*channel.txt*      For Vim version 7.4.  Last change: 2016 Feb 04


		  VIM REFERENCE MANUAL	  by Bram Moolenaar


		      Inter-process communication		*channel*

DRAFT  DRAFT  DRAFT  DRAFT  DRAFT  DRAFT  DRAFT  DRAFT  DRAFT  DRAFT

Vim uses channels to communicate with other processes.
A channel uses a socket.				*socket-interface*

Vim current supports up to 10 simultaneous channels.
The Netbeans interface also uses a channel. |netbeans|

1. Demo					|channel-demo|
2. Opening a channel			|channel-open|
3. Using a JSON channel			|channel-use|
4. Vim commands				|channel-commands|
5. Using a raw channel			|channel-use|
6. Job control				|job-control|

{Vi does not have any of these features}
{only available when compiled with the |+channel| feature}

==============================================================================
1. Demo							*channel-demo*

This requires Python.  The demo program can be found in
$VIMRUNTIME/tools/demoserver.py
Run it in one terminal.  We will call this T1.

Run Vim in another terminal.  Connect to the demo server with: >
	let handle = ch_open('localhost:8765', 'json')

In T1 you should see:
	=== socket opened === ~

You can now send a message to the server: >
	echo ch_sendexpr(handle, 'hello!')

The message is received in T1 and a response is sent back to Vim.
You can see the raw messages in T1.  What Vim sends is:
	[1,"hello!"] ~
And the response is:
	[1,"got it"] ~
The number will increase every time you send a message.

The server can send a command to Vim.  Type this on T1 (literally, including
the quotes):
	["ex","echo 'hi there'"] ~
And you should see the message in Vim. You can move the cursor a word forward:
	["normal","w"] ~

To handle asynchronous communication a callback needs to be used: >
	func MyHandler(handle, msg)
	  echo "from the handler: " . a:msg
	endfunc
	call ch_sendexpr(handle, 'hello!', "MyHandler")

Instead of giving a callback with every send call, it can also be specified
when opening the channel: >
	call ch_close(handle)
	let handle = ch_open('localhost:8765', 'json', "MyHandler")
	call ch_sendexpr(handle, 'hello!', 0)

==============================================================================
2. Opening a channel					*channel-open*

To open a channel: >
    let handle = ch_open({address}, {mode}, {callback})

{address} has the form "hostname:port".  E.g., "localhost:8765".

{mode} can be:						*channel-mode*
	"json" - Use JSON, see below; most convenient way
	"raw"  - Use raw messages

							*channel-callback*
{callback} is a function that is called when a message is received that is not
handled otherwise.  It gets two arguments: the channel handle and the received
message. Example: >
	func Handle(handle, msg)
	  echo 'Received: ' . a:msg
	endfunc
	let handle = ch_open("localhost:8765", 'json', "Handle")

When {mode} is "json" the "msg" argument is the body of the received message,
converted to Vim types.
When {mode} is "raw" the "msg" argument is the whole message as a string.

When {mode} is "json" the {callback} is optional.  When omitted it is only
possible to receive a message after sending one.

The handler can be added or changed later: >
    call ch_setcallback(handle, {callback})
When {callback} is empty (zero or an empty string) the handler is removed.

Once done with the channel, disconnect it like this: >
    call ch_close(handle)

Currently up to 10 channels can be in use at the same time. *E897*

When the channel can't be opened you will get an error message.
*E898* *E899* *E900* *E901* *E902*

If there is an error reading or writing a channel it will be closed.
*E896* *E630* *E631* 

==============================================================================
3. Using a JSON channel					*channel-use*

If {mode} is "json" then a message can be sent synchronously like this: >
    let response = ch_sendexpr(handle, {expr})
This awaits a response from the other side.

