Notifier

`TrackerNotifier` allows receiving notification on changes in the data stored by a [class@SparqlConnection].

This object may be created through [method@SparqlConnection.create_notifier], events can then be listened for by connecting to the [ signal@Notifier:TrackerNotifier:eventss] signal.

Not every change is notified, only RDF resources with a class that has the [nrl:notify](nrl-ontology.html#nrl:notify) property defined by the ontology will be notified upon changes.

Database changes are communicated through [struct@NotifierEvent] events on individual graph/resource pairs. The event type obtained through [method@NotifierEvent.get_event_type] will determine the type of event. Insertion of new resources is notified through tracker_notifier_event_create events, deletion of resources is notified through tracker_notifier_event_delete events, and changes on any property of the resource is notified through tracker_notifier_event_update events.

The events happen in reaction to database changes, after a `TrackerNotifier` received an event of type tracker_notifier_event_delete, the resource will not exist anymore and only the information in the [struct@NotifierEvent] will remain.

Similarly, when receiving an event of type tracker_notifier_event_update, the resource will have already changed, so the data previous to the update is no longer available.

The [signal@Notifier:TrackerNotifier:eventss] signal is emitted in the thread-default main context of the thread where the `TrackerNotifier` instance was created.