How to create null node by XMLString Method

[kamishiroAlice]

1.I want to create a null node Like ，But when I call the method [body setValue:nil forKey:@"nullnode"]，The dictionary no have this key and value，So I can't create this node by call the [body XMLString]，How can I create the null node?

2.I wan‘t create CDATA node By "setvalue for key" method，But now I should call method like [header setValue:" forKey:@"user_id"]; ，But when I call the XMLString method ,the result string will like this : "<![CDATA[yezhichao]]> "，So Can we create a method to create the CDATA node?