(Progress): New node.

lispref/display.texi

@@ -16,6 +16,7 @@ that Emacs presents to the user.
 * Truncation::          Folding or wrapping long text lines.
 * The Echo Area::       Where messages are displayed.
 * Warnings::            Displaying warning messages for the user.
+* Progress::            Informing user about progress of a long operation.
 * Invisible Text::      Hiding part of the buffer text.
 * Selective Display::   Hiding part of the buffer text (the old way).
 * Overlay Arrow::       Display of an arrow to indicate position.
@@ -533,6 +534,104 @@ symbols.  If it matches the first few elements in a warning type, then
 that warning is not logged.
 @end defopt
 
+@node Progress
+@section Reporting Operation Progress
+@cindex progress reporting
+
+When an operation can take a while to finish, you should inform the
+user about the progress it makes.  This way the user can estimate
+remaining time and clearly see that Emacs is busy working, not hung.
+
+Functions listed in this section provide simple and efficient way of
+reporting operation progress.  Here is a working example that does
+nothing useful:
+
+@example
+(let ((progress-reporter
+       (make-progress-reporter "Collecting some mana for Emacs..."
+                               0  500)))
+  (dotimes (k 500)
+    (sit-for 0.01)
+    (progress-reporter-update progress-reporter k))
+  (progress-reporter-done progress-reporter))
+@end example
+
+@defun make-progress-reporter message min-value max-value &optional current-value min-change min-time
+This function creates a progress reporter---the object you will use as
+an argument for all other functions listed here.  The idea is to
+precompute as much data as possible to make progress reporting very
+fast.
+
+The @var{message} will be displayed in the echo area, followed by
+progress percentage.  @var{message} is treated as a simple string.  If
+you need it to depend on a filename, for instance, use @code{format}
+before calling this function.
+
+@var{min-value} and @var{max-value} arguments stand for starting and
+final states of your operation.  For instance, if you scan a buffer,
+they should be the results of @code{point-min} and @code{point-max}
+correspondingly.  It is required that @var{max-value} is greater than
+@var{min-value}.  If you create progress reporter when some part of
+the operation has already been completed, then specify
+@var{current-value} argument.  But normally you should omit it or set
+it to @code{nil}---it will default to @var{min-value} then.
+
+Remaining arguments control the rate of echo area updates.  Progress
+reporter will wait for at least @var{min-change} more percents of the
+operation to be completed before printing next message.
+@var{min-time} specifies the minimum time in seconds to pass between
+successive prints.  It can be fractional.  Depending on Emacs and
+system capabilities, progress reporter may or may not respect this
+last argument or do it with varying precision.  Default value for
+@var{min-change} is 1 (one percent), for @var{min-time}---0.2
+(seconds.)
+
+This function calls @code{progress-reporter-update}, so the first
+message is printed immediately.
+@end defun
+
+@defun progress-reporter-update reporter value
+This function does the main work of reporting progress of your
+operation.  It print the message of @var{reporter} followed by
+progress percentage determined by @var{value}.  If percentage is zero,
+then it is not printed at all.
+
+@var{reporter} must be the result of a call to
+@code{make-progress-reporter}.  @var{value} specifies the current
+state of your operation and must be between @var{min-value} and
+@var{max-value} (inclusive) as passed to
+@code{make-progress-reporter}.  For instance, if you scan a buffer,
+then @var{value} should be the result of a call to @code{point}.
+
+This function respects @var{min-change} and @var{min-time} as passed
+to @code{make-progress-reporter} and so does not output new messages
+on every invocation.  It is thus very fast and normally you should not
+try to reduce the number of calls to it: resulting overhead will most
+likely negate your effort.
+@end defun
+
+@defun progress-reporter-force-update reporter value &optional new-message
+This function is similar to @code{progress-reporter-update} except
+that it prints a message in the echo area unconditionally.
+
+The first two arguments have the same meaning as for
+@code{progress-reporter-update}.  Optional @var{new-message} allows
+you to change the message of the @var{reporter}.  Since this functions
+always updates the echo area, such a change will be immediately
+presented to the user.
+@end defun
+
+@defun progress-reporter-done reporter
+This function should be called when the operation is finished.  It
+prints the message of @var{reporter} followed by word ``done'' in the
+echo area.
+
+You should always call this function and not hope for
+@code{progress-reporter-update} to print ``100%.''  Firstly, it may
+never print it, there are many good reasons for this not to happen.
+Secondly, ``done'' is more explicit.
+@end defun
+
 @node Invisible Text
 @section Invisible Text