以自定义类（不是viewcontroller）编程调用PopoverPresentationController

Question

我试图从一个不是viewcontroller的自定义类中呈现一个popoverpresentationcontroller，但这似乎不起作用。除了我为popover创建的viewcontroller之外，我只有一个viewcontroller。我想以编程方式实现这一目标。有什么想法吗？

以下是我的示例代码：

func showErrorPopup() {
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let parentViewController =    UIApplication.sharedApplication().keyWindow?.rootViewController
    if let contentView  = storyboard.instantiateViewControllerWithIdentifier("ErrorPopUpView") as? ErrorViewController {
        contentView.modalPresentationStyle = .Popover
        contentView.preferredContentSize = CGSizeMake(400.0, 500.0)
        if let _popoverPresentationController = contentView.popoverPresentationController {
            _popoverPresentationController.delegate = self
            _popoverPresentationController.permittedArrowDirections = UIPopoverArrowDirection.Any
            _popoverPresentationController.sourceView = parentViewController?.view
            _popoverPresentationController.sourceRect = CGRectMake(-30, -280, 320, 400)
            parentViewController?.presentViewController(contentView, animated: true, completion: nil)
            }
        }