How can I cancel reading a file into a buffer on BufReadPre?

Question

In order to open PDFs using an external application, one can use the following script:

augroup nonvim
   au!
   au BufRead *.pdf sil exe "!open " . shellescape(expand("<afile>")) | bd  
augroup end

However, this is not optimal, because the pdf would still be loaded into vim, which will take a while if it is large. More importantly, your window layout will be messed up if you have more than one window showing, because of bd.

So, I thought of using BufReadPre instead of using BufRead. This way, I may still be able to cancel reading the file into a buffer. The question is: how?

Answer 1

A better option would be BufReadCmd:

                                                        BufReadCmd
BufReadCmd                      Before starting to edit a new buffer.  Should
                                read the file into the buffer. Cmd-event

Since this is supposed to read the file into the buffer, you can just not read it. :)

If you try to do a bd in a BufReadPre autocmd, you'll get an error:

E201: *ReadPre autocommands must not change current buffer

With BufReadCmd:

augroup nonvim
   au!
   au BufReadCmd *.pdf sil exe '!open' shellescape(expand("<afile>")) | bd
augroup end

I don't use a Mac, but replacing open with xdg-open, it worked fairly well enough.

Answer 2

Building on muru's answer, here is a solution that makes opening PDF files through vim seamless. The problem with :bd is that it messes up the window layout, but aside from that you may end up at a completely different buffer than where you opened the PDF from. This uses :b# and then deletes the empty buffer.

One last annoyance - that the buffer is centerd on re-entry - can be fixed with this script. (Use the original version or remove the check for atStartOfFile.)

augroup nonvim
   au!
   au BufReadCmd *.pdf sil exe "!xdg-open " . shellescape(expand("<afile>")) . "&>/dev/null" |
       \ let tobedeleted = bufnr('%') | b# | exe "bd! " . tobedeleted
augroup end

Works with NERDtree.