controller_websocket

Controller which opens a WebSocket connection to the browser.

The controller provides persistent WebSocket connections between the client and the server. The default implementation is used by mod_base when the {% stream %} tag is placed on a page.

See Transporting data between browser and server for more information about transporting data between the server and the browser.

Defining Custom Websocket Behaviour

You can provide your own websocket_start similar too controller websocket by setting a ws_handler containing the name of a websocket handler module in the zotonic context.

Example:

websocket_start(ReqData, Context) ->
    Context1 = z_context:set(ws_handler, ?MODULE, Context),
    controller_websocket:websocket_start(ReqData, Context).

When passing a custom handler module, the default handler websocket will not be used, but the specified one. Controller websocket contains the code for the default zotonic handler. It attaches itself as websocket handler to the page session.

It is also possible to configure a custom ws_handler by specifying it in a dispatch rule.:

{customws, ["socket", "custom"], controller_websocket, [{ws_handler, my_ws_handler}]}

WebSocket Handler API

In order to implement your own websocket handler you have to implement four callback functions. When you want to sent a message to the client you call controller_websocket:send_data/2.

Example:

-module(my_ws_handler).

-export([websocket_init/1,
         websocket_message/3,
         websocket_info/2,
         websocket_terminate/2]).

%% @doc Called when the websocket is initialized.
websocket_init(_Context) ->
    erlang:send_after(1000, self(), <<"Hello!">>),
    ok.

%% @doc Called when a message arrives on the websocket.
websocket_message(Msg, From, Context) ->
    controller_websocket:websocket_send_data(From, ["You said: ", Msg]).

%% @doc Called when another type of message arrives.
websocket_info(Msg, _Context) ->
    controller_websocket:websocket_send_data(self(), Msg),
    erlang:send_after(5000, self(), <<"Hello again!">>).

%% @doc Called when the websocket terminates.
websocket_terminate(Reason, Context) ->
    ok.

The websocket_init, websocket_info and websocket_terminate callbacks are called from within the controller’s receive loop, so to send a message to the websocket, you send it to self(), as in the example above.

The websocket_message function however gets a From argument passed to it because it is called from another process. To send a message to the socket, you need to send it to the From pid.

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