Mapping snippet trigger in visual mode

Question

I'm using the UltiSnips plugin. When selecting text, you can press the trigger key to enter Insert mode, and the deleted text can then be used in a subsequent snippet. The snippet I'm using is as following:

snippet # "Put in brackets" i
\\${1:emph}{${VISUAL}$2}$0
endsnippet

So the way I use it is I select some text, press my trigger key (<TAB>), enter #, and press tab again. That works fine, but I want to map it a key (<C-S-E>), such that I can simply select the text and press that key to do the same thing.

I've tried different ways, but none of them work:
vnoremap <buffer> <C-S-E> #
vnoremap <buffer> <C-S-E> :call UltiSnips#ExpandSnippet()#(<C-O>):call UltiSnips#ExpandSnippet()

function! WrapBrack()
    call UltiSnips#ExpandSnippet()
    normal i#
    call UltiSnips#ExpandSnippet()
    return ''
endfunction

vnoremap <buffer><expr> <C-S-E> WrapBrack()

This last one says that it won't allow autocommand to write because it's not safe, and I don't want to overrule that.

I don't know what to try next, and I can't find helpful documentation. Also I'm not very good with VimScript.

Here's the plugin settings I have:

Plug 'sirver/ultisnips'
let g:UltiSnipsExpandTrigger = '<tab>'
let g:UltiSnipsJumpForwardTrigger = '<tab>'
let g:UltiSnipsJumpBackwardTrigger = '<s-tab>'
let g:UltiSnipsSnippetDirectories=["UltiSnips", "my_snippets"]

Answer 1

The function to call would be UltiSnips#SaveLastVisualSelection(), as can be seen here, or by inspecting the output of :xnoremap <tab>.

Modifying from your mapping, this would be

vnoremap <buffer> <C-S-E> :call UltiSnips#SaveLastVisualSelection()<CR>gvs#(<C-O>):call UltiSnips#ExpandSnippet()<CR>

(remember to add a <CR> after a function is :called)

Note that, from my testing, the snippet expansion only occurs when the cursor is placed one character after the snippet trigger (# in your case). This means that using and expanding in insert-normal mode may not work as expected (since the cursor would be on #, instead of after it). A more robust solution would be to use the expression register <C-R>=, that is

xnoremap <buffer> <C-S-E> :<C-U>call UltiSnips#SaveLastVisualSelection()<CR>gvs#<C-R>=UltiSnips#ExpandSnippet()<CR>