toggle preview between selection and file preview

[rbusfield]

Original post was rather wide with 3 questions.
The final Answer is regarding ability to toggle preview. I will repost a new discussion with the fzf.vim question.

Editing the original post:

New to fzf.vim and vim plugins in general...
I have a config file (yml-like) that I edit in vim. One section of the config file takes items from a huge list of items (thousands).
Ideally, a custom vim command binding (:MyCmd) would run my list generator and feed list into fzf. The selected items should be inserted into the config file with preceding spaces.
foo
bar
baz
similar to a :r! cat list.txt | grep -e "foo" -e "ba*" | sed -i -e 's/^/ /'

(answered) Extra points for showing the current selection of items in a preview window.
(answered) Even more points for ability to toggle between custom item preview and current selection preview.

[LangLangBart]

Even more points for ability to toggle between custom item preview and current selection preview.

One potential solution involves defining two hotkeys to display the preferred preview. For instance,

seq 12 | fzf --multi --preview 'echo {+}' --preview-window 'nohidden' \
    --bind 'load:select-all' \
    --bind 'ctrl-t:change-preview:echo {+}' \
    --bind 'ctrl-g:change-preview:cat {+f}'

---

In the most recent fzf version¹, a transform action was introduced that enables the toggling
between two previews with a single hotkey².

seq 12 | fzf --multi --preview 'echo {+}' --preview-window 'nohidden' \
    --bind 'load:select-all' \
    --prompt 'First Preview > ' \
    --bind 'ctrl-t:transform:[[ ! {fzf:prompt} =~ First ]] &&
        echo "change-prompt(First Preview > )+change-preview:echo {+}" ||
        echo "change-prompt(Second Preview > )+change-preview:echo {}"'

Warning

Please be aware that there is currently a problem³ with the {+f} placeholder in the transform
action. And, the preview will not update during a focus⁴ shift (↑/↓).
The {} placeholder needs to be escaped in the above example.

       --bind 'ctrl-t:transform:[[ ! {fzf:prompt} =~ First ]] &&
-        echo "change-prompt(First Preview > )+change-preview:echo {+}" ||
-        echo "change-prompt(Second Preview > )+change-preview:echo {}"'
+        echo "change-prompt(First Preview > )+change-preview:echo \{+}" ||
+        echo "change-prompt(Second Preview > )+change-preview:echo \{}"'

Footnotes

1. CHANGELOG.md 0.45.0 ↩

2. ADVANCED.md Toggling with a single key binding ↩

3. Issue #3575 ↩

4. Issue #3579 ↩

[rbusfield]

HMMM. I can see how this works but think I am still stuck with the two Issues above.
Here is my testcase script fzf-transform.sh but still unsure of the escape for the preview action inside the transform.:

#!/bin/bash
# fzf testcase for Preview transform/toggle between Selected_Files and File_Preview
# Requirements:
#   1.1 One input list without reload
#   1.2 preview on current item (cat {})
#   1.3 toggle to preview of the selected list of items, one per line (echo {+f})
#   1.4 toggle between current item and selected set (CTRL-T)
#   1.5 retain selected item list between transforms
#   1.6 return seleted list as stdout

#create list
dir=`mktemp -d`
cat << EOF > $dir/list.txt
foo
bar
baz
EOF

# create files
echo "# file is foo" > $dir/foo
echo "# file is bar" > $dir/bar
echo "# file is baz" > $dir/baz

pushd $dir
cat list.txt | \
fzf --multi --prompt 'Items> ' \
  --header 'CTRL-T: Toggle Preview, [TAB]: multi-select' \
  --preview-label ' File_Preview ' \
  --bind 'ctrl-t:transform:[[ ! {fzf:preview-label} =~ Selected_Files ]] &&
      echo "change-preview-label(" Selected_Files ")+change-preview:echo \{+f}" ||
      echo "change-preview-label(" File_Preview ")+change-preview:cat \{}"' \
  --preview '[[ {fzf:preview-label} =~ File_Preview ]] && cat {} || echo {+}'

popd
rm -rf $dir

[LangLangBart]

As of fzf version 0.45.0, the {fzf:preview-label} is not a valid placeholder.

Placeholder	Description
{}	current line
{n}	current line number (zero index)
{q} /{fzf:query}	current query
{fzf:prompt}	current prompt
{fzf:action}	name of the last action performed

Placeholder expression flags	Description
+	space-separated list of the selected lines
f	temporary file
s	preserve whitespace

The {fzf:prompt} was added so that users could use it as a state¹, instead of creating an external file. There's also a discussion about changing the {fzf:prompt} to $FZF_PROMPT in the next version². If you prefer not to use the {fzf:prompt}, you might need to use an external file to manage your state, like so:

# File_Preview: state 0
# Selected_Files: state 1
echo 0 >/tmp/state
cat list.txt | fzf --multi --prompt 'Items> ' \
	--header 'CTRL-T: Toggle Preview, [TAB]: multi-select' \
	--preview-label ' File_Preview ' \
	--preview 'cat {}' \
	--bind 'ctrl-t:transform:[[ $(</tmp/state) -eq 0 ]] &&
		echo "execute-silent(echo 1 > /tmp/state)+change-preview-label( Selected_Files )+change-preview:cat \{+f}" ||
		echo "execute-silent(echo 0 > /tmp/state)+change-preview-label( File_Preview )+change-preview:cat \{}"'

Footnotes

1. Possibility to access the current prompt through a placeholder · Issue #3354 ↩

2. Expose states to the subprocesses via environment variables · Issue #3582 ↩

[rbusfield]

Changing the Subject line to reflect the solution for toggle preview