Browser waits for ajax call to complete even after

问题:

I have some (potentially) long-running ajax calls that I would like to abort if the user navigates to another page. The following jQuery code calls abort on all pending XMLHttpRequest objects upon navigating away from the page:

$.ajaxSetup({
    beforeSend: function(xhr) {
        $(window).bind('beforeunload', function() {
            xhr.abort();
        });
    }
});

In a test case, I force a 10-second wait on the server-side operation being called. Using Firebug, I confirmed that the above code does indeed cause all pending ajax calls to halt immediately when I click any link on the page. However, the browser still waits the full 10 seconds before moving on to the next page. IE appears to exhibit the same behavior. Is this a known browser behavior? Is there anything I can do allow the user to navigate away from the page immediately in this situation? Thanks in advance.

回答1:

Thank you for your replies! It turns out I was completely wrong about this being a browser issue - the problem was on the server. ASP.NET serializes requests of the same session that require session state, so in this case, the next page didn't begin processing on the server until those ajax-initiated requests completed.

Unfortunately, in this case, session state is required in the http handler that responded to the ajax calls. But read-only access is good enough, so by marking the handler with IReadOnlySessionState instead of IRequiresSessionState, session locks are not held and the problem is fixed.

Hope this information proves useful to others.

回答2:

Regarding Todd's own answer to this question...

I just had this issue with PHP and the same solution would have worked. However I needed the information in the session. For PHP developers you can call session_write_close() to close and write out your session in the middle of the request. This will free up the session for the other requests.

回答3:

You might want to check a weird side effect of abort()

When the abort() method is used, the readystatechange event fires in Explorer and Mozilla. Worse, readyState = 4, which means that the average xmlhttp script assumes the data has been loaded correctly. This can give very weird effects.

documented here:

http://www.quirksmode.org/blog/archives/2005/09/xmlhttp_notes_a_1.html

回答4:

Are you sure that you are using an asychronous request? If the browser blocks during the entire request, you are using a synchronous request (async parameter is false)

回答5:

The server issue is also the case with the Apache/Php server that I am working with. Removed the session_start on a script that did not need it (AJAX) and everything is working as expected. Thanks to Todd who pointed out a like problem!

回答6:

Reference to the accepted answer:

http://improve.dk/optimizing-performance-programmatically-setting-readonlysessionstate/

About IRequiresSessionState

What this means is that, for a given session, only one request can execute concurrently. Any other requests, from that same session, will block, waiting for the session to be released