Undo redo is blank


Edition : 4th.
I type in the code in page 151 and 52, add two functions :insertObject… and removeObject … into RMDocument.h and RMDocument.m
but the undo, redo is still blank.
In page 152, it is said “These methods will be called automatically when the NSArrayController wishes to inset or remove Person objects.” I want to know why these two method will be automatically called? Because in practice, they are not been called.
Also, the book said “for example, when the Add Employee and Remove buttons sent it insert: and remove: message”. I think “it” is the employees in the RMDocument, but how can employees receive message from the button.

Many thanks.