KeyboardObserverUsage.swift

final class ViewController: UIViewController {
    
    let keyboard = KeyboardObserver()
    weak var firstRespounder: UIView?

    override func viewDidLoad() {
        super.viewDidLoad()

        keyboard.handler = {
            [weak self] state in
            switch state {
            case .show(let rect):
                let y = self!.view.bounds.height - rect.height - self!.firstRespounder!.convert(self!.firstRespounder!.frame, to: self?.view!).maxY
                if y > 0 {
                    return
                }
                self?.view.transform = CGAffineTransform.identity.translatedBy(x: 0, y: y)
            default:
                self?.view.transform = .identity
            }
        }
    }
  
}