Web Progress Notifications in Fennec

Working on OpenAttribute for Firefox Mobile (Fennec) yesterday, one of the first challenges I faced was how to get notification that a page had finished loading. In the desktop version, I attach a listener for all tabs using gBrowser.addTabsProgressListener. Unfortunately with browsers running in their own processes, this approach doesn’t work on Fennec. I spent quite a bit of time trying different approaches, all with the intent of creating a progress listener and attaching it myself. The Electrolysis wiki page says that one of the nice side effects of the message passing model is that this problem is easy to solve, but it sure didn’t feel easy.

I don’t remember why I eventually started looking at the mobile-browser source tree, but as I looked through browser.js, there it was:

messageManager.addMessageListener("Content:StateChange", this);

It turns out it is really easy: you can listen for Content:StateChange or Content:LocationChange, and get access to the same details you’d normally have in the WebProgressListener implementation.