To send a message, without handling a response: >
    call ch_sendexpr(handle, {expr}, 0)

To send a message and letting the response handled by a specific function,
asynchronously: >
    call ch_sendexpr(handle, {expr}, {callback})

The {expr} is converted to JSON and wrapped in an array.  An example of the
message that the receiver will get when {expr} is the string "hello":
	[12,"hello"] ~

The format of the JSON sent is:
    [{number},{expr}]

In which {number} is different every time.  It must be used in the response
(if any):

    [{number},{response}]

This way Vim knows which sent message matches with which received message and
can call the right handler.  Also when the messages arrive out of order.

The sender must always send valid JSON to Vim.  Vim can check for the end of
the message by parsing the JSON.  It will only accept the message if the end
was received.

When the process wants to send a message to Vim without first receiving a
message, it must use the number zero:
    [0,{response}]

Then channel handler will then get {response} converted to Vim types.  If the
channel does not have a handler the message is dropped.

On read error or ch_close() the string "DETACH" is sent, if still possible.
The channel will then be inactive.

==============================================================================
4. Vim commands						*channel-commands*

PARTLY IMPLEMENTED: only "ex" and "normal" work

With a "json" channel the process can send commands to Vim that will be
handled by Vim internally, it does not require a handler for the channel.

Possible commands are:				*E903* *E904* *E905*
    ["redraw"  {forced}]
    ["ex",     {Ex command}]
    ["normal", {Normal mode command}]
    ["eval",   {expression}, {number}]
    ["expr",   {expression}]

With all of these: Be careful what these commands do!  You can easily
interfere with what the user is doing.  To avoid trouble use |mode()| to check
that the editor is in the expected state.  E.g., to send keys that must be
inserted as text, not executed as a command:
    ["ex","if mode() == 'i' | call feedkeys('ClassName') | endif"] ~

Errors in these commands are normally not reported to avoid them messing up
the display.  If you do want to see them, set the 'verbose' option to 3 or
higher.


Command "redraw" ~

The other commands do not update the screen, so that you can send a sequence
of commands without the cursor moving around.  You must end with the "redraw"
command to show any changed text and show the cursor where it belongs.

The argument is normally an empty string:
	["redraw", ""] ~
To first clear the screen pass "force":
	["redraw", "force"] ~


Command "ex" ~

The "ex" command is executed as any Ex command.  There is no response for
completion or error.  You could use functions in an |autoload| script:
	["ex","call myscript#MyFunc(arg)"]

You can also use "call |feedkeys()|" to insert any key sequence.


Command "normal" ~

The "normal" command is executed like with ":normal!", commands are not
mapped.  Example to open the folds under the cursor:
	["normal" "zO"]


Command "eval" ~

The "eval" command an be used to get the result of an expression.  For
example, to get the number of lines in the current buffer:
	["eval","line('$')"] ~

it will send back the result of the expression:
	[{number}, {result}]
Here {number} is the same as what was in the request.  Use a negative number
to avoid confusion with message that Vim sends.

{result} is the result of the evaluation and is JSON encoded.  If the
evaluation fails it is the string "ERROR".


Command "expr" ~

The "expr" command is similar to "eval", but does not send back any response.
Example:
	["expr","setline('$', ['one', 'two', 'three'])"] ~

==============================================================================
5. Using a raw channel					*channel-raw*

If {mode} is "raw" then a message can be send like this: >
    let response = ch_sendraw(handle, {string})
The {string} is sent as-is.  The response will be what can be read from the
channel right away.  Since Vim doesn't know how to recognize the end of the
message you need to take care of it yourself.

To send a message, without expecting a response: >
    call ch_sendraw(handle, {string}, 0)
The process can send back a response, the channel handler will be called with
it.

To send a message and letting the response handled by a specific function,
asynchronously: >
    call ch_sendraw(handle, {string}, {callback})

This {string} can also be JSON, use |jsonencode()| to create it and
|jsondecode()| to handle a received JSON message.

==============================================================================
6. Job control						*job-control*

NOT IMPLEMENTED YET

To start another process: >
    call startjob({command})

This does not wait for {command} to exit.

TODO:

    let handle = startjob({command}, 's')            # uses stdin/stdout
    let handle = startjob({command}, '', {address})  # uses socket
    let handle = startjob({command}, 'd', {address}) # start if connect fails


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