What are the factors that virtualizingstackpanel consider when it decides visualizing items around the visible item?
Ex: Having a listview, when viewing item 7, item 6& 8 will be visualized too although they are not seen.
How can limit visualized items, so that ONLY visible item is visualized?
I have solved the problem by overriding MeasureOverride
function, we already purchased the book Sheridan has mentioned in his answer, depending on virtualization chapter, here is what I did to my class that extends VirtualizingStackPanel
: and it worked!
private ItemsControl ItemsOwner { get; set; }
private int StartIndex { get; set; }
private int EndIndex { get; set; }
protected override void OnInitialized(EventArgs e)
{
ItemsOwner = ItemsControl.GetItemsOwner(this) as ItemsControl;
}
protected override Size MeasureOverride(Size availableSize)
{
ItemsControl itemsControl = ItemsControl.GetItemsOwner(this);
// we can set StartIndex& EndIndex to mimic
// VirtualizingStackPanel.CacheSize in .Net 4.5
// for my problem, I just fixed them at the index of item to be shown
StartIndex = PagesView.Instance.SelectedIndex;
EndIndex = StartIndex;
// Virtualize items
IItemContainerGenerator generator = ItemsOwner.ItemContainerGenerator;
GeneratorPosition startPos = generator.GeneratorPositionFromIndex(StartIndex);
int childIndex = startPos.Offset == 0 ? startPos.Index : startPos.Index + 1;
using (generator.StartAt(startPos, GeneratorDirection.Forward, true))
{
for (int i = StartIndex; i <= EndIndex; i++, childIndex++)
{
bool isNewlyRealized;
UIElement child = generator.GenerateNext(out isNewlyRealized) as UIElement;
if (isNewlyRealized)
{
if (childIndex >= InternalChildren.Count)
{
AddInternalChild(child);
}
else
{
InsertInternalChild(childIndex, child);
}
generator.PrepareItemContainer(child);
}
}
}
//DumpGeneratorContent();
// Measure
foreach (UIElement child in InternalChildren)
{
child.Measure(availableSize);
}
// Clean up
CleanupItems();
return availableSize;
}
private void CleanupItems()
{
IItemContainerGenerator generator = ItemsOwner.ItemContainerGenerator;
for (int i = InternalChildren.Count - 1; i >= 0; i--)
{
GeneratorPosition position = new GeneratorPosition(i, 0);
int itemIndex = generator.IndexFromGeneratorPosition(position);
if (itemIndex < StartIndex || itemIndex > EndIndex)
{
generator.Remove(position, 1);
RemoveInternalChildRange(i, 1);
}
}
}