Fix VirtualizingStackPanel and nth-child for the currently realizing item container #12957
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…item container (#12957) * Added failing tests for VirtualizingStackPanel and nth-child * VirtualizingStackPanel: support index of currently realizing item
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What does the pull request do?
This PR ensures that
VirtualizingStackPanel.ContainerFromIndex/IndexFromContainerhandles the container currently being realized, so index-based styles (nth-childandnth-last-child) are correctly applied.What is the current behavior?
Currently, any style targeting a descendant element of an item container having itself an index-based style activator isn't applied when the item container is created by a
VirtualizingStackPanel. See #12838 for details.What is the updated/expected behavior with this PR?
Styles on item container descendants are correctly applied.
How was the solution implemented (if it's not obvious)?
The container currently being realized is stored in a field, so
ContainerFromIndexandIndexFromContainercan return it correctly. Before the PR, these methods looked only at the already realized items, but the currently realizing item isn't in that list yet, soNthChildActivatorfails on it.Why did this work previously for container styles (
ListBoxItem:nth-child(even)) but not for styles targeting descendants inside the item template (ListBoxItem:nth-child(even) > TextBlock)?Well, it didn't at first: a matching
NthChildActivatorwasn't activated during the container creation (CreateContainer), similar to this issue. However, after creation the panel raisesChildIndexChanged(inItemContainerPrepared), handled by the activator which now gets a proper index.Elements from the item template aren't created at this point though, as this only happens when the container is measured, which is after
ItemContainerPrepared(so theChildIndexChangedevent was already raised and handled).While this PR adds a special case for the container being realized, ideally we should handle the indexes properly for all containers that have just been realized but aren't part of the
_realizedElementsyet (they're in_measureElementstemporarily duringRealizeElements()), as their index might be requested by some code. Pinging @grokys for thoughts on this.Unit tests have been added.
Fixed issues