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#### 18.5.3 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 suitable `revert-buffer-function`. See Reverting in the Emacs Lisp Reference Manual.

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

Variable: buffer-stale-function

  The value of this variable is a function to check whether a 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reverted without asking the user and `nil` if the function is just going to be used to warn the user that the buffer is out of date.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:

      (setq-local 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 suitable `revert-buffer-function` and `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 section to this chapter. This section 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).

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