Elegantly move a UIView to a keyboard's inputAccessoryView

Question

Is there an elegant way to seamlessly move a UIView (i.e. a UIButton), that lives on the bottom of it's parent view, to a keyboard's inputAccessoryView?

I envision the keyboard to basically pick up the button and drag it up while it is sliding in. Please see the the mock-up below. Obviously, I'd like this to work the other way around as well.

I know how to do both separately, but I have no idea how to combine it. Anyone out there who did that before?

Thanks!

Update: I forgot to mention that I'm mainly working in the context of UIScrollViews. While André's answer below works fine for ordinary views, it breaks when using it with scroll views.

Answer 1

in this case you could ignore using the inputaccessoryview and instead adjust the sign in button's bottom constraint on keyboard notifications.

class ViewController: UIViewController {
  @IBOutlet weak var signInButtonBottomConstraint: NSLayoutConstraint!

  override func viewDidLoad() {
    super.viewDidLoad()

    NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillShow:"), name: UIKeyboardWillShowNotification, object: nil)
    NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillHide:"), name: UIKeyboardWillHideNotification, object: nil)
  }

  func keyboardWillShow(notification: NSNotification) {
    let userInfo = notification.userInfo!

    let animationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as! Double
    let keyboardEndFrame = (userInfo[UIKeyboardFrameEndUserInfoKey] as! NSValue).CGRectValue()

    signInButtonBottomConstraint.constant = keyboardEndFrame.height
    UIView.animateWithDuration(animationDuration) { () -> Void in
      self.view.layoutIfNeeded()
    }
  }

  func keyboardWillHide(notification: NSNotification) {
    let userInfo = notification.userInfo!

    let animationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as! Double

    signInButtonBottomConstraint.constant = 0.0
    UIView.animateWithDuration(animationDuration) { () -> Void in
      self.view.layoutIfNeeded()
    }
  }
}