Expand Root in TreeView when ItemSource changed?

Try to register for your ObservableCollection's CollectionChange Event and do your actions there.

Derive from TreeView and create IsRootItemExpanded property and create same name property in ViewModel. In set accesser check if value true then call your method like this ExpandToDepth(1) When itemssource is need to update then set IsRootItemExpanded property true. You must to bind IsRootItemExpanded of TreeView to IsRootItemExpanded of ViewModel.

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