How to present popover properly in iOS 8

Actually it is much simpler than that. In the storyboard you should make the viewcontroller you want to use as popover and make a viewcontroller class for it as usual. Make a segue as shown below from the object you want to open the popover, in this case the UIBarButton named "Config".

In the "mother viewcontroller" implement the UIPopoverPresentationControllerDelegate and the delegate method:

func popoverPresentationControllerDidDismissPopover(popoverPresentationController: UIPopoverPresentationController) {
    //do som stuff from the popover
}

Override the prepareForSeque method like this:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
     //segue for the popover configuration window
    if segue.identifier == "yourSegueIdentifierForPopOver" {
        if let controller = segue.destinationViewController as? UIViewController {
            controller.popoverPresentationController!.delegate = self
            controller.preferredContentSize = CGSize(width: 320, height: 186)
        }
    }
}

And you're done. And you can now treat the popover view as any other view, ie. add fields and what not! And you get hold of the the content controller by using the popoverPresentationController.presentedViewController method in the UIPopoverPresentationController.

Also on an iPhone you would have to overwrite

func adaptivePresentationStyle(for controller: UIPresentationController) -> UIModalPresentationStyle {

        return UIModalPresentationStyle.none
    } 

Okay, A housemate took a look at it and figured it out:

 func addCategory() {

    var popoverContent = self.storyboard?.instantiateViewControllerWithIdentifier("NewCategory") as UIViewController
    var nav = UINavigationController(rootViewController: popoverContent)
    nav.modalPresentationStyle = UIModalPresentationStyle.Popover
    var popover = nav.popoverPresentationController
    popoverContent.preferredContentSize = CGSizeMake(500,600)
    popover.delegate = self
    popover.sourceView = self.view
    popover.sourceRect = CGRectMake(100,100,0,0)

    self.presentViewController(nav, animated: true, completion: nil)

}

That's the way.

You don't talk to the popover itself anymore, you talk to the view controller inside of it to set the content size, by calling the property preferredContentSize