Handle iOS keyboard with autolayout

KeyboardObserver.swift

private final class KeyboardObserver {
    private static var key: UInt8 = 0
    
    let guide = UILayoutGuide()
    
    private var isAttached = false
    private let top: NSLayoutConstraint
    private let left: NSLayoutConstraint
    private let width: NSLayoutConstraint
    private let height: NSLayoutConstraint
    
    static func layoutGuide(for viewController: UIViewController) -> UILayoutGuide {
        if let observer = objc_getAssociatedObject(viewController, &KeyboardObserver.key) as? KeyboardObserver { return observer.guide }
        let observer = KeyboardObserver(in: viewController.view)
        objc_setAssociatedObject(viewController, &KeyboardObserver.key, observer, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        return observer.guide
    }
    
    private init(in view: UIView) {
        view.addLayoutGuide(guide)
        
        top = guide.topAnchor.constraint(equalTo: view.topAnchor)
        left = guide.leadingAnchor.constraint(equalTo: view.leadingAnchor)
        width = guide.widthAnchor.constraint(equalToConstant: 0)
        height = guide.heightAnchor.constraint(equalToConstant: 0)
        
        // default constraints to bottom
        NSLayoutConstraint.activate([
            guide.widthAnchor.constraint(equalTo: view.widthAnchor).priority(UILayoutPriorityDefaultLow),
            guide.heightAnchor.constraint(equalToConstant: 0).priority(UILayoutPriorityDefaultLow),
            guide.leadingAnchor.constraint(equalTo: view.leadingAnchor).priority(UILayoutPriorityDefaultLow),
            guide.topAnchor.constraint(equalTo: view.bottomAnchor).priority(UILayoutPriorityDefaultLow)
        ])
        
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(keyboardWillChange(notification:)),
                                               name: .UIKeyboardWillChangeFrame,
                                               object: nil)
    }
    
    @objc private func keyboardWillChange(notification: NSNotification) {
        guard let view = guide.owningView else { return }
        
        let beginFrame = (notification.userInfo![UIKeyboardFrameBeginUserInfoKey] as! NSValue).cgRectValue
        let endFrame = (notification.userInfo![UIKeyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
        let animationDuration = notification.userInfo![UIKeyboardAnimationDurationUserInfoKey] as! Double
        let animationCurve = notification.userInfo![UIKeyboardAnimationCurveUserInfoKey] as! UInt
        
        let fromFrameInView = view.convert(beginFrame, from: nil)
        let toFrameInView = view.convert(endFrame, from: nil)
        
        update(fromFrameInView)
        UIView.animate(withDuration: animationDuration,
                       delay: 0,
                       options: UIViewAnimationOptions(rawValue: animationCurve),
                       animations: {
                        self.update(toFrameInView)
        }, completion: nil)
    }
    
    private func update(_ rect: CGRect) {
        top.constant = rect.origin.y
        left.constant = rect.origin.x
        width.constant = rect.size.width
        height.constant = rect.size.height
        
        if !isAttached {
            isAttached = true
            NSLayoutConstraint.activate([ top, left, width, height ])
        }
        
        guide.owningView?.layoutIfNeeded()
    }
}