API: Document buffer updates

Originally written by @phodge in
https://github.com/neovim/neovim/pull/5269.

runtime/doc/msgpack_rpc.txt

@@ -241,4 +241,192 @@ Even for statically compiled clients it is good practice to avoid hardcoding
 the type codes, because a client may be built against one Nvim version but
 connect to another with different type codes.
 
+==============================================================================
+6. Live Updates				      *live-updates* *rpc-live-updates*
+
+A dedicated API has been created to allow co-processes to be notified in
+real-time when the user changes a buffer in any way. It is difficult and
+error-prone to try and do this with autocommands such as |TextChanged|.
+
+							*live-updates-enabling*
+Setting Up~
+
+If your API client is a standalone co-process, it can use the
+`"nvim_buf_live_updates"`API method to activate Live Update events for a
+specific buffer. For example, in python >
+
+	import sys, neovim
+	nvim = neovim.attach('stdio')
+	bufnr = sys.argv[1]
+	nvim.buffers[bufnr].live_updates(True)
+
+After the `"nvim_buf_live_updates"` method is called, neovim will send a
+series of notifications containing the entire buffer's contents and any
+subsequent changes. The buffer's contents are sent via notifications because
+if you were to use the other API methods to retrieve the buffer contents, the
+buffer could be changed again before you turn on live updates. This can cause
+a delay if your plugin activates live updates for a very large buffer, but it
+is the the most efficient way to maintain a copy of the entire buffer's
+contents inside your plugin.
+
+						      *live-updates-disabling*
+Turning Off~
+
+You can use `"nvim_buf_live_updates"` with an argument of `False` to turn off
+notifications. One final notification will be sent to indicate that live
+updates are no longer active for the specified buffer. Alternatively, you can
+just close the channel.
+
+						    *live-updates-limitations*
+Limitations~
+
+Note that any of the following actions will also turn off live updates because
+the buffer contents are unloaded from memory:
+
+  - Closing all a buffer's windows (unless 'hidden' is enabled).
+  - Using |:edit| to reload the buffer
+  - reloading the buffer after it is changed from outside neovim.
+
+							  *live-updates-events*
+Handling Events~
+
+The co-process will start receiving the notification events which will be
+equivilent to the following |rpcnotify()| calls:
+
+1. rpcnotify({channel}, "LiveUpdateStart",		      *LiveUpdateStart*
+             [{buf}, {changedtick}, {linedata}, {more}])
+
+	Neovim will send at least one of these notifications to provide you
+	with the original buffer contents. If the buffer is very large, neovim
+	will send the contents through in multiple events to avoid loading the
+	entire buffer's contents into memory at once.
+
+	{buf} is an API handle for the buffer.
+
+	{changedtick} is the value of |b:changedtick| for the buffer. If you
+	send an API command back to neovim you can check the value of
+	|b:changedtick| as part of your request to ensure that no other
+	changes have been made. See |live-update-race-conditions| for more
+	information.
+
+	{linedata} is a list of strings containing the buffer's contents. If
+	this list contains 100 strings, then they represent lines 1-100 of the
+	buffer.  Newline characters are not included in the strings, so empty
+	lines will be given as empty strings. If you receive another
+	`"LiveUpdateStart"` notification with another {linedata} list, then
+	these lines represent the next N lines of the buffer. I.e., a second
+	notification with another list of 100 strings will represent lines
+	101-200 of the buffer.
+
+	{linedata} will always have at least 1 item, but the maximum length is
+	determined by neovim and not guaranteed to be any particular size.
+	Also the number of {linedata} items may vary between notifications, so
+	your plugin must be prepared to receive the line data in whatever size
+	lists neovim decides to split it into.
+
+	{more} is a boolean which tells you whether or not to expect more
+	`"LiveUpdateStart"` notifications. When {more} is false, you can
+	be certain that you now have the entire buffer's contents.
