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   1 @c This is part of the Emacs manual.
   2 @c Copyright (C) 2004-2016 Free Software Foundation, Inc.
   3 @c See file emacs.texi for copying conditions.
   4 @c
   5 @c This file is included either in emacs-xtra.texi (when producing the
   6 @c printed version) or in the main Emacs manual (for the on-line version).
   7 @node Autorevert
   8 @section Auto Reverting Non-File Buffers
   9
  10 Global Auto Revert Mode normally only reverts file buffers.  There are
  11 two ways to auto-revert certain non-file buffers: by enabling Auto
  12 Revert Mode in those buffers (using @kbd{M-x auto-revert-mode}); and
  13 by setting @code{global-auto-revert-non-file-buffers} to a
  14 non-@code{nil} value.  The latter enables Auto Reverting for all types
  15 of buffers for which it is implemented (listed in the menu below).
  16
  17 Like file buffers, non-file buffers should normally not revert while
  18 you are working on them, or while they contain information that might
  19 get lost after reverting.  Therefore, they do not revert if they are
  20 modified.  This can get tricky, because deciding when a non-file
  21 buffer should be marked modified is usually more difficult than for
  22 file buffers.
  23
  24 Another tricky detail is that, for efficiency reasons, Auto Revert
  25 often does not try to detect all possible changes in the buffer, only
  26 changes that are major or easy to detect.  Hence, enabling
  27 auto-reverting for a non-file buffer does not always guarantee that
  28 all information in the buffer is up-to-date, and does not necessarily
  29 make manual reverts useless.
  30
  31 At the other extreme, certain buffers automatically revert every
  32 @code{auto-revert-interval} seconds.  (This currently only applies to
  33 the Buffer Menu.)  In this case, Auto Revert does not print any
  34 messages while reverting, even when @code{auto-revert-verbose} is
  35 non-@code{nil}.
  36
  37 The details depend on the particular types of buffers and are
  38 explained in the corresponding sections.
  39
  40 @menu
  41 * Auto Reverting the Buffer Menu:: Auto Revert of the Buffer Menu.
  42 * Auto Reverting Dired::           Auto Revert of Dired buffers.
  43 * Supporting additional buffers::  How to add more Auto Revert support.
  44 @end menu
  45
  46 @node Auto Reverting the Buffer Menu
  47 @subsection Auto Reverting the Buffer Menu
  48
  49 If auto-reverting of non-file buffers is enabled, the Buffer Menu
  50 (@pxref{Several Buffers}) automatically reverts every
  51 @code{auto-revert-interval} seconds, whether there is a need for it or
  52 not.  (It would probably take longer to check whether there is a need
  53 than to actually revert.)
  54
  55 If the Buffer Menu inappropriately gets marked modified, just revert
  56 it manually using @kbd{g} and auto-reverting will resume.  However, if
  57 you marked certain buffers to get deleted or to be displayed, you have
  58 to be careful, because reverting erases all marks.  The fact that
  59 adding marks sets the buffer's modified flag prevents Auto Revert from
  60 automatically erasing the marks.
  61
  62 @node Auto Reverting Dired
  63 @subsection Auto Reverting Dired buffers
  64
  65 Auto-reverting Dired buffers currently works on GNU or Unix style
  66 operating systems.  It may not work satisfactorily on some other
  67 systems.
  68
  69 Dired buffers only auto-revert when the file list of the buffer's main
  70 directory changes (e.g., when a new file is added).  They do not
  71 auto-revert when information about a particular file changes
  72 (e.g., when the size changes) or when inserted subdirectories change.
  73 To be sure that @emph{all} listed information is up to date, you have
  74 to manually revert using @kbd{g}, @emph{even} if auto-reverting is
  75 enabled in the Dired buffer.  Sometimes, you might get the impression
  76 that modifying or saving files listed in the main directory actually
  77 does cause auto-reverting.  This is because making changes to a file,
  78 or saving it, very often causes changes in the directory itself; for
  79 instance, through backup files or auto-save files.  However, this is
  80 not guaranteed.
  81
  82 If the Dired buffer is marked modified and there are no changes you
  83 want to protect, then most of the time you can make auto-reverting
  84 resume by manually reverting the buffer using @kbd{g}.  There is one
  85 exception.  If you flag or mark files, you can safely revert the
  86 buffer.  This will not erase the flags or marks (unless the marked
  87 file has been deleted, of course).  However, the buffer will stay
  88 modified, even after reverting, and auto-reverting will not resume.
  89 This is because, if you flag or mark files, you may be working on the
  90 buffer and you might not want the buffer to change without warning.
  91 If you want auto-reverting to resume in the presence of marks and
  92 flags, mark the buffer non-modified using @kbd{M-~}.  However, adding,
  93 deleting or changing marks or flags will mark it modified again.
