How to use associated object is swift

How to create a loading view in UIView using extension

import UIKit
import ObjectiveC

extension UIView {

    private struct AssociatedKeys {
        static var LoadingDescriptiveName = "loading_nsh_DescriptiveName"
        static var LoadingViewDescriptiveName = "loading_view_nsh_DescriptiveName"
    }

    private func updateLoadingView(shouldShow: Bool) {
        if shouldShow {
            loadingView.startAnimating()
            addSubview(loadingView)
            loadingView.isHidden = false

            if let superview = loadingView.superview {
                loadingView.backgroundColor = superview.backgroundColor
                loadingView.centerXAnchor.constraint(equalTo: superview.centerXAnchor).isActive = true
                loadingView.centerYAnchor.constraint(equalTo: superview.centerYAnchor).isActive = true

            }
        } else {
            UIView.transition(with: self,
                              duration: 0.35,
                              options: .transitionCrossDissolve,
                              animations: { [weak self] in
                                self?.loadingView.removeFromSuperview()
            }, completion: nil)
        }
    }

    @objc var loading: Bool {
        get {
            guard let isLoading = objc_getAssociatedObject(self, &AssociatedKeys.LoadingDescriptiveName) as? Bool else { return false }
            return isLoading
        }
        set {
            objc_setAssociatedObject(self,
                                     &AssociatedKeys.LoadingDescriptiveName,
                                     newValue,
                                     .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            updateLoadingView(shouldShow: newValue)
        }
    }


    private var loadingView: UIActivityIndicatorView {

        if let view = objc_getAssociatedObject(self, &AssociatedKeys.LoadingViewDescriptiveName) as? UIActivityIndicatorView {
            return view
        } else {
            let loadingView = UIActivityIndicatorView(activityIndicatorStyle: .gray)
            loadingView.hidesWhenStopped = true
            loadingView.clipsToBounds = true
            loadingView.stopAnimating()
            loadingView.translatesAutoresizingMaskIntoConstraints = false
            objc_setAssociatedObject(self,
                                     &AssociatedKeys.LoadingViewDescriptiveName,
                                     loadingView,
                                     .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            return loadingView
        }
    }
} 

source:

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