fzf.vim - How to create git branch command with git log preview?

I'm trying to create neovim fzf command FzfGBranches to use git branch -a as input, and use git log --oneline as fzf preview:

command! -bang -nargs=0 FzfGBranches
            \ call fzf#vim#grep(
            \ "git branch -a", 1,
            \ fzf#vim#with_preview({
            \   'options': [
            \       '--prompt', '*Branches> ',
            \       '--bind', 'ctrl-d:page-down,ctrl-u:page-up',
            \   ],
            \   'placeholder': "echo {} | rev | cut -d'*' -f1 | rev | git log --oneline --graph --date=short --color=always --pretty=\"format:%C(auto)%cd %h%d %s\"",
            \ }), <bang>0)

When call FzfGBranches command, it show up and seems good, but when I press up/down keys, the preview (on the right side) does not refresh. See below screenshot:

How should I specify the --preview option in fzf.vim?

Solution

tl;dr…

Simplify this with fzf#run
Make your git log command accept the branch selected from FZF

Deets

First off, placeholder is undocumented, so I didn't spend time down this path. Conceptually, this is possible, so there has to be a simpler way. Enter fzf#run. This is a thin wrapper around the actual FZF tool. It only requires a source, sink, and options. Most of the magic would be happening in your options string.

Secondly, your last segment in the placeholder — to show a log of the changes in the branch — will always show the log of the branch you're currently on.

git log
  \ --oneline
  \ --graph
  \ --date=short
  \ --color=always
  \ --pretty=\"format:%C(auto)%cd %h%d %s\"

What you want to do is to pipe the current branch highlighted in FZF through to your git log command. Something along the lines of

[alias]
  branch-name = rev-parse --abbrev-ref HEAD
  smart-log = !git log {1:-$(git branch-name)}

Now, we can pipe the branch name from stdin into the git command.

$ echo your-branch-name | git smart-log

With this working, we are ready to build a working command in Vim

command! -bang -nargs=0 FzfTBranches
            \ call fzf#run({
            \   'source': "git branch -a",
            \   'sink': 'git checkout',
            \   'down': '40%',
            \   'options': '--prompt="*Branches> " --bind="ctrl-d:page-down,ctrl-u:page-up" --preview="echo {} | sed \"s/\*//\" | sed \"s/^ *//;s/ *$//\" | xargs git smart-log"'
            \ })

Breaking down the --preview segments…

echo {} to report the current FZF selectoin
sed \"s/\*//\" to remove any * characters, if any. This is the case for the current branch
sed \"s/^ *//;s/ *$//\" to remove any whitespaces from the beginning and the end of the string gotten from echo {}
xargs git smart-log pipes in the result of the previous three as an argument into our smart-log