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HTML SSE API


Server-Sent Events (SSE) allow a web page to get updates from a server.

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.

Browser Support
The numbers in the table specify the first browser version that fully support server-sent events.


API







SSE
6.0
79.0
6.0
5.0
11.5



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>";};
Try it Yourself »

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):

<?phpheader('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: The server time is: " & 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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Reference: https://www.w3schools.com/html/html5_se ... events.asp