let keyboardNotifications: [Notification.Name] = [
    .UIKeyboardWillShow,
    .UIKeyboardWillHide,
    .UIKeyboardWillChangeFrame,
]

override func viewDidLoad() {
    super.viewDidLoad()
}

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

    keyboardNotifications.forEach {
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardEventNotified:), name: $0, object: nil)
    }
}

override func viewWillDisappear(animated: Bool) {
    super.viewWillDisappear(animated)

    keyboardNotifications.forEach {
        NotificationCenter.default.removeObserver(self, name: $0, object: nil)
    }
}

@objc func keyboardEventNotified(notification: NSNotification) {
    guard let userInfo = notification.userInfo else { return }
    let keyboardFrameEnd = (userInfo[UIResponder.keyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
    let curve = UIView.AnimationCurve(rawValue: (userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as! NSNumber).intValue)!
    let options = UIView.AnimationOptions(rawValue: UInt(curve.rawValue << 16))
    let duration = TimeInterval(truncating: userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as! NSNumber)
    let bottom = keyboardFrameEnd.height - bottomLayoutGuide.length
    
    UIView.animate(withDuration: duration, delay: 0.0, options: [options], animations:
        { () -> Void in
            self.textView.contentInset.bottom = bottom
            self.textView.scrollIndicatorInsets.bottom = bottom
        } , completion: nil)
}

let keyboard = KeyboardObserver()

override func viewDidLoad() {
    super.viewDidLoad()

    keyboard.observe { [weak self] (event) -> Void in
        guard let self = self else { return }
        switch event.type {
        case .willShow, .willHide, .willChangeFrame:
            let keyboardFrameEnd = event.keyboardFrameEnd
            let bottom = keyboardFrameEnd.height - self.bottomLayoutGuide.length
            
            UIView.animate(withDuration: event.duration, delay: 0.0, options: [event.options], animations: { () -> Void in
                self.textView.contentInset.bottom = bottom
                self.textView.scrollIndicatorInsets.bottom = bottom
                }, completion: nil)
        default:
            break
        }
    }
}

public struct KeyboardEvent {
    public let type: KeyboardEventType
    public let keyboardFrameBegin: CGRect
    public let keyboardFrameEnd: CGRect
    public let curve: UIViewAnimationCurve
    public let duration: NSTimeInterval
    public var isLocal: Bool?

    public var options: UIViewAnimationOptions {
        return UIViewAnimationOptions(rawValue: UInt(curve.rawValue << 16))
    }
    ...

public enum KeyboardEventType {
    case willShow
    case didShow
    case willHide
    case didHide
    case willChangeFrame
    case didChangeFrame
    ...
}