Receive trigger when active NSView focus changes

Question

I'm surprised that I haven't been able to find an answer for this from searching. So if there is a page describing how to do this, let me know, but I've been unable to find it.

I have 3 sibling NSOutlineViews all inside an NSSplitView I have added observers for NSOutlineViewSelectionDidChange which, when triggered update an NSTextView editor. I even test for negative row values to indicate that the user has unselected the row and clear the text.

The NSOutlineViews are connected to custom datasource objects and the NSSplitView is created by my custom NSWindowController in turn created by my custom NSDocument and custom NSDocumentController (I'm not using .nibs)

However I cannot seem to receive triggers for the NSOutlineViews changing their active status, it works if the user selects a different row in a different NSOutlineView, as the selection has changed but if they click the selected row of a different view, I don't receive any event or notification that anything has changed. Visually I can see the change as the row selection highlight colour changes from coloured to grey, in the outline view that has lost focus and the selection row colour changes from grey to coloured in the newly activated view.

I've tried to catch mouseDown events, tried becoming first responder, tried observing changes in the NSSplitView I've been right through the NSObject hierarchy from NSOutlineView to NSResponder looking for the appropriate notification or method. I found deprecated documentation regarding a focus change notification. I've tried combinations of nsview, nsnotification, nsoutlineview and various actions in google but can't find the 'this is how you do it'

EDIT: this is the code I've added to my NSOutlineView subclass (along with prototypes in the headers) to become first responder, but it is never triggered.

- (BOOL)acceptsFirstResponder { return YES; }
- (BOOL)becomeFirstResponder {
    NSLog(@"becomefirstResponder %@",self);
    return YES;
}