如何在iOS9中展示PopOver

我正在尝试创建一个弹出窗口,当我呈现视图控制器时,背景总是纯黑色,大小是全屏.

我似乎无法弄清楚什么是错的,这是我的代码

  @IBAction func distancePopOver( sender : UIBarButtonItem){

    //a UIViewController that I created in the storyboard
    let controller = storyboard!.instantiateViewControllerWithIdentifier("distancePopOver")
    controller.modalPresentationStyle= UIModalPresentationSTyle.PopOver
    controller.preferredContentSize = CGSizeMake(200,30)
    self.presentViewController(controller, animated: true, completion: nil)

    //Configure the Popover presentation controller
    let popController = (controller.popoverPresentationController)!
    popController.permittedArrowDirections = UIPopoverArrowDirection.Down
    popController.barButtonItem = sender
    popController.delegate = self
  }

每当我点击UIBarButtonItem时,它都会以全屏显示视图,但它不应该是我在第5行中指定的大小吗？