Vimscript: how to detect selection of a text object in visual mode?

Question

I'm writing what may become a plugin that needs to know what is currently visually selected.

To do it, I'm using the autocmd CursorMoved event to detect the creation of visual mode selections on the fly. So far, it works well enough, but for some reason if I make a visual selection using vip or vap this isn't picked up by the autocmd even though the cursor moves position.

Edit!

I think this is because in my code below I only check for mode() == 'v' (i.e. is visual mode character-wise) and vip could be a line-wise visual selection.

...continued OP

Other text objects (like vi], viw and the like) work fine as do regular motions (like v$ v}).

I wonder if perhaps it's a bug in Vim? If not, what is the correct way to detect that vip has been executed in visual mode? I can't find any other autocmd that does the job.

Here are the relevant sections of code if it helps:

function! Update_visual_mark_list()
    if mode() ==# 'v'
        " exit visual mode
        normal! v

        " get end points of last visual selection
        let [_, l1, c1, _] = getpos("'<")
        let [_, l2, c2, _] = getpos("'>")

        " do some stuff

        " go back to last visual selection
        normal! gv
    endif
endfunction

augroup visual_walk
    autocmd!
    autocmd CursorMoved * call Update_visual_mark_list()
augroup END

Answer 1

The "less-and-greater" marks are only guaranteed to be up-to-date after the visual selection finished and the mode has been switched.

If still in Visual mode then getpos('v') and getpos('.') must be used instead. Also to note that the selection could be expanded towards any direction, so it's not guaranteed which mark is the "left" and which is the "right" one. This is told under :h line() (that is also linked by :h getpos()).

UPD.

I think this is because in my code below I only check for mode() == 'v' (i.e. is visual mode character-wise) and vip could be a line-wise visual selection.

Yes, that's right. v_ip is always linewise.

I wonder if perhaps it's a bug in Vim? If not, what is the correct way to detect that vip has been executed in visual mode? I can't find any other autocmd that does the job.

In recent Vim there's :h ModeChanged event that allows to track all mode changes (including v to V). Perhaps, you can make use of this instead of CursorMoved?