == null)
{
int count = 0;
foreach (ListViewItemPresenter presenter in presenters)
{
if (IsListViewItemVisible(presenter, scrollViewer, itemsLayoutOrientation))
{
if (firstVisibleItemIndex == -1)
{
firstVisibleItemIndex = count;
}
lastVisibleItemIndex = count;
}
count++;
}
}
}
double center = (lastVisibleItemIndex + firstVisibleItemIndex) / 2.0;
int centerItemIndex = goingNext ? (int)Math.Ceiling(center) : (int)Math.Floor(center);
return(firstVisibleItemIndex, lastVisibleItemIndex, centerItemIndex);
}
