Drop-in keyboard handler

HandlesKeyboard.swift

//
//  HandlesKeyboard.swift
//
//  Created by Scott Gardner on 1/14/19.
//  Copyright © 2019 Scott Gardner. All rights reserved.
//

import UIKit

protocol HandlesKeyboard where Self: UIViewController {
    
    var bottomConstraintForKeyboardHandling: NSLayoutConstraint! { get }
    func startHandlingKeyboardChanges()
    func stopHandlingKeyboardChanges()
}

extension HandlesKeyboard {
    
    /// Registers caller to start handling keyboard will show and will hide notifications
    /// - Warning: Caller *must* implement `stopHandlingKeyboardChanges()` to unregister, e.g., in `deinit`
    /// ```
    /// func stopHandlingKeyboardChanges() {
    ///     NotificationCenter.default.removeObserver(self)
    /// }
    ///
    /// deinit {
    ///     stopHandlingKeyboardChanges()
    /// }
    /// ```
    func startHandlingKeyboardChanges() {
        NotificationCenter.default.addObserver(forName: UIResponder.keyboardWillShowNotification, object: nil, queue: nil) { [weak self] in
            self?.updateBottomConstraint(notification: $0)
        }
        
        NotificationCenter.default.addObserver(forName: UIResponder.keyboardWillHideNotification, object: nil, queue: nil) { [weak self] in
            self?.updateBottomConstraint(notification: $0)
        }
    }
    
    // Intentionally not implemented to ensure caller unregisters
//    func stopHandlingKeyboardChanges() {
//        NotificationCenter.default.removeObserver(self)
//    }
    
    func updateBottomConstraint(notification: Notification) {
        guard let userInfo = notification.userInfo,
            let animationDuration = (userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as? NSNumber)?.doubleValue,
            let keyboardEndFrame = (userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue,
            let rawAnimationCurve = (userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as? NSNumber)?.uint32Value
            else { return }
        
        if notification.name == UIResponder.keyboardWillHideNotification {
            bottomConstraintForKeyboardHandling.constant = 0.0
        } else {
            bottomConstraintForKeyboardHandling.constant = view.bounds.maxY - view.convert(keyboardEndFrame, to: view.window).minY
        }
        
        let animationCurve = UIView.AnimationOptions(rawValue: UInt(rawAnimationCurve << 16))
        
        UIView.animate(withDuration: animationDuration, delay: 0.0, options: [.beginFromCurrentState, animationCurve], animations: { [weak self] in
            self?.view.layoutIfNeeded()
            }, completion: nil)
    }
}