Swift Extension 能够给已有类添加计算型属性，但是仍然不能添加存储属性。Swift 中也可以使用 Objective-C runtime 的关联对象（Associated Objects）的方式添加属性，弥补这一痛点。
extension UIView {
// 嵌套结构体
private struct AssociatedKeys {
static var displayNameKey = "displayNameKey"
}
// String 类型
var displayName: String? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.displayNameKey) as? String
}
set {
objc_setAssociatedObject(self, &AssociatedKeys.displayNameKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)
}
}
}
// 关联闭包属性
typealias pushCompletionClosure = ()->()
extension UIViewController {
private struct AssociatedKeys {
static var pushCompletionKey = "pushCompletionKey"
}
var pushCompletion: pushCompletionClosure? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.pushCompletionKey) as? pushCompletionClosure
}
set {
objc_setAssociatedObject(self, &AssociatedKeys.pushCompletionKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_COPY_NONATOMIC)
}
}
}
