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Websocket

note

If you are unfamiliar with websockets, this is what shiny uses for inputs: it allows sending messages from the client to the browser and vice versa.

You can listen to incoming messages with the receive method which takes 1) the name of the message to handle and 2) a callback function to run when a message with name is received. The callback function must accept the message as first argument and optionally the socket as second argument.

Below a handler listening to the message hello, prints the message and uses the websocket to send a response.

# listen to incoming messages
app$receive("hello", \(msg, ws){
  print(msg)
  ws$send("hello", "Hello back! (sent from R)")
})

JavaScript​

Messages sent from the server can be handled client-side with the JavaScript websocket library or using the Ambiorix class. It provides a static method to send messages through the websocket, like other methods in R it accepts 1) the name of the message and 2) the message itself: Ambiorix.send('hello', 'Hello from the client').

One can also instantiate the class to add handlers with receive method then run start to have the handlers actually listen to incoming messages.

var wss = new Ambiorix();
wss.receive("hello", (msg) => {
  alert(msg);
});
wss.start();

When setting up a project with create_ambiorix an ambiorix.js file is placed in the static directory, this contains a class that will allow receiving and sending messages through the websocket.

The Ambiorix object has two classes, send which is static and thus can be used without instantiating the class.

Ambiorix.send('messageName', 'Sent from the server')

And receive, a method to add listeners, very much like the receive method in R, this also takes the name of the message as first argument and the callback function as second argument.

var wss = new Ambiorix();
wss.receive("hello", (msg) => {
  alert(msg);
});

This must then be "started," this actually attaches the event listeners created with receive.

wss.start()

Example​

Here we put in practice all that was explained in the previous sections. This example simply sends a message from the client to the server at the click of a button, this message is then printed by the server which responds with another message that shows an alert.

<!-- templates/home.html -->
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" href="static/style.css">
  <script src="static/ambiorix.js"></script>
  <script>
    var wss = new Ambiorix();
    wss.receive("hello", (msg) => {
      alert(msg);
    });
    wss.start();
  </script>
  <title>Ambiorix</title>
</head>
<body>
  <h1 class="brand">Websocket example</h1>
  <button onclick="Ambiorix.send('hello', 'Hi from the client')">Send a message</button>
</body>
</html>

Below we use receive to pass a callback function that receives the message and sends a response (that triggers the alert).

library(ambiorix)

app <- Ambiorix$new()

# homepage
app$get("/", \(req, res){
  res$send_file("home.html")
})

# socket 
app$receive("hello", \(msg, ws){
  print(msg)
  ws$send("hello", "Hello back! (sent from R)")
})

app$start()

Bypass Ambiorix​

The above made use of ambiorix's convenience, if you wish to bypass it you can specify your own handler function which it must accept the websocket.

app$websocket <- \(ws){
  ws$onMessage(\(binary, message){
    cat("Received a message:", message, "\n")
  })
}