WebSocket Handling

WebSocket Handlers

A WebSocket handler is a function for processing WebSocket requests. A WebSocket handler must have one argument typed WebSocket. The WebSocket protocol is a full-duplex communications channel. Thus, the WebSocket object implements both Stream and StreamSink for reading and writing.

Here is an example:

//A WebSocket handler
Future getContentType(WebSocket socket) {
  socket.listen((event) { //receiving the query from the client
    socket.add(contentTypes[event].value); //sending back result to the client
  return socket.done;

Notice that the queries and replies can happen continuously until the connection is closed. Also notice that a WebSocket handler usually returns socket.done to indicate the connection is closed when WebSocket.done completes. If you'd like to process further, you can return another Future instance instead.

Map URI to WebSocket Handlers

To map a URI to a WebSocket handler, you have to prefix the URI with "ws:". For example,

new StreamServer(uriMapping: {
  "ws:/contentType": getContentType

Of course, you can have any number of URI mapping, WebSocket or not.

Client-side Example

If you implement the client in Dart, you can use WebSocket to establish the connection. However, for demonstration purpose, we implement the client in JavaScript here:

<script type="text/javascript">
var ws, mimeType;
function getContentType(type) {
  if (ws == null) {
    ws = new WebSocket("ws://localhost:8080/contentType");
    ws.onopen = function() {
      console.log("connection started");
    ws.onmessage = function (evt) { 
      mimeType = evt.data;
      console.log("Echo back: " + mimeType);
    ws.onclose = function() {
      console.log("connection closed");
  } else {

For Dart implementation, you can refer to Seth's Blog.

Notice that WebSocket used at the client is under dart:html library, while WebSocket used at the server is under dart:io library.