Extension Properties

I’m looking for a more swifty way to do what the following iOS swift code does.

extension UITextField {
  private static var NextFieldKey = "nextField"
  @IBOutlet var nextField: UITextField? {
    get {
      return objc_getAssociatedObject(self, &UITextField.NextFieldKey) as? UITextField
    set {
      objc_setAssociatedObject(self, &UITextField.NextFieldKey, newValue, .OBJC_ASSOCIATION_ASSIGN)

class ViewController: UIViewController, UITextFieldDelegate {
  func textFieldShouldReturn(textField: UITextField) -> Bool {
    if let next = textField.nextField {
    } else {
    return true

I think this is actually close to as Swifty as possible. I sometimes use private global dictionaries to fake stored extension properties, but that necessitates boxing UIKit objects in weak referencing objects, and managing the cleanup of the dictionaries. Not tidy.