how to center a popoverview in swift

Another way for Swift 3 (Xcode 8, iOS 9) is this:

Called from somewhere:

self.performSegue(withIdentifier: "showPopupSegue", sender: yourButton)

Function that gets called before segue gets fired:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let popoverPresentationController = segue.destination.popoverPresentationController {
        let controller = popoverPresentationController
        controller.permittedArrowDirections = UIPopoverArrowDirection(rawValue: 0)
        controller.sourceView = self.view
        controller.sourceRect = CGRect(x: UIScreen.main.bounds.width * 0.5 - 200, y: UIScreen.main.bounds.height * 0.5 - 100, width: 400, height: 200)
        segue.destination.preferredContentSize=CGSize(width: 400, height: 200)
    }
}

Remember to set the storyboard segue's Kind attribute to "Present as Popover" and Anchor attribute to any view in your previous view controller.

You need to provide the source rect for the popover.

From the apple documentation: the source rect is the rectangle in the specified view in which to anchor the popover. Use this property in conjunction with the sourceView property to specify the anchor location for the popover.

In your case, under

_popoverPresentationController.sourceView = self.view;

add:

_popoverPresentationController.sourceRect = CGRectMake(CGRectGetMidX(self.view.bounds), CGRectGetMidY(self.view.bounds),0,0)

It will do the trick!

Swift 4 implementation :

popover.popoverPresentationController?.sourceRect = CGRect(x: view.center.x, y: view.center.y, width: 0, height: 0)
popover.popoverPresentationController?.sourceView = view
popover.popoverPresentationController?.permittedArrowDirections = UIPopoverArrowDirection(rawValue: 0)

Here's an implementation using Swift 3

let popover = storyboard?.instantiateViewController(withIdentifier: "popover") as! PopoverVC

    popover.modalPresentationStyle = UIModalPresentationStyle.popover
    popover.popoverPresentationController?.backgroundColor = UIColor.green
    popover.popoverPresentationController?.delegate = self

    popover.popoverPresentationController?.sourceView = self.view
    popover.popoverPresentationController?.sourceRect = CGRect(x: self.view.bounds.midX, y: self.view.bounds.midY, width: 0, height: 0)

    popover.popoverPresentationController?.permittedArrowDirections = UIPopoverArrowDirection(rawValue: 0)

    self.present(popover, animated: true)

Based on Istvan's answer