How to write the contents of the current buffer back to file on each key press in Vim?

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 solution

The answers below helped to 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

These hacks are not needed any more. Vim can automatically write a file to disk whenever it is changed. Just add this to your $MYVIMRC:

autocmd TextChanged,TextChangedI <buffer> write

I believe you need Vim 7.4. In contrast to autosave=1, this will save your file as soon as you change it.