private void ItemsCollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
// Provides a subset of the full items collection to bind to from a GroupedItemsPage
// for two reasons: GridView will not virtualize large items collections, and it
// improves the user experience when browsing through groups with large numbers of
// items.
//
// A maximum of 12 items are displayed because it results in filled grid columns
// whether there are 1, 2, 3, 4, or 6 rows displayed
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
if (e.NewStartingIndex < 12)
{
TopItems.Insert(e.NewStartingIndex,Items[e.NewStartingIndex]);
if (TopItems.Count > 12)
{
TopItems.RemoveAt(12);
}
}
break;
case NotifyCollectionChangedAction.Move:
if (e.OldStartingIndex < 12 && e.NewStartingIndex < 12)
{
TopItems.Move(e.OldStartingIndex, e.NewStartingIndex);
}
else if (e.OldStartingIndex < 12)
{
TopItems.RemoveAt(e.OldStartingIndex);
TopItems.Add(Items[11]);
}
else if (e.NewStartingIndex < 12)
{
TopItems.Insert(e.NewStartingIndex, Items[e.NewStartingIndex]);
TopItems.RemoveAt(12);
}
break;
case NotifyCollectionChangedAction.Remove:
if (e.OldStartingIndex < 12)
{
TopItems.RemoveAt(e.OldStartingIndex);
if (Items.Count >= 12)
{
TopItems.Add(Items[11]);
}
}
break;
case NotifyCollectionChangedAction.Replace:
if (e.OldStartingIndex < 12)
{
TopItems[e.OldStartingIndex] = Items[e.OldStartingIndex];
}
break;
case NotifyCollectionChangedAction.Reset:
TopItems.Clear();
while (TopItems.Count < Items.Count && TopItems.Count < 12)
{
TopItems.Add(Items[TopItems.Count]);
}
break;
}
}
