HTML5 ChannelMessaging Tutorial

Source

(c) 2013 kevin languedoc (klanguedoc)

HTML5 ChannelMessaging is another type of data streaming between two domains, computers and clients. Channel Messaging in HTML5 uses ports and endpoints to attach to each other and maintain a communication open to send and receive data asynchronously using the postMessage method.

Channel object can have two ports, port1 and port2 endpoints. The port1 is for local use while the port, port2, is sent to the remote window or web worker. The ChannelMessage API provides several properties and methods to provide efficient cross document bi-directional messaging across domains without exposing the DOM to malicious content.

Process Overview

To initiate the ChannelMessaging process you will need to create a ChannelMessage object. Then transfer one port to be used by the other document. Next create an eventListener to capture the message coming from the other window. Finally open the remaining port to receive the incoming message.

As an alternative you can use a Web Worker to send a port to another window as in the following code snippet:

var worker = new WebWorker(âjobMsg.jsâ);
worker.postMessage({code:âportâ}, [channel.port2]);

ChannelMessaging API

Object
Type
Description
port1
MessagePort
The first port attribute
port2
MessagePort
The second port attribute
postMessage
method
Method to send a message through the channel
start
method
Start the messaging
close
method
Close the connection and channel
onmessage
event
Message data received which can include the origin, data, lastEventId, source and ports

Example Application

The following example will demonstrate how to implement channel messaging in a web page using two iframes. You can follow along with the client and javascript code below. For the client (web page), create a html file in an existing or new site. In the source code view, add two iframes. Set the IDs to msg1 and msg2. Also set the width to 480px and the height to 320px. Finally set the src attribute of the first iframe to channelMsg1.html and the second iframe src to channelMsg2.

In the head element add a script element with a type equal to “text/javascript” and the src to “port1ChannelHandler.js” for the first iframe document, "port2ChannelHandler.js" for the second iframe document and finally "parentChannelHandler.js" for the parent document. We will use the javascript script files for all html pages sequentially in the next section.

The two child documents that will be hosted in the two separate iframe windows will have identical coding design so I will explain only once.

The two child documents that will be hosted in the two separate iframe windows will have identical coding design so I will explain only once. In the msg1.html and msg2.html files, add a script element to the head section and as before set the type and src attributes. In the body of both files add a div element with an id set to cMsg1 and cMsg2.

HTML Parent Document

	<header>
	   <h1>main</h1>
	</header>
	<nav>
		<p>
			<a href="/">Channeler</a>
		</p>
			
	</nav>
			
	<div>
		<iframe id="Msg1" src="channelMsg1.html" width="480px" height="320px"></iframe><br/>
		<iframe id="Msg2" src="channelMsg1.html"  width="480px" height="320px"></iframe>

In the javascript script file, define a function as sendMessage() that will be added to the first iframe and an event listener that we will create last. In this function create a new ChannelMessage. Next get a handle on the parent window and send the port2 to the parent document so that the parent can forward it to the other iframe window as in the code below. Also get a handle on the div element in the iframe1 page, cMsg1. Now create an event handler to insert the value of the incoming message into the div on the page. Finally create an event handler for the port1 to listen for incoming messages and start (open) the connection.

function sendMessage()

/**
 * @author Kevin Languedoc
 */
function sendMessage(){
	
	var cMessage = new ChannelMessage();
	var port1Div = document.getElementById("cMsg1");
	
	//send the port to the other document in iframe 2
	cMessage=parent.window.postMessage("message","Http://locallost", [cMessage.port2]);
	
	//define the message handler for port1
	var msgHandler = function(e){
		port1Div.value = e.data;
	}
	
	//Add the event listener
	cMessage.port1.addEventListener("MsgArrived", "http://localhost/",false);
	
	//Open the port
	cMessage.port1.start();
}
window.addEventListener('Content', sendMessage, false);

function parentMessageHandler()

Now we need to define the code for the parent document to receive the message from the first iframe and send it to the second iframe. The pattern of the code is very similar to the first iframe, we will need to define a message handler and get the content from iframes[0] and transfer the content to iframes[1]. We will encapsulate this code into another function.

function parentChannelHandler()

/**
 * @author Kevin Languedoc
 */
function parentChannelHandler(){
	var iframes = window.iframes;
	// Define the message handler.
	var msgHandler = function(msgEvent){
        if( msgEvent.ports.length > 0 ){
           
            iframes[1].postMessage('port2','http://localhost',msgEvent.ports);
        }
   };
	
	
  
    // Listen for the message from iframe[0]
    window.addEventListener('port1message',msgHandler,false);
}

window.addEventListener('ContentReceived',parentChannelHandler,false);
}

iframes[1] Message Handler

The bit of code is for our second iframe document, iframe[1]. The function will be called port2MessageHandler and will follow a similar as the first two functions. First define a message handler and its body add a form handler for the incoming message on submit.

function port2ChannelHandler()

/**
 * @author Kevin Languedoc
 */
function port2ChannelHandler(){
    
    var msgHandler = function(evt){

        var formHandler = function(){
            var message = 'hello world form port 1';
            evt.ports[0].postMessage(message);
        }
        document.forms[0].addEventListener('submit',formHandler,false);
    }
    window.addEventListener('msg',msgHandler,false);
}

window.addEventListener('ContentReceived',port2ChannelHandler,false);

Conclusion

Channel Messaging can have some greater applications. the first that comes to mind is customer and technical chats with current end users or future end users. You also provide in web app chatting which could be useful for games and or to provide live collaboration in applications like Cloud based IDEs.

The possibilities are really endless.

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Comments 3 comments

keirnanholland profile image

keirnanholland 3 years ago from nowhere

What's with the â in the code examples, I think it's a character that is appearing as a double quote on your end, it's appearing on my mac mini right now as an a with a short vowel hat accent on top.


keirnanholland profile image

keirnanholland 3 years ago from nowhere


klanguedoc profile image

klanguedoc 3 years ago from Canada Author

keirnanholland ,

I will fix the typos. the html editor is swapping characters when I am pasting from gdoc.

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