Nvim documentation: channel

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*channel.txt*    Nvim

		 NVIM REFERENCE MANUAL    by Thiago de Arruda

Nvim asynchronous IO					*channel*

				      Type |gO| to see the table of contents.


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
     `jobstart(..., {'pty': v:true})` or |termopen()|.

  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.


on_stdout({chan-id}, {data}, {name})		*on_stdout* 

on_stderr({chan-id}, {data}, {name})		*on_stderr* 

on_stdin({chan-id}, {data}, {name})		*on_stdin* 

on_data({chan-id}, {data}, {name})		*on_data*
    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.

      {chan-id}     Channel handle. |channel-id|
      {data}	    Raw data (|readfile()|-style list of strings) read from
		    the channel. EOF is a single-item list: `['']`. First and
		    last items may be partial lines! |channel-lines|
      {name}	    Stream name (string) like "stdout", so the same function
		    can handle multiple streams. Event names depend on how the
		    channel was opened and in what mode/protocol.

    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.

    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.

    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):
      - `foobar` may arrive as `['fo'], ['obar']` 
      - `foo\nbar` may arrive as
	or `['foo',''], ['bar']`
	or `['foo'], ['','bar']`
	or `['fo'], ['o','bar']`
    There are two ways to deal with this:
    1. To wait for the entire output, use |channel-buffered| mode.
    2. To read line-by-line, use the following code:
	let s:lines = ['']
	func! s:on_event(job_id, data, event) dict
	  let eof = (a:data == [''])
	  " Complete the previous line.
	  let s:lines[-1] .= a:data[0]
	  " Append (last item may be a partial line, until EOF).
	  call extend(s:lines, a:data[1:])

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

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
    let id = jobstart(['cat'], {'on_stdout': function('s:OnEvent') } )
    call chansend(id, "hello!")

Here is a 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)
    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|.

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.

    func! OnEvent(id, data, event)
      if a:data == [""]
      call chansend(a:id, map(a:data, {i,v -> toupper(v)}))
    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:
- Use a normal buffer and handle all possible commands yourself.
  This will be complicated, since there are so many possible commands.
- Use a terminal window.  This works well if what you type goes directly to
  the job and the job output is directly displayed in the window.
  See |terminal|.
- Use a window with a prompt buffer. This works well when entering a line for
  the job in Vim while displaying (possibly filtered) output from the job.

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

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

	" 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, ''])

	" Function handling output from the shell: Add it above the prompt.
	func GotOutput(channel, msg, name)
	  call append(line("$") - 1, a:msg)

	" Function handling the shell exits: close the window.
	func JobExit(job, status, event)

	" Start a shell in the background.
	let shell_job = jobstart(["/bin/sh"], #{
		\ on_stdout: function('GotOutput'),
		\ on_stderr: function('GotOutput'),
		\ on_exit: function('JobExit'),
		\ })

	set buftype=prompt
	let buf = bufnr('')
	call prompt_setcallback(buf, function("TextEntered"))
	call prompt_setprompt(buf, "shell command: ")

	" start accepting shell commands

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