Detect active NSSplitViewItem

I have a SplitView with (several) SplitViewItems. Echt SplitViewItem has a ViewController with multiple views in it.

I need to detect which of the SplitViewItems has the focus of the user.

For example: if the user clicks on any control/view (or navigates to it in any other way), the background of the SplitViewItem that contains that view item should change. As I do not know which/how many views will be enclosed in the ViewController in the SplitViewItem, I'd prefer to detect which SplitViewItem is the 'active' one in the SplitViewController.

I have been searching for a solution all day long. I could not find any kind of notification, nor found a way to solve this managing the responder chain.

Can anybody please point me in the right direction? A (swift) code example would be highly appreciated.

Thanks!

It took me quite some research, but I found a working solution. Not the most elegant, but working.

I found the best approach to be to add an event monitor to the SplitViewController.

In viewDidLoad() add the following code:

    NSEvent.addLocalMonitorForEvents(matching: [.keyDown, .leftMouseDown, .flagsChanged]) { [unowned self] (theEvent) -> NSEvent? in
        let eventLocation = theEvent.locationInWindow
        let numberOfSplitViews = self.splitViewItems.count
        var activeIndex: Int?

        for index in 0..<numberOfSplitViews {
            let view = self.splitViewItems[index].viewController.view
            let locationInView = view.convert(eventLocation, from: nil)
            if ((locationInView.x > 0) && (locationInView.x < view.bounds.maxX) && (locationInView.y > 0) && (locationInView.y < view.bounds.maxY)) {
                activeIndex = index
                break
            }
        }

        switch theEvent.type {
        case .keyDown:
            print("key down in pane \(activeIndex)")
            self.keyDown(with: theEvent)
        case .leftMouseDown, .rightMouseDown:
            print("mouse down in pane \(activeIndex)")
            self.mouseDown(with: theEvent)
        case .flagsChanged:
            print("flags changed in pane \(activeIndex)")
            self.flagsChanged(with: theEvent)
        default:
            print("captured some unhandled event in pane \(activeIndex)")
        }
        return theEvent
    }

(you may have to tweak the relevant events to your own liking. Also, you may need to remover the monitor using NSEvent.removeMonitor(_:)).

In addition (out of scope of this question), you might also want to consider making the variable activeIndex an observable class variable (I did this using RxSwift), allowing you to easily react on any change happening inside the 'active pane'.

Any more elegant/simple solutions are welcome!