Protesilaos Stavrou

Change Log of Dired Preview

Automatically preview files in Dired

This document contains the release notes for each tagged commit on the project’s main git repository: https://github.com/protesilaos/dired-preview.

The newest release is at the top. For further details, please consult the manual: https://protesilaos.com/emacs/dired-preview.

1. Version 0.5.0 on 2025-04-09

This version contains a few bug fixes and minor refinements that should improve the behaviour of the package.

1.1. Revised how preview windows are deleted

In the past, dired-preview-mode could delete windows that held another buffer, thus undoing the window layout that was present before a preview buffer was displayed. Now dired-preview-mode makes sure to only delete windows that have not had another buffer shown in them, i.e. windows that were created just for preview purposes.

1.2. Directories no longer get misunderstood as “large files”

We tweaked how we test the type of the buffer-to-be-preview such that directories are not mistaken for “large files”. This was an issue for Mac computers that Sean Devlin brought to my attention in issue 27: https://github.com/protesilaos/dired-preview/issues/27.

1.3. The dired-preview-delay has a 0.1 second minimum to avoid instability

A value of 0 could lead to a noticeably degraded experience while navigating the Dired buffer. Thanks to Yiyu Zhou for reporting the matter in issue 2 on the GitLab mirror: https://gitlab.com/protesilaos/dired-preview/-/issues/2.

1.4. Files without an extension can also be ignored

The user option dired-preview-ignored-extensions-regexp will now also match files without an extension, such as the .DS_Store on Mac computers.

Thanks to Sean Devlin for the contribution in pull request 26: https://github.com/protesilaos/dired-preview/pull/26. The change is within the ~15-line limit, meaning that Sean does not need to assign copyright to the Free Software Foundation.

1.5. The preview buffer is the “other window” for scrolling purposes

The commands which scroll the other window, such as scroll-other-window now operate on the preview buffer when that is displayed. Thanks to Karthik Chikmagalur for proposing this in issue 24: https://github.com/protesilaos/dired-preview/issues/24.

This is in addition to the commands we already provided for scrolling the preview window, namely, dired-preview-page-up and dired-preview-page-down. Do M-x describe-keymap and then search for dired-preview-mode-map: it is in effect when dired-preview-mode is enabled.

2. Version 0.4.0 on 2025-02-18

This version contains several refinements and bug fixes.

2.1. Preview buffers have a mode line indicator

Preview buffers have a prefix to their name to make them stand out. This is controlled by the user option dired-preview-buffer-name-indicator, which is a string that defaults to [P].

2.2. Control how preview buffers are cleaned up

The way dired-preview works is to display a buffer and then keep a list of preview buffers to economise on redisplaying it again. This list of buffers is relevant for as long as we are in the Dired buffer, otherwise all buffers therein are killed (buffers that were alive before being previewed are not touched).

By default we delete from oldest to newest the accumulated buffers when they exceed 10 in total. Though users can modify this behaviour by editing the value of the new user option dired-preview-kill-buffers-method (its doc string explains the technicalities).

Thanks to artelse for discussing this with me in issue 20: https://github.com/protesilaos/dired-preview/issues/20.

2.3. The dired-preview-display-action-alist has a new optional function

The dired-preview-display-action-alist is the user option which controls where the preview window is displayed. Its value can either be the symbol of a function or a display-buffer-alist entry.

By default, we have a “do-what-I-mean” function that tries to find a good placement for the window. The new dired-preview-display-action-alist-below function has a straightforward behaviour: it always shows the preview below the current window and it always makes the preview window 0.3 times the height of the Emacs frame.

2.4. Encypted files are no longer previewed

This is to ensure that potentially sensitive contents are not displayed by accident, such as during a video call.

2.5. We no longer preview the “self” directory

We should not trigger a preview when the cursor is over the implicit . directory, as that causes a recursion that breaks things. Thanks to Inkbottle007 for reporting the bug in issue 23: https://github.com/protesilaos/dired-preview/issues/23.

