KeyboardDodger.swift

class KeyboardDodger {

    private struct KeyboardValues {

        private let userInfo: [AnyHashable : Any]

        init(notification: Notification) {
            userInfo = (notification as NSNotification).userInfo!
        }

        var duration: TimeInterval {
            return userInfo[UIKeyboardAnimationDurationUserInfoKey] as! TimeInterval
        }

        var frame: CGRect {
            return (userInfo[UIKeyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
        }

        var curve: UIViewAnimationOptions {
            return UIViewAnimationOptions(rawValue: userInfo[UIKeyboardAnimationCurveUserInfoKey] as! UInt)
        }

    }

    private let view: UIView
    private let constraint: NSLayoutConstraint
    private let collisionView: UIView
    private let notificationCenter = NotificationCenter.default
    private let offset: CGFloat = 8

    init (view: UIView, constraint: NSLayoutConstraint, collisionView: UIView) {
        self.view = view
        self.constraint = constraint
        self.collisionView = collisionView
        addObservers()
    }

    private func addObservers() {
        notificationCenter.addObserver(self, selector: #selector(KeyboardDodger.keyboardWillChangeFrame(_:)), name: NSNotification.Name.UIKeyboardWillChangeFrame, object: nil)
        notificationCenter.addObserver(self, selector: #selector(KeyboardDodger.keyboardWillHide(_:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
    }

    deinit {
        notificationCenter.removeObserver(self)
    }

    private dynamic func keyboardWillChangeFrame(_ notification: Notification) {
        let values = KeyboardValues(notification: notification)
        updateConstraint(values)
        animateView(values)
    }

    private func updateConstraint(_ values: KeyboardValues) {
        if !collisionView.frame.intersects(values.frame) { return }
        constraint.constant = values.frame.height + offset
        view.addConstraint(constraint)
        view.setNeedsUpdateConstraints()
    }

    private func animateView(_ values: KeyboardValues) {
        UIView.animate(withDuration: values.duration, delay: 0, options: values.curve, animations: {
            self.view.layoutIfNeeded()
        }, completion: nil)
    }

    private dynamic func keyboardWillHide(_ notification: Notification) {
        let values = KeyboardValues(notification: notification)
        removeConstraint()
        animateView(values)
    }

    private func removeConstraint() {
        constraint.constant = 0
        view.removeConstraint(constraint)
        view.setNeedsUpdateConstraints()
    }

}