让 UIButton 支持设置不同状态下背景颜色，支持 xib

BackgroundButton.swift

import UIKit

// https://spin.atomicobject.com/2018/04/25/uibutton-background-color/

// Declare a global var to produce a unique address as the assoc object handle
private var normalColorHandle: UInt8 = 0
private var disabledColorHandle: UInt8 = 0
private var highlightedColorHandle: UInt8 = 0
private var selectedColorHandle: UInt8 = 0

@IBDesignable
extension UIButton {
    // https://stackoverflow.com/questions/14523348/how-to-change-the-background-color-of-a-uibutton-while-its-highlighted
    private func image(withColor color: UIColor) -> UIImage? {
        let rect = CGRect(x: 0.0, y: 0.0, width: 1.0, height: 1.0)
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()

        context?.setFillColor(color.cgColor)
        context?.fill(rect)

        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return image
    }

    func setBackgroundColor(_ color: UIColor, for state: UIControl.State) {
        self.setBackgroundImage(image(withColor: color), for: state)
    }
    
    @IBInspectable
    var normalColor: UIColor? {
        get {
            if let color = objc_getAssociatedObject(self, &normalColorHandle) as? UIColor {
                return color
            }
            return nil
        }
        set {
            if let color = newValue {
                self.setBackgroundColor(color, for: .normal)
                objc_setAssociatedObject(self, &normalColorHandle, color, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            } else {
                self.setBackgroundImage(nil, for: .normal)
                objc_setAssociatedObject(self, &normalColorHandle, nil, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }

    @IBInspectable
    var disabledColor: UIColor? {
        get {
            if let color = objc_getAssociatedObject(self, &disabledColorHandle) as? UIColor {
                return color
            }
            return nil
        }
        set {
            if let color = newValue {
                self.setBackgroundColor(color, for: .disabled)
                objc_setAssociatedObject(self, &disabledColorHandle, color, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            } else {
                self.setBackgroundImage(nil, for: .disabled)
                objc_setAssociatedObject(self, &disabledColorHandle, nil, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }
    
    @IBInspectable
    var highlightedColor: UIColor? {
        get {
            if let color = objc_getAssociatedObject(self, &highlightedColorHandle) as? UIColor {
                return color
            }
            return nil
        }
        set {
            if let color = newValue {
                self.setBackgroundColor(color, for: .highlighted)
                objc_setAssociatedObject(self, &highlightedColorHandle, color, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            } else {
                self.setBackgroundImage(nil, for: .highlighted)
                objc_setAssociatedObject(self, &highlightedColorHandle, nil, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }
    
    @IBInspectable
    var selectedColor: UIColor? {
        get {
            if let color = objc_getAssociatedObject(self, &selectedColorHandle) as? UIColor {
                return color
            }
            return nil
        }
        set {
            if let color = newValue {
                self.setBackgroundColor(color, for: .selected)
                objc_setAssociatedObject(self, &selectedColorHandle, color, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            } else {
                self.setBackgroundImage(nil, for: .selected)
                objc_setAssociatedObject(self, &selectedColorHandle, nil, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }
}