How to test if there is only default buffer open? - vim

Question

I'm writing a plugin that loads sessions only if no buffers have been open. So far I'm testing vs argc.
if (argc() != 0)
return
endif
This works fine if I pass in arguments from the command line: vim file1
However, the problem is that I have some scripts that wrap around vim:
function foo {
vim file1
}
$ foo
In the latter case, argc is empty and thus the above if condition fails.
How do you test if vim was invoked with only the default buffer open?

Answer 1

If you pass files to Vim, that will be reflected in argc (also when you wrap the Vim invocation). Rather, the corner case is launching Vim with an edit command, e.g. vim -c "edit foo". If you need to detect that, you need to check two things:
the current buffer is the default blank buffer
no other text buffers have been loaded
Here's a set of functions to implement that:
function! IsBlank( bufnr )
return (empty(bufname(a:bufnr)) &&
\ getbufvar(a:bufnr, '&modified') == 0 &&
\ empty(getbufvar(a:bufnr, '&buftype'))
\)
endfunction
function! ExistOtherBuffers( targetBufNr )
return ! empty(filter(range(1, bufnr('$')), 'buflisted(v:val) && v:val != a:targetBufNr'))
endfunction
function! IsEmptyVim()
let l:currentBufNr = bufnr('')
return IsBlank(l:currentBufNr) && ! ExistOtherBuffers(l:currentBufNr)
endfunction

Answer 2

The best I could come up with was to write a function. I thought bufnr('$') would do it but it lists that highest loaded buffer number. The bufnr('$') function returns 1 even though I have not opened a file (simply launched vim). Also buffers can be unloaded with :bw, which does not change what bufnr('$') returns.
Anyway this is the function:
function! NumBuffers()
let rc = 0
for idx in range(bufnr('$'))
if bufloaded(idx)
let rc += 1
endif
endfor
return rc
endfunction