Vim: write back my file on each key press

Question

I'd like vim to automatically write my file as often as possible. The ideal would be every keystroke.

I need to save regularly so that my background build process will see it. It's a makefile for a latex document, and I'd like the previewer to show me a nearly up-to-date document when I'm finished typing.

Eventual answer (the answers below helped make this)

" Choose your own statusline here
let g:pbstatusline="%F\ %y\ %l:%c\ %m"
set statusline=%F\ %y\ %l:%c\ %m

autocmd FileType tex setlocal autowriteall

" Save the file every 5 keypresses
autocmd FileType tex setlocal statusline=%!pb:WriteFileViaStatusLine()

" Save the file every time this event fires.
autocmd FileType tex :autocmd InsertLeave,CursorHold,CursorHoldI * call pb:WriteFileViaStatusLine("always")

" 1 optional param: "always" is only allowed value.
let s:writefilecounter = 0
function! pb:WriteFileViaStatusLine(...)
   if s:writefilecounter > 5 || (a:0 > 0 && a:1 == "always")
      if &buftype == ""
         write
      endif
      let s:writefilecounter = 0
   else
      let s:writefilecounter = s:writefilecounter + 1
   endif

   return g:pbstatusline
endfunction

Answer 1

There are CursorMoved and CursorMovedI autocmd events, but I don't think there's one that applies every single time you type in Insert mode.

You could also, were you so bold, rebind every single printable character in Insert mode to save and then type the character.

Answer 2

One hack is to use your status line:

function! WriteFile() 
  if &buftype == ""
    write
  endif
  return '%f %h%w%m%r%=%-14(%l,%c%V%) %(%P%)'
endfunction
setlocal statusline=%!WriteFile()
set laststatus=2

As long as the status line is visible, it's updated after each change to the file. When updated, the WriteFile() function is called, which writes the file (and returns my approximation at the default status line). With laststatus=2, the status line is shown even when only one window is open.

This will keep the current buffer saved after each change.