+
+2. rpcnotify({channel}, "LiveUpdate",				  *LiveUpdate*
+	     [{buf}, {changedtick}, {firstline}, {numreplaced}, {linedata}])
+
+	Indicates that {numreplaced} lines starting at line {firstline} have
+	been replaced with the new line data contained in the {linedata} list.
+	All buffer changes (even adding single characters) will be transmitted
+	as whole-line changes.
+
+	{buf} is an API handle for the buffer.
+
+	{changedtick} is the value of |b:changedtick| for the buffer. If you
+	send an API command back to neovim you can check the value of
+	|b:changedtick| as part of your request to ensure that no other
+	changes have been made.
+
+	{firstline} is the integer line number of the first line that was
+	replaced. Note that {firstline} is zero-indexed, so if line `1` was
+	replaced then {firstline} will be `0` instead of `1`. {firstline} is
+	guaranteed to always be less than or equal to the number of lines that
+	were in the buffer before the lines were replaced.
+
+	{numreplaced} is a positive integer indicating how many lines were
+	replaced. It will be `0` if new lines were added to the buffer but
+	none were replaced. If {numreplaced} is `0` then the new lines were
+	added
+	before the zero-indexed line number in {firstline}.
+
+	{linedata} is a list of strings containing the contents of the new
+	buffer lines. Newline characters are not included in the strings, so
+	empty lines will be given as empty strings. If {numreplaced} is `1` or
+	more, then {linedata} may be an empty list (indicating that lines were
+	deleted from the buffer). But if {numreplaced} is `0` (indicating that
+	lines were added to the buffer) then {linedata} is guaranteed to
+	contain at least 1 item.
+
+	Note: sometimes {changedtick} will be |v:null|, which means that the
+	buffer text *looks* like it has changed, but actually hasn't. In this
+	case the lines in {linedata} contain the modified text that is shown
+	to the user, but doesn't reflect the actual buffer contents. Currently
+	this behaviour is only used for the 'inccommand' option.
+
+3. rpcnotify({channel}, "LiveUpdateTick",		      *LiveUpdateTick*
+	     [{buf}, {changedtick}])
+
+	Indicates that |b:changedtick| was incremented for the buffer {buf},
+	but no text was changed. This is currently only used by undo/redo.
+
+	{buf} is an API handle for the buffer.
+
+	{changedtick} is the new value of |b:changedtick| for that buffer.
+
+4. rpcnotify({channel}, "LiveUpdateEnd", [{buf}])		*LiveUpdateEnd*
+
+	{buf} is an API handle for the buffer.
+
+	Indicates that live updates for the nominated buffer have been
+	disabled, either by calling the api function `"nvim_buf_live_updates"`
+	with argument `false`, or because the buffer was unloaded (see
+	|live-updates-disabling| and |live-updates-limitations| for more
+	information).
+
+							*live-updates-examples*
+Example Events~
+
+If live updates are activated a new empty buffer, the following
+|LiveUpdateStart| event will be sent: >
+
+	rpcnotify({channel}, "LiveUpdateStart", [{buf}, [""], v:false])
+
+If the user adds 2 new lines to the start of a buffer, the following event
+would be generated: >
+
+	rpcnotify({channel}, "LiveUpdate", [{buf}, 0, 0, ["line1", "line2"]])
+
+If the puts the cursor on a line containing the text `"Hello world"` and adds
+a `!` character to the end using insert mode, the following event would be
+generated: >
+
+	rpcnotify({channel}, "LiveUpdate",
+		  [{buf}, {linenr}, 1, ["Hello world!"]])
+
+If the user moves their cursor to line 3 of a buffer and deletes 20 lines
+using `20dd`, the following event will be generated: >
+
+	rpcnotify({channel}, "LiveUpdate", [{buf}, 2, 20, []])
+
+If the user selects lines 3-5 of a buffer using |linewise-visual| mode and
+then presses `p` to paste in a new block of 6 lines, then the following event
+would be sent to the co-process: >
+
+	rpcnotify({channel}, "LiveUpdate",
+		  [{buf}, 2, 3, ['pasted line 1', 'pasted line 2',
+				 'pasted line 3', 'pasted line 4',
+				 'pasted line 5', 'pasted line 6']])
+
+If the user uses :edit to reload a buffer then the following event would be
+generated: >
+
+	rpcnotify({channel}, "LiveUpdateEnd", [{buf}])
+
+
  vim:tw=78:ts=8:ft=help:norl: