Specifying filetype in ModeChanged event autocmd

Question

I'm using stylua as the code formatter for lua files. I do not usually get used to its formatting style but sometimes I do need a little help with it, so I decided to make it the formatprg during the visual modes, to format the selected code piece, and clear it for the other modes. Here's what I have currently (yes, I write comma-first lists):

-- after/ftplugin/lua.lua

local enable_stylua_formatprg_on_vV_mode_only_group
    = vim.api.nvim_create_augroup('enable_stylua_formatprg_on_vV_mode_only', {})

-- to v/V mode: enable
vim.api.nvim_create_autocmd('ModeChanged', { callback = function () vim.bo.formatprg = 'stylua --search-parent-directories -' end
                                           , pattern = '*:[vV]'
                                           , group = enable_stylua_formatprg_on_vV_mode_only_group
                                           })
-- from v/V mode: disable
vim.api.nvim_create_autocmd('ModeChanged', { callback = function () vim.bo.formatprg = '' end
                                           , pattern = '[vV]:*'
                                           , group =  enable_stylua_formatprg_on_vV_mode_only_group
                                           })

It was after some time that I came to realise that these autocmds are not limited to functioning in lua files only. I immediately thought about adding a *.lua pattern, as what you would do for BufRead, BufLeave, etc., but it seems that's not what you can do here since ModeChanged event uses some old_mode:new_mode pattern, leaving the only thing to do is to add a filetype checker within the callback function.

I wonder if my understanding is correct, or otherwise any better alternatives are available. Code suggestions are welcome.

Answer 1

Embarrassingly, it seems that *.lua is indeed acceptable by ModeChanged. So the following would work:

-- to v/V mode: enable
vim.api.nvim_create_autocmd('ModeChanged', { callback = function () vim.bo.formatprg = 'stylua --search-parent-directories -' end
                                           , pattern = { '*:[vV]', '*.lua' }
                                           , group = enable_stylua_formatprg_in_vV_mode_only_group
                                           })
-- from v/V mode: disable
vim.api.nvim_create_autocmd('ModeChanged', { callback = function () vim.bo.formatprg = '' end
                                           , pattern = { '[vV]:*', '*.lua' }
                                           , group =  enable_stylua_formatprg_in_vV_mode_only_group
                                           })