let keyboardNotifications = [
    UIKeyboardWillShowNotification,
    UIKeyboardWillHideNotification,
    UIKeyboardWillChangeFrameNotification
]

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

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

    keyboardNotifications.forEach {
        NSNotificationCenter.defaultCenter().addObserver(self, selector: "keyboardEventNotified:", name: $0, object: nil)
    }
}

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

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

func keyboardEventNotified(notification: NSNotification) {
    guard let userInfo = notification.userInfo else { return }
    let keyboardFrameEnd = (userInfo[UIKeyboardFrameEndUserInfoKey] as! NSValue).CGRectValue()
    let curve = UIViewAnimationCurve(rawValue: (userInfo[UIKeyboardAnimationCurveUserInfoKey] as! NSNumber).integerValue)!
    let options = UIViewAnimationOptions(rawValue: UInt(curve.rawValue << 16))
    let duration = NSTimeInterval(userInfo[UIKeyboardAnimationDurationUserInfoKey] as! NSNumber)
    let distance = UIScreen.mainScreen().bounds.height - keyboardFrameEnd.origin.y
    let bottom = distance >= bottomLayoutGuide.length ? distance : bottomLayoutGuide.length

    UIView.animateWithDuration(duration, delay: 0.0, options: [options], animations:
        { [weak self] () -> 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 s = self else { return }
        switch event.type {
        case .WillShow, .WillHide, .WillChangeFrame:
            let distance = UIScreen.mainScreen().bounds.height - event.keyboardFrameEnd.origin.y
            let bottom = distance >= s.bottomLayoutGuide.length ? distance : s.bottomLayoutGuide.length

            UIView.animateWithDuration(event.duration, delay: 0.0, options: [event.options], animations:
                { [weak self] () -> 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
    ...
}