While still improving the 0.6.9x version of tracker for Fremantle (last week we released 0.6.91 with tons of bugfixing), we are also working on the 0.7.x plans. Philip has already described in detail what are we doing, and i think the most exciting feature is going to be the “observe changes about resources”.
The problem is easy to explain: you write your application (e.g. email viewer), populate its views with data from tracker (e.g. “last 10 emails”), and then… how does your app know when is a new email has been received? It does so because maybe evolution has already pushed a new one into tracker!
Obviously “polling” tracker every n minutes is not a solution, and the nice live queries are very complex and hard to implement properly. BUT we have come up with an easy solution that can help.
The applications will subscribe to an object (or more) in DBus (one object = one class in nepomuk), and then they will receive a signal when something happens in that category with the relevant URIs (e.g. an email is deleted in Emails category, a music file is added in Music class, a document updated, and so on). With this information, the app can decide what to do (e.g. deprecate the view, ignore it because now it is showing other info, query tracker about the relevant metadata, …) .
In coding terms: your email viewer will ask “last 10 emails” and subscribe to signals from “/org/freedesktop/Tracker/Resources/Classes/nmo/Email”. Then when a new mail enters the mailbox, the app will receive a signal “ChangeAdded ([list of tracker uris added])”. Here are some more details of the design.
Once we have this mechanism in place, it will be easier to use tracker as backend for user data in the applications: a nice query language + update notifications! It will also allow us to switch into a model where providers (e.g. a feeds monitor, browsers) push info into tracker, and completely independient applications just query it from there… and then to write a mashup app would we soooo easy… and….
P.D. Philip told me: “before you blog about this, it will be implemented”. Well… if i waited one more day…
Ivan Frade's Blog 
Superb work, Ivan! I want to see all these pieces on my n810 🙂
I’m very glad to see this finally hapenning, I’ve been wishing for this kind of an API for a long time.
hey im a nokia n900 user, and as gd as tracker is, id love to see some kind of selection option for what and where it finds objects, as like the photo viewer, with tracker, it finds everyy photo on the phone, i think it would be amazing if you could select(or deselect) where tracker scans eg: you would only want it to scan and show photos from a specific folder as a default, instead of having to go into filtering and selecting the folder.
Hi Simon,
there is a configuration file: /home/user/.config/tracker/tracker.cfg where you will find some interesting options to play with 😉