Is it possible to configure previewer option at the picker level?

[dmy-gh]

I'd like to toggle the git_diff cmd_modified while using git_status in order to preview only staged hunks diff.
Currently the preview displays the full diff and thus can't be used to validate what will be part of a commit.

It can probably be done by extending the git_diff previewer to provide the appropriate cmd_modified command and add an action to switch to a custom picker using this custom previewer but it seems a bit heavy to only tweak an option.

Is it currently possible?

[ibhagwan]

require("fzf-lua").setup({
  previewers = {
    my_git_diff = { <copy from default and modify> }
  },
})

Then run :FzfLua git_status previewer=my_git_diff

Defaults here:

fzf-lua/lua/fzf-lua/defaults.lua

Lines 179 to 190 in 9b84b53

	git_diff = {
	pager = M._preview_pager_fn,
	cmd_deleted = "git diff --color HEAD --",
	cmd_modified = "git diff --color HEAD",
	cmd_untracked = "git diff --color --no-index /dev/null",
	-- TODO: modify previewer code to accept table cmd
	-- cmd_deleted = { "git", "diff", "--color", "HEAD", "--" },
	-- cmd_modified = { "git", "diff", "--color", "HEAD" },
	-- cmd_untracked = { "git", "diff", "--color", "--no-index", "/dev/null" },
	_fn_git_icons = function() return M.globals.git.icons end,
	_ctor = previewers.fzf.git_diff,
	},

[dmy-gh]

I tried to copy the setup git_diff definition but I was missing the pager, _fn_git_icons and especially _ctor.
It works with it, thank you.

Is it possible to reload the previewer with a new cmd_modified value and not the whole picker?
Using an action to switch to the custom version reloads the picker and loses track of the current line position.

[ibhagwan]

I tried to copy the setup git_diff definition but I was missing the pager, _fn_git_icons and especially _ctor. It works with it, thank you.

Is it possible to reload the previewer with a new cmd_modified value and not the whole picker? Using an action to switch to the custom version reloads the picker and loses track of the current line position.

For that I think you’ll need to create your own previewer (I.e. copy the git_diff previewer inner code) and send a variable in the opts that will instruct the previewer to return a different command, this is the code you’re after:

fzf-lua/lua/fzf-lua/previewer/fzf.lua

Lines 287 to 383 in 9b84b53

	function Previewer.git_diff:new(o, opts)
	Previewer.git_diff.super.new(self, o, opts)
	self.cmd_deleted = path.git_cwd(o.cmd_deleted, opts)
	self.cmd_modified = path.git_cwd(o.cmd_modified, opts)
	self.cmd_untracked = path.git_cwd(o.cmd_untracked, opts)
	self.pager = opts.preview_pager == nil and o.pager or opts.preview_pager
	if type(self.pager) == "function" then
	self.pager = self.pager()
	end
	do
	-- populate the icon mappings
	local icons_overrides = o._fn_git_icons and o._fn_git_icons()
	self.git_icons = {}
	for _, i in ipairs({ "D", "M", "R", "A", "C", "T", "?" }) do
	self.git_icons[i] =
	icons_overrides and icons_overrides[i] and
	utils.lua_regex_escape(icons_overrides[i].icon) or i
	end
	end
	return self
	end
	function Previewer.git_diff:cmdline(o)
	o = o or {}
	local act = shell.raw_preview_action_cmd(function(items, fzf_lines, fzf_columns)
	if not items or utils.tbl_isempty(items) then
	utils.warn("shell error while running preview action.")
	return
	end
	local is_deleted = items[1]:match(self.git_icons["D"] .. utils.nbsp) ~= nil
	local is_modified = items[1]:match("[" ..
	self.git_icons["M"] ..
	self.git_icons["R"] ..
	self.git_icons["A"] ..
	self.git_icons["T"] ..
	"]" .. utils.nbsp) ~= nil
	local is_untracked = items[1]:match("[" ..
	self.git_icons["?"] ..
	self.git_icons["C"] ..
	"]" .. utils.nbsp) ~= nil
	local file = items[1]
	if file:match("%s%->%s") then
	-- for renames, we take only the last part (#864)
	file = file:match("%s%->%s(.*)$")
	end
	file = path.entry_to_file(file, self.opts)
	local cmd = nil
	if is_modified then
	cmd = self.cmd_modified
	elseif is_deleted then
	cmd = self.cmd_deleted
	elseif is_untracked then
	local stat = uv.fs_stat(file.path)
	if stat and stat.type == "directory" then
	cmd = utils._if_win({ "dir" }, { "ls", "-la" })
	else
	cmd = self.cmd_untracked
	end
	end
	if not cmd then return "" end
	if type(cmd) == "table" then return table.concat(cmd, " ") end
	local pager = ""
	if self.pager and #self.pager > 0 and
	vim.fn.executable(self.pager:match("[^%s]+")) == 1 then
	-- style 2: as we are unable to use %var% within a "cmd /c" without !var! expansion
	-- https://superuser.com/questions/223104/setting-and-using-variable-within-same-command-line-in-windows-cmd-ex
	pager = "| " .. utils._if_win_normalize_vars(self.pager, 2)
	end
	-- with default commands we add the filepath at the end.
	-- If the user configured a more complex command, e.g.:
	-- git_diff = {
	-- cmd_modified = "git diff --color HEAD %s | less -SEX"
	-- }
	-- we use ':format' directly on the user's command, see
	-- issue #392 for more info (limiting diff output width)
	local fname_escaped = libuv.shellescape(file.path)
	if cmd:match("[<{]file[}>]") then
	cmd = cmd:gsub("[<{]file[}>]", fname_escaped)
	elseif cmd:match("%%s") then
	cmd = cmd:format(fname_escaped)
	else
	cmd = string.format("%s %s", cmd, fname_escaped)
	end
	local env = {
	["LINES"] = fzf_lines,
	["COLUMNS"] = fzf_columns,
	["FZF_PREVIEW_LINES"] = fzf_lines,
	["FZF_PREVIEW_COLUMNS"] = fzf_columns,
	}
	local setenv = utils.shell_setenv_str(env)
	cmd = string.format("%s %s %s", table.concat(setenv, " "), cmd, pager)
	-- TODO: exlpore why passing env (which we btw don't need anymore)
	-- makes git-delta use a different syntax theme
	return { cmd = cmd, env = nil }
	end, "{}", self.opts.debug)
	return act
	end

You also might be able to hack it by saving a ref to the previewer and modifying self.cmd_modified which will be simpler but I haven’t tested such use case.

[dmy-gh]

I'll stick with your first simpler solution for the time being but may give a try to this one.
Thank you for your help and for this invaluable plugin 🙏