KeyboardChangeViewController.swift

import UIKit

public protocol KeyboardChangeDelegate {
    func doKeyboardAnimateTo(toValue: CGFloat)
}

open class KeyboardChangeViewController: UIViewController {

    override open func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

            NotificationCenter.default
                .addObserver(self,
                             selector: #selector(keyboardWillChange(_:)),
                             name: NSNotification.Name.UIKeyboardWillChangeFrame,
                             object: nil)
            NotificationCenter.default
                .addObserver(self,
                             selector: #selector(keyboardWillChange(_:)),
                             name: NSNotification.Name.UIKeyboardWillHide,
                             object: nil)
            NotificationCenter.default
                .addObserver(self,
                             selector: #selector(keyboardWillChange(_:)),
                             name: NSNotification.Name.UIKeyboardWillShow,
                             object: nil)
    }

    override open func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)

            NotificationCenter.default
                .removeObserver(self,
                                name: NSNotification.Name.UIKeyboardWillChangeFrame,
                                object: nil)
            NotificationCenter.default
                .removeObserver(self,
                                name: NSNotification.Name.UIKeyboardWillHide,
                                object: nil)
            NotificationCenter.default
                .removeObserver(self,
                                name: NSNotification.Name.UIKeyboardWillShow,
                                object: nil)
    }

    @objc private func keyboardWillChange(_ notification: NSNotification) {
        guard
            let delegate = self as? KeyboardChangeDelegate,
            let info = notification.userInfo,
            let kbFrame = (info[UIKeyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue else {
                return
        }

        let kb = view.convert(kbFrame, from: view.window!.screen.coordinateSpace)
        let fromBottom = max(0, view.frame.size.height - kb.origin.y)

        delegate.doKeyboardAnimateTo(toValue: fromBottom)
        view.layoutIfNeeded()
    }
}