2.6. Miscellaneous

  • Fixed a scenario where we would try to delete the last available window on the current frame. This should never happen. Thanks to artelse for reporting a relevant bug in the discussion of issue 22: https://github.com/protesilaos/dired-preview/issues/22.
  • Fixed a case when hexl-follow-ascii could fail to find an overlay under certain conditions. This did not create any noticeable problems, though having an error there would interfere with any workflow that would rely on toggle-debug-on-error.
  • The preview window will automatically be closed if the user switches outside the given Dired buffer. We now do not consider a change to the minibuffer as being “outside” this context. This way, a quick M-x to, say, enable a minor mode does not have any effect on the window layout.
  • Suppressed the messaging facility of the underlying tracking of preview buffers. Otherwise, Dired would notify us that the directory has changed whenever we would preview a new one, which is superfluous.
  • The body of the dired-preview-trigger function, which determines whether a preview will be displayed, is encapsulated in a condition-case. This helps capture errors and thus have a more predictable behaviour.
  • The dired-preview-display-action-alist has a more accurate declaration which allows for its correct customisation inside the Custom UI interface. In particular, it will behave the same way as the display-buffer-alist, where relevant.

3. Version 0.3.0 on 2024-09-16

This version brings some nice new features and makes small refinements to the code base.

3.1. Run commands in the preview window

The dired-preview-mode-map is active when dired-preview-mode is enabled. Here are all the keys and corresponding commands it provides right now:

Key binding Command
C-c C-c dired-preview-hexl-toggle
C-c C-d dired-preview-page-down
C-c C-f dired-preview-find-file
C-c C-o dired-preview-open-dwim
C-c C-u dired-preview-page-up

With the exception of dired-preview-hexl-toggle, these are all new commands.

My most used command among those is dired-preview-open-dwim, which has a Do-What-I-Mean behaviour: If the file name matches dired-preview-media-extensions-regexp, dired-preview-ignored-extensions-regexp, or dired-preview-image-extensions-regexp, then it opens it externally. Otherwise, it visits the file in an Emacs buffer. Note that here we include the dired-preview-image-extensions-regexp because while Emacs can visit those in a buffer, it does not offer as much functionality as other apps that specialise in handling image files.

[ Emacs uses the system default for those files when opening them externally. ]

3.2. Advanced users can rely on the dired-preview-with-window macro

Use this in your custom functions to run some code with the preview window as the selected window. For example, here is a simple one from our code base: 

(defun dired-preview-page-down ()
  "Move a page down in the preview window.
This technically runs `scroll-up-command'."
  (interactive)
  (dired-preview-with-window
    (call-interactively 'scroll-up-command)))

Remember to add them to the dired-preview-mode-map.

3.3. Placeholder window prevents preview jumpiness

Wtih dired-preview-ignored-extensions-regexp we can exclude certain files from being previewed. This is useful because, for example, Emacs cannot display those files or do something useful with their contents, or we just want to hide them (e.g. to omit GPG-encrypted files from the preview).

In the past, dired-preview-mode would pop up a preview window for a file that could be previewed and would then hide that window for anything matching dired-preview-ignored-extensions-regexp. As a result, windows would jump in and out of the frame layout. This could be disorienting.

We now provide the user option dired-preview-ignored-show-ignored-placeholders which shows a placeholder window when trying to preview an ignored file. So as we move up and down the Dired buffer, the preview window stays in place regardless if we are on an ignored file or not.

The default value of dired-preview-ignored-show-ignored-placeholders is t because I think this is the better behaviour. Though users can set it to nil to retain the old functionality.

Thanks to Álvaro Ramírez (xenodium) for the contribution. This was done in pull request 15: https://github.com/protesilaos/dired-preview/pull/15.

The change is within the permissible limit of ~15 lines, meaning that Álvaro does not need to assign copyright to the Free Software Foundation.

