Auto Reverting Non-File Buffers
===============================

Global Auto Revert Mode normally only reverts file buffers.  There are two ways to auto-revert certain non-file buffers: by enabling Auto Revert Mode in those buffers (using M-x auto-revert-mode); and by setting global-auto-revert-non-file-buffers to a non-nil value.  The latter enables Auto Reverting for all types of buffers for which it is implemented (listed in the menu below).

Like file buffers, non-file buffers should normally not revert while you are working on them, or while they contain information that might get lost after reverting.  Therefore, they do not revert if they are ``modified''.  This can get tricky, because deciding when a non-file buffer should be marked modified is usually more difficult than for file buffers.

Another tricky detail is that, for efficiency reasons, Auto Revert often does not try to detect all possible changes in the buffer, only changes that are ``major'' or easy to detect.  Hence, enabling auto-reverting for a non-file buffer does not always guarantee that all information in the buffer is up-to-date, and does not necessarily make manual reverts useless.

At the other extreme, certain buffers automatically revert every auto-revert-interval seconds.  (This currently only applies to the Buffer Menu.)  In this case, Auto Revert does not print any messages while reverting, even when auto-revert-verbose is non-nil.

The details depend on the particular types of buffers and are explained in the corresponding sections.

Auto Reverting the Buffer Menu
------------------------------

If auto-reverting of non-file buffers is enabled, the Buffer Menu automatically reverts every auto-revert-interval seconds, whether there is a need for it or not.  (It would probably take longer to check whether there is a need than to actually revert.)

If the Buffer Menu inappropriately gets marked modified, just revert it manually using g and auto-reverting will resume.  However, if you marked certain buffers to get deleted or to be displayed, you have to be careful, because reverting erases all marks.  The fact that adding marks sets the buffer's modified flag prevents Auto Revert from automatically erasing the marks.

Auto Reverting Dired buffers
----------------------------

Auto-reverting Dired buffers currently works on GNU or Unix style operating systems.  It may not work satisfactorily on some other systems.

Dired buffers only auto-revert when the file list of the buffer's main directory changes (e.g., when a new file is added).  They do not auto-revert when information about a particular file changes (e.g., when the size changes) or when inserted subdirectories change. To be sure that all listed information is up to date, you have to manually revert using g, even if auto-reverting is enabled in the Dired buffer.  Sometimes, you might get the impression that modifying or saving files listed in the main directory actually does cause auto-reverting.  This is because making changes to a file, or saving it, very often causes changes in the directory itself; for instance, through backup files or auto-save files.  However, this is not guaranteed.

If the Dired buffer is marked modified and there are no changes you want to protect, then most of the time you can make auto-reverting resume by manually reverting the buffer using g.  There is one exception.  If you flag or mark files, you can safely revert the buffer.  This will not erase the flags or marks (unless the marked file has been deleted, of course).  However, the buffer will stay modified, even after reverting, and auto-reverting will not resume. This is because, if you flag or mark files, you may be working on the buffer and you might not want the buffer to change without warning. If you want auto-reverting to resume in the presence of marks and flags, mark the buffer non-modified using M-~.  However, adding, deleting or changing marks or flags will mark it modified again.

Remote Dired buffers are not auto-reverted (because it may be slow). Neither are Dired buffers for which you used shell wildcards or file arguments to list only some of the files.  *Find* and *Locate* buffers do not auto-revert either.

Adding Support for Auto-Reverting additional Buffers.
-----------------------------------------------------

This section is intended for Elisp programmers who would like to add support for auto-reverting new types of buffers.

To support auto-reverting the buffer must first of all have a revert-buffer-function.  See Reverting (the Emacs Lisp Reference Manual).

In addition, it must have a buffer-stale-function.

 -- Variable: buffer-stale-function
     The value of this variable is a function to check whether a non-file buffer needs reverting.  This should be a function with one optional argument NOCONFIRM.  The function should return non-nil if the buffer should be reverted.  The buffer is current when this function is called.

     While this function is mainly intended for use in auto-reverting, it could be used for other purposes as well.  For instance, if auto-reverting is not enabled, it could be used to warn the user that the buffer needs reverting.  The idea behind the NOCONFIRM argument is that it should be t if the buffer is going to be reverted without asking the user and nil if the function is just going to be used to warn the user that the buffer is out of date. In particular, for use in auto-reverting, NOCONFIRM is t. If the function is only going to be used for auto-reverting, you can ignore the NOCONFIRM argument.

     If you just want to automatically auto-revert every auto-revert-interval seconds (like the Buffer Menu), use:

          (set (make-local-variable 'buffer-stale-function)
               #'(lambda (&optional noconfirm) 'fast))

     in the buffer's mode function.

     The special return value fast tells the caller that the need for reverting was not checked, but that reverting the buffer is fast. It also tells Auto Revert not to print any revert messages, even if auto-revert-verbose is non-nil.  This is important, as getting revert messages every auto-revert-interval seconds can be very annoying.  The information provided by this return value could also be useful if the function is consulted for purposes other than auto-reverting.

Once the buffer has a revert-buffer-function and a buffer-stale-function, several problems usually remain.

The buffer will only auto-revert if it is marked unmodified.  Hence, you will have to make sure that various functions mark the buffer modified if and only if either the buffer contains information that might be lost by reverting, or there is reason to believe that the user might be inconvenienced by auto-reverting, because he is actively working on the buffer.  The user can always override this by manually adjusting the modified status of the buffer.  To support this, calling the revert-buffer-function on a buffer that is marked unmodified should always keep the buffer marked unmodified.

It is important to assure that point does not continuously jump around as a consequence of auto-reverting.  Of course, moving point might be inevitable if the buffer radically changes.

You should make sure that the revert-buffer-function does not print messages that unnecessarily duplicate Auto Revert's own messages, displayed if auto-revert-verbose is t, and effectively override a nil value for auto-revert-verbose.  Hence, adapting a mode for auto-reverting often involves getting rid of such messages.  This is especially important for buffers that automatically revert every auto-revert-interval seconds.

If the new auto-reverting is part of Emacs, you should mention it in the documentation string of global-auto-revert-non-file-buffers.

Similarly, you should add a node to this chapter's menu.  This node should at the very least make clear whether enabling auto-reverting for the buffer reliably assures that all information in the buffer is completely up to date (or will be after auto-revert-interval seconds).