Change UIButton's background colour when highlighted

UIButton+StateBGColorV2.swift

//
//  UIButton+StateBGColor.swift
//  ButtonExtension
//
//  Created by Jerapong Nampetch on 7/19/2558 BE.
//  Copyright (c) 2558 Jerapong Nampetch. All rights reserved.
//

import UIKit

private var highlightBGColorKey = 0
private var selectedBGColorKey = 0
private var disableBGColorKey = 0

extension UIButton {
    @IBInspectable var highlightBGColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &highlightBGColorKey) as? UIColor
        }
        set {
            objc_setAssociatedObject(self, &highlightBGColorKey, newValue, UInt(OBJC_ASSOCIATION_RETAIN))
            self.setBackgroundImage(self.bgColorImage(backgroundColor!), forState: .Normal)
            self.setBackgroundImage(self.bgColorImage(newValue!), forState: .Highlighted)
        }
    }
    
    @IBInspectable var selectedBGColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &selectedBGColorKey) as? UIColor
        }
        set {
            objc_setAssociatedObject(self, &selectedBGColorKey, newValue, UInt(OBJC_ASSOCIATION_RETAIN))
            self.setBackgroundImage(self.bgColorImage(backgroundColor!), forState: .Normal)
            self.setBackgroundImage(self.bgColorImage(newValue!), forState: .Selected)
        }
    }
    
    @IBInspectable var disabledBGColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &disableBGColorKey) as? UIColor
        }
        set {
            objc_setAssociatedObject(self, &disableBGColorKey, newValue, UInt(OBJC_ASSOCIATION_RETAIN))
            self.setBackgroundImage(self.bgColorImage(backgroundColor!), forState: .Normal)
            self.setBackgroundImage(self.bgColorImage(newValue!), forState: .Disabled)
        }
    }
    
    // Create 1px image from quartz, inspired from 
    // http://stackoverflow.com/questions/9151379/is-it-possible-to-use-quartz-2d-to-make-a-uiimage-on-another-thread
    func bgColorImage(color: UIColor) -> UIImage {
        let size = CGSize(width: 1, height: 1)
        let opaque = false
        let scale: CGFloat = 0
        UIGraphicsBeginImageContextWithOptions(size, opaque, scale)
        color.setFill()
        UIRectFill(CGRect(origin: CGPoint.zeroPoint, size: size))
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return image
    }
}