I have a render-heavy item template in an ItemsControl and want to minimize the recreation of child item templates when ItemsSource signals a change. I am wondering if, because ObservableCollection can tell WPF precisely what has changed (as opposed to just the whole list), if it is more efficient in rendering changes to the collection, or if WPF is smart enough to reuse previous item views if it detects the same item is still in the changed list.
Your suspicion is correct. WPF will not reuse previous views. If you replace the ItemsSource of an ItemsControl with a new List, it will create completely new views for each item in the list, even if the same items were in the old list.
You can test this yourself by putting a custom control in the ItemTemplate and adding a breakpoint or debug logging to its constructor. If you replace the ItemsSource with an identical list, you will see your control constructed once for each item in the list. On the other hand, when an item is added to an ObservableCollection you will only see it called once.
Note that the ItemsControl can reuse the container (such as ListBoxItem) if you are using a virtualizing panel and have container recycling enabled. See Link. It still can't reuse the contents of the container, however.