How do I get vim to keep its undo history?

Question

If I'm editing two files with vim, changing to the other file ( :bnext, :bprev ) seems to drop the undo history from the open file - hitting the 'u' key reports "Already at oldest change".

For example:

1. vim testfile1 testfile2
2. add some stuff to testfile1
3. :w
4. :bn
5. :bp
6. u
7. eep! can't undo!

Is there any way to keep this history for non-visible buffers?

Answer 1

The newest version of vim (7.3) has persistent undo, so that you can make a change, close vim completely (even shutdown and restart), restart vim, and undo. In your .vimrc:

" tell it to use an undo file
set undofile
" set a directory to store the undo history
set undodir=/home/yourname/.vimundo/

Answer 2

Looks like this will do it:

:set hidden

(in .vimrc)

Answer 3

You can use Viewports.
"vim -o testfile1 testfile2" - open files in splitted window.
":sp filename" - split and open "filename".
":vsp filename" - vertical split and open "filename".
"Ctrl+w+arrow" - Change viewport.