/// <summary>
/// Immediately cleans up any containers that have gone offscreen. Called by MeasureOverride.
/// When recycling this runs before generating and measuring children; otherwise it runs after.
/// </summary>
private void CleanupContainers(int firstViewport, ItemsControl itemsControl)
{
Debug.Assert(IsVirtualizing, "Can't clean up containers if not virtualizing");
Debug.Assert(InRecyclingMode || IsPixelBased,
"For backwards compat the standard virtualizing mode has its own cleanup algorithm");
Debug.Assert(itemsControl != null, "We can't cleanup if we aren't the itemshost");
//
// It removes items outside of the container cache window (a logical 'window' at
// least as large as the viewport).
//
// firstViewport is the index of first data item that will be in the viewport
// at the end of Measure. This is effectively the scroll offset.
//
// _visibleStart is index of the first data item that was previously at the top of the viewport
// At the end of a Measure pass _visibleStart == firstViewport.
//
// _visibleCount is the number of data items that were previously visible in the viewport.
int cleanupRangeStart = -1;
int cleanupCount = 0;
int itemIndex = -1; // data item index used to compare with the cache window position.
int lastItemIndex;
IList children = RealizedChildren;
int focusedChild = -1, previousFocusable = -1, nextFocusable = -1; // child indices for the focused item and before and after focus trail items
bool performCleanup = false;
UIElement child;
object item;
if (children.Count == 0)
{
return; // nothing to do
}
AdjustCacheWindow(firstViewport, itemsControl.Items.Count);
if (IsKeyboardFocusWithin && !IsPixelBased)
{
// If we're not in a hieararchy we can find the focus trail locally; for hierarchies it has already been
// precalculated.
FindFocusedChild(out focusedChild, out previousFocusable, out nextFocusable);
}
//
// Iterate over all realized children and recycle the ones that are eligible. Items NOT eligible for recycling
// have one or more of the following properties
//
// - inside the cache window
// - the item is its own container
// - has keyboard focus
// - is the first focusable item before or after the focused item
// - the CleanupVirtualizedItem event was canceled
//
for (int childIndex = 0; childIndex < children.Count; childIndex++)
{
child = (UIElement)children[childIndex];
lastItemIndex = itemIndex;
itemIndex = GetGeneratedIndex(childIndex);
// itemsControl.Items can change without notifying VirtualizingStackPanel (when ItemsSource is not an ObservableCollection or does not implement INotifyCollectionChanged).
// Fetch the item from the container instead of referencing from the Items collection.
item = itemsControl.ItemContainerGenerator.ItemFromContainer(child);
if (itemIndex - lastItemIndex != 1)
{
// There's a generated gap between the current item and the last. Clean up the last range of items.
performCleanup = true;
}
if (performCleanup)
{
if (cleanupRangeStart >= 0 && cleanupCount > 0)
{
//
// We've hit a non-virtualizable container or a non-contiguous section.
//
CleanupRange(children, Generator, cleanupRangeStart, cleanupCount);
// CleanupRange just modified the _realizedChildren list. Adjust the childIndex.
childIndex -= cleanupCount;
focusedChild -= cleanupCount;
previousFocusable -= cleanupCount;
nextFocusable -= cleanupCount;
cleanupCount = 0;
cleanupRangeStart = -1;
}
performCleanup = false;
}
if (IsOutsideCacheWindow(itemIndex) &&
!((IGeneratorHost)itemsControl).IsItemItsOwnContainer(item) &&
childIndex != focusedChild &&
childIndex != previousFocusable &&
childIndex != nextFocusable &&
!IsInFocusTrail(child) && // logically the same computation as the three above; used when in a treeview.
child != _bringIntoViewContainer && // the container we're going to bring into view must not be recycled
NotifyCleanupItem(child, itemsControl))
{
//
// The container is eligible to be virtualized
//
if (cleanupRangeStart == -1)
{
cleanupRangeStart = childIndex;
}
cleanupCount++;
//
// Save off the child's desired size if we're doing pixel-based virtualization.
// We need to save off the size when doing hierarchical (i.e. TreeView) virtualization, since containers will vary
// greatly in size. This is required both to compute the index of the first visible item in the viewport and to Arrange
// children in their proper locations.
//
if (IsPixelBased)
{
itemsControl.StoreItemValue(item, child.DesiredSize, _desiredSizeStorageIndex);
}
}
else
{
// Non-recyclable container;
performCleanup = true;
}
}
if (cleanupRangeStart >= 0 && cleanupCount > 0)
{
CleanupRange(children, Generator, cleanupRangeStart, cleanupCount);
}
}
// Tells the Generator to clear out all containers for this ItemsControl. This is called by the ItemValueStorage
// service when the ItemsControl this panel is a host for is about to be thrown away. This allows the VSP to save
// off any properties it is interested in and results in a call to ClearContainerForItem on the ItemsControl, allowing
// the Item Container Storage to do so as well.
// Note: A possible perf improvement may be to make 'fast' RemoveAll on the Generator that simply calls ClearContainerForItem
// for us without walking through its data structures to actually clean out items.
internal void ClearAllContainers(ItemsControl itemsControl)
{
Debug.Assert(itemsControl == ItemsControl.GetItemsOwner(this),
"We can only clear containers that this panel is a host for");
IItemContainerGenerator generator = Generator;
if (IsPixelBased)
{
IList children = RealizedChildren;
UIElement child;
for (int i = 0; i < children.Count; i++)
{
child = (UIElement)children[i];
itemsControl.StoreItemValue(((ItemContainerGenerator)generator).ItemFromContainer(child), child.DesiredSize, _desiredSizeStorageIndex);
}
}
if (generator != null)
{
generator.RemoveAll();
}
}