iOS - UITextView + NSUndoManager

I'm trying to integrate the undo/redo features in a UITextView (I'm building a LateX editor) with no luck and to be honest I'm really confused about this argument. I don't understand the steps involved in those two operations, I mean I need two methods

• one to remove the last inserted text
• one to restore the text removed

One doubt is where I get the last inserted text? in other words where I have to register for the undo?

• in textViewDidChange I can get the whole text
• in textViewShouldChangeTextInRange I can get the last char inserted

I know that what I wrote wasn't the best explanation ever, But I hope someone here has faced the same problem in the past and can give me an hint. Basically ,to resume, I have to add the undo/redo features to a textview, possibly having two buttons linked with those actions.

Thanks in advance