TreeView, How do you know which row is expanded or not?

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I have a Doc/View framework and everytime the Doc changes it notifies the 
View to update its displayed data.
When the View updates it needs to update among other things a TreeView & 
TreeModel based on the data of the Doc, its items can be removed, added or 
moved around.

Currrently I just clear everything in the TreeModel in the View then add 
everything again base on the
data of the Doc, but this does not preserive which rows were expanded and 
the user must then
rexpend them.

How should I keep the rows expanded, while adding/removeing/moving items?
How can I tell which rows are expanded and not?

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