How to detect when a popover is dismissed in iOS 9

I'm updating an app to use universal storyboards. I've created a popover segue to a new viewcontroller using interface builder by dragging from a button to my new viewcontroller and selecting 'Present As Popover' as the kind of segue.

When the user presses outside of the popover (dismissing it) I need to be notified in the presenting view controller so I can undo their actions. How can I do this?

Normally I would have created the popover manually and made my viewcontroller the popover's delegate; allowing me to use the popoverControllerDidDismissPopover delegate call back. However, this is deprecated in iOS9 and even if it wasn't I've no idea where to find the popover so I can set its delegate to my view controller.

Not sure which method you're referring to as being deprecated but you can still use the UIPopoverPresentationControllerDelegate to achieve this. Something like:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "popoverSegue" {
        let vc = segue.destinationViewController
        sortVC.modalPresentationStyle = .Popover
        sortVC.popoverPresentationController?.sourceRect = filterButton.bounds
        sortVC.preferredContentSize = CGSizeMake(216, 150)
        sortVC.popoverPresentationController!.delegate = self
    }
}

And then use the

func popoverPresentationControllerDidDismissPopover(popoverPresentationController: UIPopoverPresentationController)

method to handle its dismissal.