feat: add pre and post formatter hooks

[itmecho]

This is a feature I've added to my local copy of conform.nvim which I wondered if you would be interested in adding! I would've created and issue instead but I thought I'd just do a PR instead as I had already made the changes locally.

What?

Auto command triggers for ConformPreFormatter and ConformPostFormatter which fire before and after each formatter that is called. This allows hooking into the format process for each formatter.

Why?

I've added this as the formatters I have to run for work can sometimes take a while and there doesn't seem to be any feedback as to which formatter is currently running.

Adding these hooks enables me to have the following auto commands:

vim.api.nvim_create_autocmd("User", {
  pattern = 'ConformPreFormatter',
  callback = function(event)
    print('running ' .. event.data.formatter.name)
  end
})

vim.api.nvim_create_autocmd("User", {
  pattern = 'ConformPostFormatter',
  callback = function(event)
    print(string.format('finished running %s: ok=%s', event.data.formatter.name, event.data.ok))
  end
})

which results in the following:

Screen.Recording.2025-01-14.at.13.02.18.mov

[stevearc]

I'm on board with the general idea; left a bit of feedback

[stevearc]

We should try to provide the guarantee that if ConformPreFormatter is called, then ConformPostFormatter will also be called. Right now there are some error paths that won't hit the second autocmd. I believe if we move it into this util.wrap_callback function it will always get called.

[itmecho]

I've moved this into the callback and changed the event data to return the error (code and message)

[stevearc]

The whole formatter contains a lot of fields that probably aren't very useful. Could we start with just passing the name and then add other data if it's needed?

[itmecho]

Good point! I've left the structure of data.formatter.name so it can be easily extended in the future

[gegoune]

To follow naming patterns of Vim/nvim I suggest changing event names to ConformFormatPre and ConformFormatPost.

See :h autocmd-events.

[itmecho]

I'm on board with the general idea; left a bit of feedback

Thanks for the feedback! I have actioned that and also gone with the rename suggested by @gegoune

[stevearc]

LGTM thanks for the PR!