protected void UpdateData()
{
IncrementState();
MarkExistingItemsAsStale();
_fetchedPages.Clear();
_requestedPages.Clear();
UpdateCount();
var queryItemVisibilityArgs = new QueryItemVisibilityEventArgs();
OnQueryItemVisibility(queryItemVisibilityArgs);
if (queryItemVisibilityArgs.FirstVisibleIndex.HasValue)
{
var firstVisiblePage = queryItemVisibilityArgs.FirstVisibleIndex.Value / _pageSize;
var lastVisiblePage = queryItemVisibilityArgs.LastVisibleIndex.Value / _pageSize;
int numberOfVisiblePages = lastVisiblePage - firstVisiblePage + 1;
EnsurePageCacheSize(numberOfVisiblePages);
for (int i = firstVisiblePage; i <= lastVisiblePage; i++)
{
BeginGetPage(i);
}
}
else
{
// in this case we have no way of knowing which items are currently visible,
// so we signal a collection reset, and wait to see which pages are requested by the UI
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}
}
protected void OnQueryItemVisibility(QueryItemVisibilityEventArgs e)
{
EventHandler <QueryItemVisibilityEventArgs> handler = QueryItemVisibility;
if (handler != null)
{
handler(this, e);
}
}
private void HandleQueryItemVisibility(object sender, QueryItemVisibilityEventArgs e)
{
var wrapPanel = AssociatedObject.GetItemsHost() as VirtualizingWrapPanel;
if (wrapPanel != null)
{
var range = wrapPanel.GetVisibleItemsRange();
e.SetVisibleRange(range.FirstRealizedItemIndex, range.LastRealizedItemIndex);
}
}