HTML5 Server-Sent Events / PDF

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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.

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

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Browser Support

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

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Receive Server-Sent Event Notifications

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

Example

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"

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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..
  }

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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.......

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