Swift给系统类写扩展新增属性

例如要给UIImageView扩展一个url属性

import UIKit

private var key: Void?
extension UIImageView {
    
    @IBInspectable var url: String? {
        get {
            return objc_getAssociatedObject(self, &key) as? String
        }
        set{
            objc_setAssociatedObject(self, &key,newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

运用：