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39.4.2 Reporting Operation Progress

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. A convenient way to do this is to use a progress reporter.

Here is a working example that does nothing useful:

(let ((progress-reporter
       (make-progress-reporter "Collecting mana for Emacs..."
                               0  500)))
  (dotimes (k 500)
    (sit-for 0.01)
    (progress-reporter-update progress-reporter k))
  (progress-reporter-done progress-reporter))

function make-progress-reporter message \&optional min-value max-value current-value min-change min-time​

This function creates and returns a progress reporter object, which you will use as an argument for the other functions listed below. The idea is to precompute as much data as possible to make progress reporting very fast.

When this progress reporter is subsequently used, it will display message in the echo area, followed by progress percentage. message is treated as a simple string. If you need it to depend on a filename, for instance, use format-message before calling this function.

The arguments min-value and max-value should be numbers standing for the starting and final states of the operation. For instance, an operation that scans a buffer should set these to the results of point-min and point-max correspondingly. max-value should be greater than min-value.

Alternatively, you can set min-value and max-value to nil. In that case, the progress reporter does not report process percentages; it instead displays a “spinner" that rotates a notch each time you update the progress reporter.

If min-value and max-value are numbers, you can give the argument current-value a numerical value specifying the initial progress; if omitted, this defaults to min-value.

The remaining arguments control the rate of echo area updates. The progress reporter will wait for at least min-change more percents of the operation to be completed before printing next message; the default is one percent. min-time specifies the minimum time in seconds to pass between successive prints; the default is 0.2 seconds. (On some operating systems, the progress reporter may handle fractions of seconds with varying precision).

This function calls progress-reporter-update, so the first message is printed immediately.

function progress-reporter-update reporter \&optional value suffix​

This function does the main work of reporting progress of your operation. It displays the message of reporter, followed by progress percentage determined by value. If percentage is zero, or close enough according to the min-change and min-time arguments, then it is omitted from the output.

reporter must be the result of a call to make-progress-reporter. value specifies the current state of your operation and must be between min-value and max-value (inclusive) as passed to make-progress-reporter. For instance, if you scan a buffer, then value should be the result of a call to point.

Optional argument suffix is a string to be displayed after reporter’s main message and progress text. If reporter is a non-numerical reporter, then value should be nil, or a string to use instead of suffix.

This function respects min-change and min-time as passed to 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.

function progress-reporter-force-update reporter \&optional value new-message suffix​

This function is similar to progress-reporter-update except that it prints a message in the echo area unconditionally.

reporter, value, and suffix have the same meaning as for progress-reporter-update. Optional new-message allows you to change the message of the reporter. Since this function always updates the echo area, such a change will be immediately presented to the user.

function progress-reporter-done reporter​

This function should be called when the operation is finished. It prints the message of reporter followed by word ‘done’ in the echo area.

You should always call this function and not hope for 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.

macro dotimes-with-progress-reporter (var count [result]) reporter-or-message body…​

This is a convenience macro that works the same way as dotimes does, but also reports loop progress using the functions described above. It allows you to save some typing. The argument reporter-or-message can be either a string or a progress reporter object.

You can rewrite the example in the beginning of this subsection using this macro as follows:

(dotimes-with-progress-reporter
    (k 500)
    "Collecting some mana for Emacs..."
  (sit-for 0.01))

Using a reporter object as the reporter-or-message argument is useful if you want to specify the optional arguments in make-progress-reporter. For instance, you can write the previous example as follows:

(dotimes-with-progress-reporter
    (k 500)
    (make-progress-reporter "Collecting some mana for Emacs..." 0 500 0 1 1.5)
  (sit-for 0.01))

macro dolist-with-progress-reporter (var count [result]) reporter-or-message body…​

This is another convenience macro that works the same way as dolist does, but also reports loop progress using the functions described above. As in dotimes-with-progress-reporter, reporter-or-message can be a progress reporter or a string. You can rewrite the previous example with this macro as follows:

(dolist-with-progress-reporter
    (k (number-sequence 0 500))
    "Collecting some mana for Emacs..."
  (sit-for 0.01))