Mapping specific for diff mode and vimdiff

Question

My idea is that whenever &diff is true for a buffer, I would like some mappings to be active when that buffer is the current buffer.

The following attempt works for when I open a session in diff mode via vimdiff, and almost works when I do diffthis from a non-diff session; I say almost because running :diffthis does not set the mappings because the BufEnter event is not triggered, so I have to jump away from that buffer and back to it to see the mappings activated.

How can I have the autocmd be executed even when :diffthis is executed and, in general, everytime &diff is/becomes true in the window where the cursor is?

function! Toggle_diffopts(which)
  exe 'set diffopt'
        \ . ['+','-'][!!(index(split(&diffopt,','), a:which) + 1)]
        \ . '='
        \ . a:which
endfunction
function! SetDiffMappings()
  if &diff
    nnoremap <buffer> <silent> <Leader>diw :call Toggle_diffopts('iwhite')<CR>
    nnoremap <buffer> <silent> <Leader>dic :call Toggle_diffopts('icase')<CR>
    let b:diff_mappings = v:true
  else
    if exists('b:diff_mappings') && b:diff_mappings
      nunmap <buffer> <Leader>diw
      nunmap <buffer> <Leader>dic
      let b:diff_mappings = v:false
    endif
  endif
endfunction
augroup DiffMappings
  autocmd!
  autocmd BufEnter * :call SetDiffMappings()
augroup end

This does not answer my question.

Answer 1

You're doing it all wrong(tm). As mapping is buffer-local, while option is window-local (diff), you should create mapping once per buffer, and test the option value within the mapping (e.g. map-expression), not the other way round (but also try NOT to map dic/diw and other too common key combinations).

Next, your buffer-local mapping toggles global option (diffopt) - this is a very very bad design. Make sure you NEVER toggle global state due to buffer/window/tab switch. Always use global commands and mappings to change global settings.

On the contrary, creating plain global command has advantages. It's trivial to implement and easy to remember and to type due to auto-completion.

And finally, if you only need it from time to time then don't even bother writing a command. In real life typing :set dif<tab><right><right>...=<tab> is still much faster than musing about "if it's diw or what".