1. Firefox Add-on
Unfortunately, the rather simple weblin way of tracking browsers is not sufficient anymore. We want tabs (more about tabs and windows at the OVW blog).
Reliable information about visible tabs is only available inside the browser. So I need real browser addons. I started with Firefox some time ago. The addon keeps track of all tabs, when they open/close, become visible and when they navigate. It sends all the information over a local TCP connection to the client (using a simple remote procedure call protocol). This sounds complicated and it is, because the socket implementation in Firefox has its quirks. I could tell stories. But the beauty of a TCP connection is, that if the browser crashes or disappears, then the client will notice when the TCP connection goes down. It will then leave all rooms automatically, even if the browser addon does not say goodbye.
2. Tracking Browser Coordinates
Unfortunately, I don't get a real window handle (HWND) inside the browser. I need a native component to get a HWND for the tab. Why HWND? because I want to track the coordinates of the tab: resize and move events. I could add a native DLL to the addon, but then the addon is not cross-platform. Since I have a native part anyway (the client), I rather make the platform dependent processing in the client.
I implemented a BrowserInfo-module which finds the tab's HWND in the window list and does GetWindowRect() to get coordinates. The module also arranges the window stacking order with SetWindowPos() so that the avatar display is always directly on top of it's browser. That's good old Win32 coding. While most of the code is cross-platform, this part must be adapted to other operating systems, i.e. MacOS.
3. Model View Controller
The virtual presence module manages entering and leaving chat rooms, participants and chat lines. This is the "model". The view gets events from the model when participants enter/leave/chat. The view can always query the model. The model has only data structures. The view creates visible avatars.
And best of all: there can be multiple views for the same model. This means, that 2 browser windows can show the same scene.