[Late to the party...]
Another way to do this without the overhead of an Ajax call or WebService handler is to load a special ASPX page after a given amount of time (i.e., prior to the session state time-out, which is typically 20 minutes):
// Client-side JavaScript
function pingServer() {
// Force the loading of a keep-alive ASPX page
var img = new Image(1, 1);
img.src = '/KeepAlive.aspx';
}
The KeepAlive.aspx page is simply an empty page which does nothing but touch/refresh the Session state:
// KeepAlive.aspx.cs
public partial class KeepSessionAlive: System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
// Refresh the current user session
Session["refreshTime"] = DateTime.UtcNow;
}
}
This works by creating an img (image) element and forcing the browser to load its contents from the KeepAlive.aspx page. Loading that page causes the server to touch (update) the Session object, extending the session's expiration sliding time window (typically by another 20 minutes). The actual web page contents are discarded by the browser.
An alternative, and perhaps cleaner, way to do this is to create a new iframe element and load the KeepAlive.aspx page into it. The iframe element is hidden, such as by making it a child element of a hidden div element somewhere on the page.
Activity on the page itself can be detected by intercepting mouse and keyboard actions for the entire page body:
// Called when activity is detected
function activityDetected(evt) {
...
}
// Watch for mouse or keyboard activity
function watchForActivity() {
var opts = { passive: true };
document.body.addEventListener('mousemove', activityDetected, opts);
document.body.addEventListener('keydown', activityDetected, opts);
}
I cannot take credit for this idea; see: https://www.codeproject.com/Articles/227382/Alert-Session-Time-out-in-ASP-Net.