UIButtonExtensions.swift

extension UIButton {
    private var normalBackgroundColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &NormalBackgroundColorKey) as? UIColor
        }
        
        set(newValue) {
            objc_setAssociatedObject(self,
                &NormalBackgroundColorKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
    
    @IBInspectable var highlightedBackgroundColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &HighlightedBackgroundColorKey) as? UIColor
        }
        
        set(newValue) {
            objc_setAssociatedObject(self,
                &HighlightedBackgroundColorKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
    
    @IBInspectable var disabledBackgroundColor: UIColor? {
        get {
            return objc_getAssociatedObject(self, &DisabledBackgroundColorKey) as? UIColor
        }
        
        set(newValue) {
            objc_setAssociatedObject(self,
                &DisabledBackgroundColorKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }
    
    override public var backgroundColor: UIColor? {
        didSet {
            if !enabled && !highlighted {
                normalBackgroundColor = backgroundColor
            }
        }
    }
    
    private func getDisabledBackgroundColor() -> UIColor? {
        return !enabled ? disabledBackgroundColor : nil
    }
    
    private func getHighlightedBackgroundColor() -> UIColor? {
        return highlighted ? highlightedBackgroundColor : nil
    }
    
    private func setColor() {
        backgroundColor = getDisabledBackgroundColor() ?? getHighlightedBackgroundColor() ?? normalBackgroundColor
    }
    
    override public var highlighted: Bool {
        didSet {
            setColor()
        }
    }
    
    override public var enabled: Bool {
        didSet {
            setColor()
        }
    }
}