Swift Extension如何添加属性

报错Extensions must not contain stored properties
这个时候我们应该使用关联属性来实现想要的功能

public extension UIView {
    private struct AssociatedKey {
    static var clipType: CornerClipType = .None}var clipType: CornerClipType {
    get {
    return objc_getAssociatedObject(self, &AssociatedKey.clipType) as? CornerClipType ?? CornerClipType.None}set {
    objc_setAssociatedObject(self, &AssociatedKey.clipType, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)}}
}

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