3.4. The dired-preview-display-action-alist replaces dired-preview-display-action-alist-function

The old symbol is an alias for the new one and users will be notified accordingly while updating.

This user option is for advanced users who want to customise where and how display-buffer places the preview window. Before, we were accepting only the symbol of a function that would return an appropriate action alist. Now we accept either a function’s symbol or an action alist directly.

Examples of both: 

;; Use a function that returns an action alist:
(setq dired-preview-display-action-alist #'dired-preview-display-action-alist-dwim)

;; Use an action alist directly:
(setq dired-preview-display-action-alist
      '((display-buffer-in-side-window)
        (side . bottom)
        (window-height . 0.2)
        (preserve-size . (t . t))))

Check our dired-preview-display-action-alist-dwim for inspiration if you want to have a function that returns an action alist based on, say, the width of the frame.

3.5. Preview of the next file works when marking for deletion

When we mark a file for deletion, Dired moves the point to the next line. Before, this would not trigger a preview of the updated file-at-point. Now it should work as expected.

Technically, we check if the last command is among those stored in the variable dired-preview-trigger-commands. If you think there are more commands we need to include there, just let me know.

3.6. The dired-preview-ignored-extensions-regexp can be nil

This was always the intention, but the relevant code was not accounting for that scenario. It should work now as intended.

4. Version 0.2.0 on 2024-05-07

This release brings many small improvements and fixes some bugs.

4.1. The preview shows up right away on new directories

In the past, entering a new directory would not trigger a preview: it would wait for some motion before doing so. This could make it seem that the mode was not enabled in the current buffer. Now, the preview shows up immediately.

Thanks to Nofint for the initial contribution in pull request 8: https://github.com/protesilaos/dired-preview/pull/8. The changes are small and do not require copyright assignment to the Free Software Foundation.

4.2. Directories are previewed as well

The contents are displayed in a Dired buffer, using the current settings for it. This means that colours are possible icons are shown too.

4.3. Large files are previewed in hexl-mode

As the documentation of this mode suggests:

A mode for editing binary files in hex dump format. This is not an ordinary major mode; it alters some aspects of the current mode’s behavior, but not all; also, you can exit Hexl mode and return to the previous mode using C-c C-c.

The C-c C-c key binding toggles the mode, in case you want to see the raw output.

Thanks to Karthik Chikmagalur for the contribution. This was done on the now defunct mailing list: https://lists.sr.ht/~protesilaos/general-issues/%3C871qeb56bw.fsf@gmail.com%3E.

4.4. Previews are displayed in a full-featured mode

Before, we would delay the execution of the mode hooks to speed things up. Though this came at the cost of (i) making the buffer less capable and (ii) forcing us to handle the execution of those hooks at a later stage when they would be needed, which proved to be error-prone in a number of scenaria.

4.5. Files without an extension are still previewed

This is a bug fix. Before, they would be considered as part of the files to be ignored, per dired-preview-ignored-extensions-regexp.

4.6. Previews are not added to what recentf-mode tracks

Thanks to Juergen Hoetzel for reporting this issue and for tweaking my suggested patch for it. This was done in pull request 12: https://github.com/protesilaos/dired-preview/pull/12. The change is small, so Juergen does not need to assign copyright to the Free Software Foundation.

4.7. A nil value for split-width-threshold still works

We no longer consider this an error and adapt the preview accordingly. Thanks to Juergen Hoetzel for the contribution. This was done in pull request 11 and does not require copyright assignment: https://github.com/protesilaos/dired-preview/pull/11.

4.8. For developers: previews are done with cl-defmethod

Each file type will thus have its own method on how to display the contents in a buffer. The idea is to make this easier to extend. The goal is to have methods that can preview PDFs and images without blocking Emacs. Other file types can be considered as well, though those two are the immediate priority.

5. Version 0.1.0 on 2023-07-08

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.

5.1. 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.

5.2. 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.

5.3. 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.

5.4. 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.

5.5. 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.

5.6. 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.

5.7. 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.