There are two possible approaches.

One is to maintain an undo-stack and a redo-stack of action object that have
undo and redo methods. This usually results in very tedious internal
application structure/logic but is sometimes the only affordable way to do
this (in terms of resources/time required).

The simpler more brute force approach is simply maintaining an undo and a
redo stack of complete document (whatever the document may be) states. This
may involve lots of copying etc. but is far more simple to do. Hence the
numerous applications with limited undo-depths.

Silvio Bierman