Notify View from ViewModel

Hello,

I would like to figure out how to solve this:

I have a ViewModel. If inside the ViewModel a certain state occurs such as property1 and property2 have some values. I need to send a notification to the View. The View then gets notified and executes some view-based logic.

To be more specific this is the usecase where I need to send messages from ViewModel to View.

I have a ViewModel with 10 properties. One of the properties is a list which is bound to TreeView.

Now I would like to send a notification to TreeView from ViewModel. When TreeView gets notified it should execute the method called expend which expends a visible node or the method called collapse which closes a node.

So to sum up. I would like to control the methods expand and collapse from ViewModel. The ViewModel is only able to figure out when that one specific state happens, then it should send notification to View, which will be catched by TreeView and then the expand or collapsed method of TreeView will get called.

How do I solve this?