iOS: Active view controller with a closure

ActiveViewControllerClosure.swift

//
// Moving keyboard for item
//
// Note: Comments with this label are to explain semantics of iOS
//       Do not use comments like these in your projects

import UIKit

class ViewController: UIViewController, UITextFieldDelegate {

    // email address entry
    // Note: Must create text field in storyboard, and connect to this IBOutlet
    @IBOutlet weak var emailText: UITextField!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.emailText.delegate = self
        
        // shifts the view up for space for the keyboard
        NotificationCenter.default.addObserver(forName: .UIKeyboardWillShow, object: nil, queue: nil) {
            notification in

            // find the size of the keyboard
            guard let keyboardSize = (notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue else {
                print("Error: Unable to get keyboard size")
                return
            }
            
            // find the active view
            guard let activeView = self.view.subviews.first(where: { $0.isFirstResponder }) else {
                print("Error: no view is active")
                return
            }

            // move the whole thing up if necessary
            if keyboardSize.contains(activeView.frame) {
                self.view.frame.origin.y = -keyboardSize.height
            }
        }
    
        // shifts the view up for space for the keyboard
        NotificationCenter.default.addObserver(forName: .UIKeyboardWillHide, object: nil, queue: nil) {
            notification in
            
            // move back to original position
            self.view.frame.origin.y = 0
        }
    }

    // Make the keyboard disappear once return is pressed
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        
        textField.resignFirstResponder()
        
        // Note: implements default return key behavior
        return true
    }
}