  94
  95 Remote Dired buffers are not auto-reverted (because it may be slow).
  96 Neither are Dired buffers for which you used shell wildcards or file
  97 arguments to list only some of the files.  @file{*Find*} and
  98 @file{*Locate*} buffers do not auto-revert either.
  99
 100 @c FIXME?  This should be in the elisp manual?
 101 @node Supporting additional buffers
 102 @subsection Adding Support for Auto-Reverting additional Buffers.
 103
 104 This section is intended for Elisp programmers who would like to add
 105 support for auto-reverting new types of buffers.
 106
 107 To support auto-reverting the buffer must first of all have a suitable
 108 @code{revert-buffer-function}.  @xref{Definition of
 109 revert-buffer-function,, Reverting, elisp, the Emacs Lisp Reference Manual}.
 110
 111 In addition, it must have a suitable @code{buffer-stale-function}.
 112
 113 @c FIXME only defvar in all of doc/emacs!
 114 @defvar buffer-stale-function
 115 The value of this variable is a function to check whether a
 116 buffer needs reverting.  This should be a function with one optional
 117 argument @var{noconfirm}.  The function should return non-@code{nil}
 118 if the buffer should be reverted.  The buffer is current when this
 119 function is called.
 120
 121 While this function is mainly intended for use in auto-reverting, it
 122 could be used for other purposes as well.  For instance, if
 123 auto-reverting is not enabled, it could be used to warn the user that
 124 the buffer needs reverting.  The idea behind the @var{noconfirm}
 125 argument is that it should be @code{t} if the buffer is going to be
 126 reverted without asking the user and @code{nil} if the function is
 127 just going to be used to warn the user that the buffer is out of date.
 128 In particular, for use in auto-reverting, @var{noconfirm} is @code{t}.
 129 If the function is only going to be used for auto-reverting, you can
 130 ignore the @var{noconfirm} argument.
 131
 132 If you just want to automatically auto-revert every
 133 @code{auto-revert-interval} seconds (like the Buffer Menu), use:
 134
 135 @example
 136 (setq-local buffer-stale-function
 137      #'(lambda (&optional noconfirm) 'fast))
 138 @end example
 139
 140 @noindent
 141 in the buffer's mode function.
 142
 143 The special return value @samp{fast} tells the caller that the need
 144 for reverting was not checked, but that reverting the buffer is fast.
 145 It also tells Auto Revert not to print any revert messages, even if
 146 @code{auto-revert-verbose} is non-@code{nil}.  This is important, as
 147 getting revert messages every @code{auto-revert-interval} seconds can
 148 be very annoying.  The information provided by this return value could
 149 also be useful if the function is consulted for purposes other than
 150 auto-reverting.
 151 @end defvar
 152
 153 Once the buffer has a suitable @code{revert-buffer-function} and
 154 @code{buffer-stale-function}, several problems usually remain.
 155
 156 The buffer will only auto-revert if it is marked unmodified.  Hence,
 157 you will have to make sure that various functions mark the buffer
 158 modified if and only if either the buffer contains information that
 159 might be lost by reverting, or there is reason to believe that the user
 160 might be inconvenienced by auto-reverting, because he is actively
 161 working on the buffer.  The user can always override this by manually
 162 adjusting the modified status of the buffer.  To support this, calling
 163 the @code{revert-buffer-function} on a buffer that is marked
 164 unmodified should always keep the buffer marked unmodified.
 165
 166 It is important to assure that point does not continuously jump around
 167 as a consequence of auto-reverting.  Of course, moving point might be
 168 inevitable if the buffer radically changes.
 169
 170 You should make sure that the @code{revert-buffer-function} does not
 171 print messages that unnecessarily duplicate Auto Revert's own messages,
 172 displayed if @code{auto-revert-verbose} is @code{t}, and effectively
 173 override a @code{nil} value for @code{auto-revert-verbose}.  Hence,
 174 adapting a mode for auto-reverting often involves getting rid of such
 175 messages.  This is especially important for buffers that automatically
 176 revert every @code{auto-revert-interval} seconds.
 177
 178 If the new auto-reverting is part of Emacs, you should mention it
 179 in the documentation string of @code{global-auto-revert-non-file-buffers}.
 180
 181 @ifinfo
 182 Similarly, you should add a node to this chapter's menu.  This node
 183 @end ifinfo
 184 @ifnotinfo
 185 Similarly, you should add a section to this chapter.  This section
 186 @end ifnotinfo
 187 should at the very least make clear whether enabling auto-reverting
 188 for the buffer reliably assures that all information in the buffer is
 189 completely up to date (or will be after @code{auto-revert-interval}
 190 seconds).