Is there a way to detect the start of a download in JavaScript?

Question

In our current project we are providing a PDF download that can be customized by the user through an HTML form he submits. It takes several seconds to dynamically generate t

Answer 1

If I was you I would do an AJAX call to the server with the information, generate the file, then return the file name/id/whatever back to the javascript, which then makes window.location something like download.php?id=x (but the file was already generated, so it is just setting the headers and reading it out) at which point you can re-enable the submit.

Answer 2

The simplest solution would be to estimate the time (generously) and do it that way. It's a hack, but it gives the desired effect. The other option might be to submit the form with a callback using Ajax to submit the form and have the generator return details to the calling page. http://www.jquery.com/ might be a good place to start for that option.

Answer 3

What you want is to be able to detect when the size of the downloaded file changes from 0 to a positive value. As far as I know it is impossible to do that with javascript - you need a plug-in that can access the client's file system.

A recommended work around: Create a session per download. Have the client poll the server about the status of the download. This could be "non existed", "not started", "started", "finished". You need some server's side work to persist and update the status of the download plus an AJAX framework.