How do I trigger an autocommand when user enters/leaves command mode

Question

I'm using a really clever plugin that changes the line number style (relative/normal) contextually. Basically when you enter insert mode you have normal line numbers and when you leave insert mode it switches to relative numbers. A missing feature is to have normal line numbers when your in command-line mode.

Looking at how it is implemented it uses Vim's events system to trigger the change. I've looked but Vim's documentation seems to only offer events on insert enter/leave. (Funny aside: UserGettingBored).

At first I thought of adding nnoremap : :call MyFunc()<CR>: which works on enter but not on exit. Also relying on a map seems flaky.

How do I trigger a function when the user enters command-line mode (presses :)? And how do I trigger a function when the user leaves this mode (presses enter, ctrlc or esc)?

Answer 1

You can use mappings to override :, /, and ? to do some action first:

nnoremap : :set number<CR>:
nnoremap / :set number<CR>/
nnoremap ? :set number<CR>?

Because we use non-recursive mappings, the : at the end will call the original built-in :.

To hide the numbers again, you can use command-line mappings to override <CR>, <Esc>, and <C-c>:

cnoremap <silent> <CR> <CR>:set nonumber<CR>
cnoremap <silent> <Esc> <Esc>:set nonumber<CR>
cnoremap <silent> <C-c> <C-c>:set nonumber<CR>

This same trick should also work with the commands from your plugin.

Answer 2

As of version 8.0.1206, Vim supports this with CmdlineEnter and CmdlineLeave.

Answer 3

I haven't tried it for your situation, but likely you're looking for 'doautocmd'. Here's the relevant info from :h doauto

                    *:do* *:doau* *:doautocmd* *E217*
:do[autocmd] [group] {event} [fname]
            Apply the autocommands matching [fname] (default:
            current file name) for {event} to the current buffer.
            You can use this when the current file name does not
            match the right pattern, after changing settings, or
            to execute autocommands for a certain event.
            It's possible to use this inside an autocommand too,
            so you can base the autocommands for one extension on
            another extension.  Example:
                :au Bufenter *.cpp so ~/.vimrc_cpp
                :au Bufenter *.cpp doau BufEnter x.c
            Be careful to avoid endless loops.  See
            |autocmd-nested|.

            When the [group] argument is not given, Vim executes
            the autocommands for all groups.  When the [group]
            argument is included, Vim executes only the matching
            autocommands for that group.  Note: if you use an
            undefined group name, Vim gives you an error message.

            After applying the autocommands the modelines are
            processed, so that their settings overrule the
            settings from autocommands, like what happens when
            editing a file.