{Tutorial}HTML5 Server-Sent Events.Step 35.

[MrHotFire]

Server-Sent Events - One Way Messaging

 

A server-sent event is when a web page automatically gets updates from a server.

 

This was also possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates come automatically.

Example: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.

Browser Support

 

 

Server-Sent Events are supported in all major browsers, except Internet Explorer.

Receive Server-Sent Event Notifications

 

The EventSource object is used to receive server-sent event notifications:

var source=new EventSource("demo_sse.php");
source.onmessage=function(event)
  {
  document.getElementById("result").innerHTML+=event.data + "<br>";
  };

 

Example explained:

 

    Create a new EventSource object, and specify the URL of the page sending the updates (in this example "demo_sse.php")

    Each time an update is received, the onmessage event occurs

    When an onmessage event occurs, put the received data into the element with id="result"

 

Check Server-Sent Events Support

 

In the tryit example above there were some extra lines of code to check browser support for server-sent events:

if(typeof(EventSource)!=="undefined")
  {
  // Yes! Server-sent events support!
  // Some code.....
  }
else
  {
  // Sorry! No server-sent events support..
  } 

 

Server-Side Code Example

 

For the example above to work, you need a server capable of sending data updates (like PHP or ASP).

 

The server-side event stream syntax is simple. Set the "Content-Type" header to "text/event-stream". Now you can start sending event streams.

 

Code in PHP (demo_sse.php):

<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');

$time = date('r');
echo "data: The server time is: {$time}\n\n";
flush();
?> 

 

Code in ASP (VB) (demo_sse.asp):

<%
Response.ContentType="text/event-stream"
Response.Expires=-1
Response.Write("data: " & now())
Response.Flush()
%> 

 

Code explained:

 

    Set the "Content-Type" header to "text/event-stream"

    Specify that the page should not cache

    Output the data to send (Always start with "data: ")

    Flush the output data back to the web page

 

The EventSource Object

 

In the examples above we used the onmessage event to get messages. But other events are also available:

Events 	Description
onopen 	When a connection to the server is opened
onmessage 	When a message is received
onerror 	When an error occurs

 

 

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