id <nscoding，nscopying =“”>到NSString

Question

我有一些NSURL *url，我想获取它的GenerationId：

NSDictionary *dictionary = [url resourceValuesForKeys:@[NSURLGenerationIdentifierKey] error:&error];
id generatonIdKey = dictionary[NSURLGenerationIdentifierKey]; //Returned as an id <NSCopying, NSCoding, NSObject> (read-only)

现在我想通过将generatonIdKey转换为NSNumber或NSString来显示。

有可能吗？如果是，怎么办？