Show UIActivityIndicatorView inside UIImageView

UIImageView+Extentions.swift

//
//  UIImageView+Extentions.swift
//  MealSpecials
//
//  Created by Aruna on 8/9/15.
//  Copyright (c) 2015 com.mealspecials. All rights reserved.
//

import UIKit
import ObjectiveC

private var activityIndicatorAssociationKey: UInt8 = 0
private var imageStoredAssociationKey: UInt8 = 0
private var isLoadingAssociationKey: UInt8 = 0

extension UIImageView {

    var activityIndicator: UIActivityIndicatorView {
        get {
            // swiftlint:disable:next line_length
            if let activityIndicator = objc_getAssociatedObject(self, &activityIndicatorAssociationKey) as? UIActivityIndicatorView {
                return activityIndicator
            }

            let activityIndicator: UIActivityIndicatorView = .init(style: .gray)

            activityIndicator.hidesWhenStopped = true
            activityIndicator.center = .init(x: self.frame.size.width / 2,
                                             y: self.frame.size.height / 2)
            activityIndicator.autoresizingMask = [.flexibleLeftMargin,
                                                  .flexibleRightMargin,
                                                  .flexibleTopMargin,
                                                  .flexibleBottomMargin]
            activityIndicator.isUserInteractionEnabled = false
            imageStored = self.image
            self.activityIndicator = activityIndicator
            addSubview(self.activityIndicator)

            return activityIndicator
        }
        set (newValue) {
            objc_setAssociatedObject(self,
                                     &activityIndicatorAssociationKey,
                                     newValue,
                                     .OBJC_ASSOCIATION_RETAIN)
        }
    }

    private var imageStored: UIImage? {
        get {
            return objc_getAssociatedObject(self, &imageStoredAssociationKey) as? UIImage
        }
        set (newValue) {
            objc_setAssociatedObject(self,
                                     &imageStoredAssociationKey,
                                     newValue,
                                     .OBJC_ASSOCIATION_RETAIN)
        }
    }

    var isLoading: Bool {
        get {
            return objc_getAssociatedObject(self, &isLoadingAssociationKey) as? Bool ?? false
        }
        set (newValue) {
            newValue ? showActivityIndicator() : hideActivityIndicator()
            objc_setAssociatedObject(self,
                                     &isLoadingAssociationKey,
                                     newValue,
                                     objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
    }

    private func showActivityIndicator() {
        OperationQueue.main.addOperation { [weak self] () -> Void in
            self?.activityIndicator.startAnimating()
            self?.image = nil
        }
    }

    private func hideActivityIndicator() {
        OperationQueue.main.addOperation { [weak self] () -> Void in
            self?.activityIndicator.stopAnimating()
            self?.image = self?.imageStored
        }
    }
}