Make any view controller popover controller.

PopoverController.swift

import UIKit

class PopoverController: UIViewController {

    enum Source {
        case barButtonItem(item: UIBarButtonItem)
        case sourceView(view: UIView?)
    }
    
    private var contentViewController: UIViewController?
    private var source: Source = .sourceView(view: nil)
    private var permittedArrowDirection: UIPopoverArrowDirection = .any
    private var contentSize: CGSize = .zero
    
    convenience init(contentViewController: UIViewController, source: Source, permittedArrowDirection: UIPopoverArrowDirection = .any, preferredContentSize: CGSize) {
        self.init(nibName: nil, bundle: nil)
        self.contentViewController = contentViewController
        self.source = source
        self.permittedArrowDirection = permittedArrowDirection
        self.contentSize = preferredContentSize
        prepareForPopover()
    }
    
    fileprivate func addAsChild(_ contentViewController: UIViewController) {
        contentViewController.willMove(toParent: self)
        
        self.view.addSubview(contentViewController.view)
        contentViewController.view.translatesAutoresizingMaskIntoConstraints = false
        contentViewController.view.topAnchor.constraint(equalTo: self.view.topAnchor).isActive = true
        contentViewController.view.leftAnchor.constraint(equalTo: self.view.leftAnchor).isActive = true
        contentViewController.view.rightAnchor.constraint(equalTo: self.view.rightAnchor).isActive = true
        contentViewController.view.bottomAnchor.constraint(equalTo: self.view.bottomAnchor).isActive = true
        
        self.addChild(contentViewController)
        contentViewController.didMove(toParent: self)
    }
    
    private func removeFromChild() {
        contentViewController?.willMove(toParent: nil)
        contentViewController?.view?.removeFromSuperview()
        contentViewController?.removeFromParent()
        contentViewController?.didMove(toParent: nil)

    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        guard let contentViewController = self.contentViewController else {
            fatalError("There must be a view controller to present in popover.")
        }
        addAsChild(contentViewController)
    }
    
    deinit {
        self.removeFromChild()
    }
    
    private func prepareForPopover() {
        self.preferredContentSize = self.contentSize
        self.modalPresentationStyle = .popover
        self.popoverPresentationController?.delegate = self
    }

}

extension PopoverController: UIPopoverPresentationControllerDelegate {
    func adaptivePresentationStyle(for controller: UIPresentationController) -> UIModalPresentationStyle {
        return .none
    }
    
    func prepareForPopoverPresentation(_ popoverPresentationController: UIPopoverPresentationController) {
        popoverPresentationController.permittedArrowDirections = self.permittedArrowDirection
        switch source {
        case .barButtonItem(let item):
            popoverPresentationController.barButtonItem = item
        case .sourceView(let view):
            popoverPresentationController.sourceView = view
            popoverPresentationController.sourceRect = view?.bounds ?? .zero
        }
    }
}