Scrollable previews

[joelim-work]

There has been a number of requests from users to implement preview scrolling:

• Scroll preview output #568
• Scrolling preview #610
• Scroll preview #925
• preview column how can scroll #1014

Given that the contents of directories can be scrolled, it is understandable why users would request the same for (text) files. However, the reality is that while scrolling is implemented for directories, the same implementation does not work for files. In lf, the entire directory tree is read once and stored in memory, and it is simple enough to implement scrolling (and other commands like top and bottom). The same approach can't be used for files though - if the user selects a file that is several GB in size, then storing the entire contents of the file just for previewing can cause out-of-memory issues as RAM is a limited resource.

That being said, it is not impossible to implement scrolling for file contents. After all, pagers and text editors exist which can handle files that are several GB in size. But then the question is "why?" - if there are already other programs that can scroll previews, why does lf need to include it as a feature as well? From the very start, lf has been designed to be a minimalistic application, with pager-like features listed as a non-feature, see #610 (comment). This means writing code such that only core functionality for file management is provided, while maintaining the flexibility to use additional programs for other less-important features, as opposed to writing a monolithic program where lf can do everything by itself.

---

Despite all of this, after some experimenting I have managed to find a way (albeit somewhat hacky) to implement preview scrolling entirely from user configuration. I believe that this was always the intended approach, and that a built-in implementation for scrolling was never intended when the file previews was added as a feature.

lfrc:

# replace with the path to the actual previewer script
set previewer /path/to/previewer.sh

# for debugging purposes only
set rulerfmt 'lf_user_preview_offset = %{lf_user_preview_offset}'

# scroll down (increase the preview offset)
# `set preview true` is a hack used to reload the preview
map <a-j> &{{
    lf -remote "send $id :set user_preview_offset $((lf_user_preview_offset + 1)); set preview true"
}}

# scroll up (decrease the preview offset)
# ensure that the offset does not go below the value of 1
map <a-k> &{{
    if [ "$lf_user_preview_offset" -gt 1 ]; then
        lf -remote "send $id :set user_preview_offset $((lf_user_preview_offset - 1)); set preview true"
    fi
}}

# reset the preview offset when changing to a different file
cmd on-select set user_preview_offset 1

previewer.sh:

#!/bin/sh

# used for text files only, but theoretically the preview offset value is just
# an abstract integer value, and can be used for other types of files

# display starting from the line corresponding to the preview offset value
tail -n "+$lf_user_preview_offset" "$1"

# do not cache preview output
exit 1

The way it works is by storing the preview offset as user-defined variable, and then using it in the previewer script. The output is never cached, so that the previewer script will be invoked again when the offset is changed. This is just a proof-of-concept implementation, just for working with text files. However, in theory this approach could also be used for scrolling other types of files, such as pages of PDFs, or frames of a video.

[joelim-work]

This is now added as a tip in the wiki.

[joelim-work]

For PDF files I can see you're using pdftoppm, you just need to use the -f option to specify the page number. You'll also want to make sure that the value of preview offset is taken into account when determining the path of the cached thumbnail, otherwise the same cached thumbnail will be shown regardless of which page you want.

[og900aero]

I think found a solution:

CACHE="$HOME/.cache/lf/thumbnail.$(stat --printf '%n\0%i\0%F\0%s\0%W\0%Y' -- "$(readlink -f "$1")" | sha256sum | awk '{print $1}')_p${lf_user_preview_offset:-1}"

 */pdf)
    	[ ! -f "${CACHE}.jpg" ] && pdftoppm -jpeg -f "${lf_user_preview_offset:-1}" -singlefile "$1" "${CACHE%.jpg}" >/dev/null 2>&1
    	chafa -f sixel -s "$2x$3" --animate off "${CACHE}.jpg"
    ;;
esac
exit 1

I have one problem. It doesn't notice that I'm on the last page, and keeps scrolling through as many blank pages as I press. When I have time, I'll add the pdfinfo command to get the last page number.

[og900aero]

I tried to get it to look at the last page number of the pdf file, but it's not right.
I ran into 2 problems:

• When I reach the last page and if I keep pressing through, although it shows the last page, the lf_user_preview_offset variable keeps increasing, so if I want to go back to the before the last page, I have to press back as many times as I pressed past the last page
• For each overrun, it caches the last page over and over again, so it can be up to 10 times in the cache folder

CACHE="$HOME/.cache/lf/thumbnail.$(stat --printf '%n\0%i\0%F\0%s\0%W\0%Y' -- "$(readlink -f "$1")" | sha256sum | awk '{print $1}')_p${lf_user_preview_offset:-1}"
total_pages=$(pdfinfo "$1" 2>/dev/null | awk '/^Pages:/ {print $2}')

    */pdf)
	if [ ! -f "${CACHE}.jpg" ] && [ "${lf_user_preview_offset:-1}" -gt "$total_pages" ]; then
	    pdftoppm -jpeg -f "$total_pages" -singlefile "$1" "${CACHE%.jpg}" >/dev/null 2>&1
	else
		pdftoppm -jpeg -f "${lf_user_preview_offset:-1}" -singlefile "$1" "${CACHE%.jpg}" >/dev/null 2>&1
	fi
    	chafa -f sixel -s "$2x$3" --animate off "${CACHE}.jpg"
    ;;
esac
exit 1

[joelim-work]

You can use lf -remote in the previewer script to set the value of the preview offset to the value of total_pages, effectively clamping it to a maximum value.

[og900aero]

Somehow it doesn't seem to work. Although it already works to set the offset to the last page, if I want to move on from the last page, it still shows the last page, but caches the last page over and over again, so it is cached in multiple copies, taking up extra space.

[joelim-work]

At this stage I should point out that the ability to scroll previews should be viewed as a way of quickly verifying the contents of a file. It is not intended to be a replacement for a file viewer - that functionality is still better suited for more dedicated programs like less/vim for text files, or zathura for PDF files.

When I first looked into this issue, I thought that it would be nice to somehow configure keybindings like <a-j>/<a-k> to scroll the preview content. Then later on I realized that it wasn't any more efficient (at least for me) than just pressing v (replace with any suitable key) to open the file in more suitable program and directly scroll from there. In other words, I found that pressing vj represented a more natural and modular workflow compared to <a-j> - it was just as fast, and I could also do other things like v/foo to open and search for foo.