RxKeyboardController.swift

final class ViewController: UIViewController {

    @IBOutlet weak var bottomConstraint: NSLayoutConstraint!

    private let disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        // avoid keyboard.
        NotificationCenter.default.rx.notification(UIResponder.keyboardWillChangeFrameNotification)
            .bind(onNext: { [view, bottomConstraint] notification in
                adjust(view: view, notification: notification, bottomLayoutConstraint: bottomConstraint, buffer: 0)
            })
            .disposed(by: disposeBag)
    }
}

private func adjust(view: UIView?, notification: Notification, bottomLayoutConstraint: NSLayoutConstraint?, buffer: CGFloat) {
    guard let view = view else { return }
    guard let bottomLayoutConstraint = bottomLayoutConstraint else { return }
    guard let keyboardFrame = (notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue else { return }
    guard let duration = notification.userInfo?[UIResponder.keyboardAnimationDurationUserInfoKey] as? TimeInterval else { return }
    view.layoutIfNeeded()
    let window = UIApplication.shared.keyWindow
    let bottomPadding = window?.safeAreaInsets.bottom ?? 0
    UIView.animate(withDuration: duration, animations: {
        bottomLayoutConstraint.constant = UIScreen.main.bounds.height - keyboardFrame.minY + buffer - bottomPadding
        view.layoutIfNeeded()
    })
}