How do I save the order of sourcelist items?

Question

I have an NSOutlineView as a sourcelist, like Mail.app. Its content is bound to an NSTreeController, which manages an array of my model objects. I've implemented drag-and-drop on the sourcelist and this is working perfectly.

Now I'd like to save the new ordering when the user finishes a drag-and-drop action in the sidebar. However, I can't find anything describing how this would usually be done.

The best I can come up with is:

• add a treeIndex property to my model class, to store the model's position in the sourcelist
• after a drop action, go through each node in the NSTreeController, and store each node's index in the model's treeIndex property

The problem is, I can't find how to retrieve the arranged objects from the NSTreeController. I can't get treeController.arrangedObjects to give me anything useful!

Searching through Github repos I've seen numerous uses of treeController.arrangedObjects.childNodes!! but that just throws a Value of type 'Any' has no member 'children' syntax error for me.

The following pseudo-code might help describe what I am trying to do after each drop action:

let parentNode = treeController.nodeAtIndex([0, 0])
for childNode in parentNode.children {
   (childNode.representedObject as SidebarItem).indexPosition = childNode.index
}