Emacs: dired-preview version 0.1.0

This is a simple and robust package to automatically preview in a side window the file at point in Dired buffers. Preview windows are closed when they are no longer relevant, while preview buffers are killed if they have not been used for other purposes beside previewing. The package provides several customisation options to control its behaviour.

Below are the release notes.

The dired-preview package was in a public testing phase from 2023-06-25 until today. In the meantime, lots of changes have been made in the interest of usability and robustness.

Global and buffer-local modes

The dired-preview-mode is a local minor mode, while dired-preview-global-mode is its global counterpart. Both only take effect in Dired buffers.

The idea for a global and a local mode is to empower the user to toggle the functionality on demand, such as when they are in a meeting and want to disable/enable previews in a given context.

During the development phase, I had made an error regarding the scope of what should be a local minor mode. Thanks to Christian Tietze for pointing it out: https://lists.sr.ht/~protesilaos/general-issues/%3Cm1zg4noej2.fsf%40christiantietze.de%3E.

Preview delay runs on an idle timer

Originally, previews would run on a timer that would block Emacs. Whereas we now arrange to only trigger a preview when Emacs is idle for a customisable amount of seconds. Refer to the user option dired-preview-delay.

Trigger a preview in the post-command phase

In the original design, previews were triggered by bespoke dired-preview commands that were remapped to n and p in Dired buffers. This had several downsides, namely, (i) the other motions would not pick up the trigger, (ii) we would have to remap all possible motions, and (iii) the code was needlessly complex.

Currently, we install a local hook in the post-command phase, which will trigger a preview if the previous command was a Dired motion. In future versions, we may expand the list of commands that we check for.

Thanks to Peter Prevos for reporting this in issue 1 on the GitHub mirror: https://github.com/protesilaos/dired-preview/issues/1.

Thanks to Christian Tietze and Ed Hamilton for discussing this topic with me on the mailing list: https://lists.sr.ht/~protesilaos/general-issues/%3Cm1zg4noej2.fsf%40christiantietze.de%3E. Commit ae93720 by Christian Tietze is based on this discussion, although the implementation details have since been redone.

During the development phase, I had made the mistake of checking the last-command, whereas I should be testing against the this-command. Thanks to Karthik Chikmagalur for pointing out my error: https://lists.sr.ht/~protesilaos/general-issues/%3C87sfab8ixn.fsf%40gmail.com%3E.

The placement of the preview window is customisable

We arrange to display previews in a side window. Due to the inherent complexity of the display-buffer function and its accoutrements, a user option is necessarily reserved for experienced users. To this end, we provide the dired-preview-display-action-alist-function. Refer to the dired-preview-display-action-alist-dwim function for the implementation details.

Thanks to Karthik Chikmagalur for making an initial suggestion about such a feature: https://lists.sr.ht/~protesilaos/general-issues/%3C87jzvp484n.fsf%40gmail.com%3E

Thanks to Bruno Boal for discussing the user option and concomitant function with me and for checking the relevant definitions. This was done via a private channel and the information is shared with permission.

Window placement and deletion is more robust

The idea of “preview” windows is that they are not ordinary windows that the user can interact with. As such, they are to be deleted when some non-preview mode of action is taken.

Testing for such cases was extensive and required lots of changes to the code base. Thanks to Bruno Boal for performing the tests with me, for brainstorming possible solutions, and for inspecting the implementation details. This was done via a private channel and the information is shared with permission.

We no longer try to preview irregular files

Before, dired-preview would attempt to produce a preview of named pipes and sockets. This was not intended and has since been addressed. Use ‘file-regular-p’ instead of ‘file-exists-p’

Thanks to Karthik Chikmagalur for bringing this matter to my attention and for recommending the use of file-regular-p instead of file-exists-p: https://lists.sr.ht/~protesilaos/general-issues/%3C87pm5cnpaf.fsf%40gmail.com%3E.

Preview buffers are killed up to a cumulative size threshold

In the original design, we were killing preview buffers all at once. This was wasteful because all the work we were doing in the background to, for example, fetch a large file was discarded even though the user could theoretically request another preview of it.

The current approach is to keep around the newer buffers in order to speed up potential requests for another preview. Older buffers are discarded starting from the oldest. The clearance of older buffers is done until we reach a cumulative size of what is specified as the value of the variable dired-preview--buffers-threshold.

Note that the symbol includes double dashes, meaning that it is intended for “private” (internal) purposes. I am mentioning it here, because this seems like a good candidate for a future user option, subject to further refinements.

Thanks to Bruno Boal for suggesting this idea and checking its implementation with me. This was done via a private channel and the information is shared with permission.