Sunday, July 29, 2012

Simple Dart + WebSocket demo

[EDIT: Updated on 2013-03-05]

I just posted a simple demo of Dart with WebSockets. Not only does it show how to connect to a WebSocket server and send messages, it also shows off a few neat features of the Dart language.

(The code follows below, or check out the full open source project.)

Look mom, no classes!

Dart is a class-based object oriented language, but that doesn't mean you are required to wrap everything in a class. No sir, this is Dart, not Java. In Dart, you can use top level functions and top level variables.

Libraries for modularity

Dart organizes code into libraries for modularity. For example, this code uses the dart:html library for the WebSocket code. Concerned about bloat here? Don't be! Thanks to tree-shaking, the code that you don't use is eliminated during a (optional) compile step. The Dart to JavaScript compiler uses tree shaking to help generate minimal JavaScript code (still more work to do here). An upcoming Dart to Dart script will also generate a minimal Dart file for your app. So include all the libraries that you want, and stop worrying about how many bytes a dependencies adds.

Inline function definitions

Dart's syntax for inline function definitions is very handy for callbacks. For example, the code for the reconnection logic reads:

new Timer(new Duration(seconds:retrySeconds),
  () => initWebSocket(url, retrySeconds));

Optional parameters with default values

Make function parameters optional by wrapping them in square brackets. In the code below, I made retrySeconds optional, with a default value of 2.

Main is where it starts

I know where every Dart program starts. This might sound minor, but understanding the structure of your program is key for building larger apps. Look for the main() function to learn where the script begins.

The code

You can also check out the full open source code for the Dart + WebSockets demo. Here's a snippet.


/*
 * See LICENSE file.
 */

import 'dart:html';
import 'dart:async';

WebSocket ws;

outputMsg(String msg) {
  var output = query('#output');
  output.text = "${output.text}\n${msg}";
}

void initWebSocket(String url, [int retrySeconds = 2]) {
  var encounteredError = false;
  
  outputMsg("Connecting to Web socket");
  ws = new WebSocket(url);
  
  ws.onOpen.listen((e) {
    outputMsg('Connected');
    ws.send('Hello from Dart!');
  });
  
  ws.onClose.listen((e) {
    outputMsg('web socket closed, retrying in $retrySeconds seconds');
    if (!encounteredError) {
      new Timer(new Duration(seconds:retrySeconds),
          () => initWebSocket(url, retrySeconds*2));
    }
    encounteredError = true;
  });
  
  ws.onError.listen((e) {
    outputMsg("Error connecting to ws");
    if (!encounteredError) {
      new Timer(new Duration(seconds:retrySeconds),
          () => initWebSocket(url, retrySeconds*2));
    }
    encounteredError = true;
  });
  
  ws.onMessage.listen((MessageEvent e) {
    outputMsg('received message ${e.data}');
  });
}

void main() {
  //initWebSocket('ws://echo.websocket.org');
  initWebSocket('ws://127.0.0.1:8889/ws');
}

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Disclaimer

I'm probably required to say that the views expressed in this blog are my own, and do not necessarily reflect those of my employer. Also, except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 3.0 License, and code samples are licensed under the BSD License.