HTML and CSS Reference
In-Depth Information
This code first checks whether the browser supports server-sent events. It does so by checking for the
existence of the window.EventSource object.
The code then proceeds to wire the click event handler of the Start Listening button. This event
handler creates a new EventSource object. The path of the ASP.NET generic handler that sends the events is
passed as the parameter while creating the EventSource .
Event handlers are then wired for the EventSource object's three events: open , message , and error . The
open event is raised when the browser makes a request to the server resource for the first time. When the
data sent by the server arrives at the client, the message event is raised. In case of an error, such as closed
connection, the error event is raised. The open and error event handlers add a message to the targetDiv
<div> element. The error handler uses the eventPhase property to decide whether the underlying
connection is closed. The possible values for eventPhase are CONNECTING ( 0 ), OPEN ( 1 ), and CLOSED ( 2 ). The
message event handler receives an event parameter, and its data property returns the data sent by the
The server-side code that sends the events to the client resides in the ASP.NET generic handler,
ClientNotifier.ashx. . This code is shown in Listing 11-14.
Listing 11-14. Sending Events to the Client
public void ProcessRequest(HttpContext context)
HttpResponse Response = context.Response;
DateTime startDate = DateTime.Now;
Response.ContentType = “text/event-stream”;
while (startDate.AddMinutes(1) > DateTime.Now)
Response.Write(string.Format(“data: {0}\n\n”, DateTime.Now.ToString(“hh:mm:ss”)));
This code sets the ContentType of the Response to text/event-stream . This way, the client browser
knows that this response belongs to server-sent events. A while loop then iterates for 1 minute. Within the
loop, the event data is dispatched to the client in batches. The Response.Write() method writes the event
data on the response stream, and Response.Flush() ensures that it's sent directly to the client without any
buffering. The event data sent to the client must be in a predefined format. A sample format is as follows:
data: Hello World!\n\n
Every piece of event data should begin with data: and end with two newline characters ( \n\n ). The
event data can also be sent in multiple-line format:
data: {\n
data: “CustomerID”: “ALFKI”,\n
data: “Country”: “USA”\n
data: }\n\n
This markup sends data to the client in JSON format. The client can parse the data to construct a JSON
The operation is halted for 15 seconds using the Thread.Sleep() method, to introduce a delay
between notifications. In addition to the developer-defined data, the server also sends the client a unique
Search WWH ::

Custom Search