Channel

Nvim asynchronous IO

1. Introduction channel-intro

Channels are nvim's way of communicating with external processes.

There are several ways to open a channel:

1. Through stdin/stdout when nvim is started with --headless, and a startup script or --cmd command opens the stdio channel using stdioopen().

2. Through stdin, stdout and stderr of a process spawned by jobstart().

3. Through the PTY master end of a PTY opened with nvim_open_term() or jobstart(…, {'pty': v:true}).

4. By connecting to a TCP/IP socket or named pipe with sockconnect().

5. By another process connecting to a socket listened to by nvim. This only supports RPC channels, see rpc-connecting.

Channels support multiple modes or protocols. In the most basic mode of operation, raw bytes are read and written to the channel. The RPC protocol, based on the msgpack-rpc standard, enables nvim and the process at the other end to send remote calls and events to each other. The builtin terminal-emulator is also implemented on top of PTY channels.

Channel Id channel-id

Each channel is identified by an integer id, unique for the life of the current Nvim session. Functions like stdioopen() return channel ids; functions like chansend() consume channel ids.

2. Reading and writing raw bytes channel-bytes

Channels opened by Vimscript functions operate with raw bytes by default. For a job channel using RPC, bytes can still be read over its stderr. Similarly, only bytes can be written to Nvim's own stderr.

channel-callback

Scripts can react to channel activity (received data) via callback functions assigned to the on_stdout, on_stderr, on_stdin, or on_data option keys. Callbacks should be fast: avoid potentially slow/expensive work.

channel-buffered
The callback is invoked immediately as data is available, where a single-item list [''] indicates EOF (stream closed). Alternatively set the stdout_buffered, stderr_buffered, stdin_buffered, or data_buffered option keys to invoke the callback only after all output was gathered and the stream was closed. E5210
If a buffering mode is used without a callback, the data is saved in the stream {name} key of the options dict. It is an error if the key exists.

channel-lines
Stream event handlers receive data as it becomes available from the OS, thus the first and last items in the {data} list may be partial lines. Empty string completes the previous partial line. Examples (not including the final [''] emitted at EOF):

There are two ways to deal with this:

If the callback functions are Dictionary-functions, self refers to the options dictionary containing the callbacks. Partials can also be used as callbacks.

Data can be sent to the channel using the chansend() function. Here is a simple example, echoing some data through a cat-process:

function! s:OnEvent(id, data, event) dict
  let str = join(a:data, "\n")
  echomsg str
endfunction
let id = jobstart(['cat'], {'on_stdout': function('s:OnEvent') } )
call chansend(id, "hello!")

Here is an example of setting a buffer to the result of grep, but only after all data has been processed:

function! s:OnEvent(id, data, event) dict
  call nvim_buf_set_lines(2, 0, -1, v:true, a:data)
endfunction
let id = jobstart(['grep', '^[0-9]'], { 'on_stdout': function('s:OnEvent'),
                                      \ 'stdout_buffered':v:true } )
call chansend(id, "stuff\n10 PRINT \"NVIM\"\nxx")
" no output is received, buffer is empty
call chansend(id, "xx\n20 GOTO 10\nzz\n")
call chanclose(id, 'stdin')
" now buffer has result

For additional examples with jobs, see job-control.

channel-pty
Special case: PTY channels opened with jobstart(..., {'pty': v:true}) do not preprocess ANSI escape sequences, these will be sent raw to the callback. However, change of PTY size can be signaled to the slave using jobresize(). See also terminal-emulator.

Terminal characteristics (termios) for :terminal and PTY channels are copied from the host TTY, or if Nvim is --headless it uses default values:

:echo system('nvim --headless +"te stty -a" +"sleep 1" +"1,/^$/print" +q')

3. Communicating using msgpack-rpc channel-rpc

When channels are opened with the rpc option set to true, the channel can be used for remote method calls in both directions, see msgpack-rpc. Note that rpc channels are implicitly trusted and the process at the other end can invoke any API function!

4. Standard IO channel channel-stdio

Nvim uses stdin/stdout to interact with the user over the terminal interface (TUI). If Nvim is --headless the TUI is not started and stdin/stdout can be used as a channel. See also --embed.

Call stdioopen() during startup to open the stdio channel as channel-id 1. Nvim's stderr is always available as v:stderr, a write-only bytes channel.

Example:

func! OnEvent(id, data, event)
  if a:data == [""]
    quit
  end
  call chansend(a:id, map(a:data, {i,v -> toupper(v)}))
endfunc
call stdioopen({'on_stdin': 'OnEvent'})

Put this in uppercase.vim and run:

nvim --headless --cmd "source uppercase.vim"

5. Using a prompt buffer prompt-buffer

If you want to type input for the job in a Vim window you have a few options:

A prompt buffer is created by setting 'buftype' to "prompt". You would normally only do that in a newly created buffer.

The user can edit and enter one line of text at the very last line of the buffer. When pressing Enter in the prompt line the callback set with prompt_setcallback() is invoked. It would normally send the line to a job. Another callback would receive the output from the job and display it in the buffer, below the prompt (and above the next prompt).

Only the text in the last line, after the prompt, is editable. The rest of the buffer is not modifiable with Normal mode commands. It can be modified by calling functions, such as append(). Using other commands may mess up the buffer.

After setting 'buftype' to "prompt" Vim does not automatically start Insert mode, use :startinsert if you want to enter Insert mode, so that the user can start typing a line.

The text of the prompt can be set with the prompt_setprompt() function. If no prompt is set with prompt_setprompt(), "% " is used. You can get the effective prompt text for a buffer, with prompt_getprompt().

The user can go to Normal mode and navigate through the buffer. This can be useful to see older output or copy text.

The CTRL-W key can be used to start a window command, such as CTRL-W w to switch to the next window. This also works in Insert mode (use Shift-CTRL-W to delete a word). When leaving the window Insert mode will be stopped. When coming back to the prompt window Insert mode will be restored.

Any command that starts Insert mode, such as "a", "i", "A" and "I", will move the cursor to the last line. "A" will move to the end of the line, "I" to the start of the line.

Here is an example for Unix. It starts a shell in the background and prompts for the next shell command. Output from the shell is displayed above the prompt.

" Function handling a line of text that has been typed.
func TextEntered(text)
  " Send the text to a shell with Enter appended.
  call chansend(g:shell_job, [a:text, ''])
endfunc
" Function handling output from the shell: Add it above the prompt.
func GotOutput(channel, msg, name)
  call append(line("$") - 1, a:msg)
endfunc
" Function handling the shell exits: close the window.
func JobExit(job, status, event)
  quit!
endfunc
" Start a shell in the background.
let shell_job = jobstart(["/bin/sh"], #{
        \ on_stdout: function('GotOutput'),
        \ on_stderr: function('GotOutput'),
        \ on_exit: function('JobExit'),
        \ })
new
set buftype=prompt
let buf = bufnr('')
call prompt_setcallback(buf, function("TextEntered"))
call prompt_setprompt(buf, "shell command: ")
" start accepting shell commands
startinsert