Someone once posted a patch to add a new event called GetChar to receive an event for every keypress. This trick is not as powerful and flexible as that, but it can be very useful for a plugin, and is supported in Vim 7.0 with no patches.

Often there are questions on how to capture every key press from a user. The answer is that you can't, unless you map all keys. But even if you map all keys, it is not flexible enough. Here is a trick with recursive <expr> maps and getchar() to have all keys pass through your function. You can do whatever you want with the keys, swallow them or pass them to Vim.

Here is a demo that shows how to use it in insert mode. What the function does is to double every key you press, except <Esc> and <C-C>, when it breaks the loop.

imap <buffer> <silent> <expr> <F12> Double("\<F12>")
function! Double(mymap)
  try
    let char = getchar()
  catch /^Vim:Interrupt$/
    let char = "\<Esc>"
  endtry
  "exec BPBreakIf(char == 32, 1)
  if char == '^\d\+$' || type(char) == 0
    let char = nr2char(char)
  endif " It is the ascii code.
  if char == "\<Esc>"
    return ''
  endif
  redraw
  return char.char."\<C-R>=Redraw()\<CR>".a:mymap
endfunction

function! Redraw()
  redraw
  return ''
endfunction

You can do almost anything that you can do normally in an insert mode, press <BS>, <C-U> etc.

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