== Websocket

This chapter describes how to use the Gun client for
communicating with a Websocket server.

@todo recovering from connection failure
reconnecting to Websocket etc.

=== HTTP upgrade

Websocket is a protocol built on top of HTTP. To use Websocket,
you must first request for the connection to be upgraded. Only
HTTP/1.1 connections can be upgraded to Websocket, so you might
need to restrict the protocol to HTTP/1.1 if you are planning
to use Websocket over TLS.

You must use the `gun_ws:upgrade/{2,3,4}` function to upgrade
to Websocket. This function can be called anytime after connection,
so you can send HTTP requests before upgrading to Websocket.

.Upgrade to Websocket

[source,erlang]
gun:ws_upgrade(ConnPid, "/websocket").

Gun will set all the necessary headers for performing the
Websocket upgrade, but you can specify additional headers
if needed. For example you can request a custom sub-protocol.

.Upgrade to Websocket and request a protocol

[source,erlang]
gun:ws_upgrade(ConnPid, "/websocket", [
	{<<"sec-websocket-protocol">>, "mychat"}
]).

You can pass the Websocket options as part of the `gun:open/{2,3}`
call when opening the connection, or using the `gun:ws_upgrade/4`.
The fourth argument is those same options. This function call
will crash if the options are incorrect, unlike when passing
them through `gun:open/{2,3}`.

When the upgrade succeeds, a `gun_ws_upgrade` message is sent.
If the server does not understand Websocket or refused the
upgrade, a `gun_response` message is sent. If Gun couldn't
perform the upgrade due to an error (for example attempting
to upgrade to Websocket on an HTTP/1.0 connection) then a
`gun_error` message is sent.

When the server does not understand Websocket, it may send
a meaningful response which should be processed. In the
following example we however ignore it:

[source,erlang]
receive
	{gun_ws_upgrade, ConnPid, ok, Headers} ->
		upgrade_success(ConnPid);
	{gun_response, ConnPid, _, _, Status, Headers} ->
		exit({ws_upgrade_failed, Status, Headers});
	{gun_error, ConnPid, StreamRef, Reason} ->
		exit({ws_upgrade_failed, Reason})
	%% More clauses here as needed.
after 1000 ->
	exit(timeout)
end.

=== Sending data

Once the Websocket upgrade has completed successfully, you no
longer have access to functions for performing requests. You
can only send and receive Websocket messages.

Use `gun:ws_send/2` to send one or more messages to the server.

@todo Implement sending of N frames

.Send a text frame

[source,erlang]
gun:ws_send(ConnPid, {text, "Hello!"}).

.Send a text frame, a binary frame and then close the connection

[source,erlang]
gun:ws_send(ConnPid, [
	{text, "Hello!"},
	{binary, BinaryValue},
	close
]).

Note that if you send a close frame, Gun will close the connection
cleanly and will not attempt to reconnect afterwards, similar to
calling `gun:shutdown/1`.

=== Receiving data

Gun sends an Erlang message to the owner process for every
Websocket message it receives.

[source,erlang]
receive
	{gun_ws, ConnPid, Frame} ->
		handle_frame(ConnPid, Frame)
end.

@todo auto ping has not been implemented yet

Gun will automatically send ping messages to the server to keep
the connection alive, however if the connection dies and Gun has
to reconnect it will not upgrade to Websocket automatically, you
need to perform the operation when you receive the `gun_error`
message.
