you may want to check observable_getter example from dart-polymer-dart-examples github repo.
class App extends Object with ObservableMixin {
@observable
DateTime timestamp;
App() {
bindProperty(this, const Symbol('timestamp'),
() => notifyProperty(this, const Symbol('second')));
}
int get second => timestamp.second;
}
main() {
App app = new App();
new Timer.periodic(const Duration(seconds: 1), (_) {
app.timestamp = new DateTime.now();
});
query('#tmpl').model = app;
}
Also check discussion at: https://code.google.com/p/dart/issues/detail?id=12473