The third type of observer involved in a template builder is an nsIRDFObserver. The template builder implements this interface to listen for RDF modifications. When the datasource is modified, the datasource will notify any observers of the change. The template builder uses these notifcations to update the template as necessary based on the new or removed information. You don't need to implement this observer yourself, although you may add an observer to the datasource if you want to be notified when the data changes.

There are two main situations when the notifications are made. The first is when the modification functions on the datasource are called. There are four such functions: 'Assert', to add a new triple (or arrow) to the RDF graph, 'Unassert' to remove a triple, 'Change' to adjust the target of a triple, and 'Move' to adjust the source of a 'triple'. For Mozilla's datasources, the latter two just Unassert the old triple and add a new one, creating the effect of changing the value. However, only one notification is made.

For instance, an Assert call looks like the following:

var source = RDF.GetResource("http://www.xulplanet.com/ndeakin/images/t/obelisk.jpg");
var predicate = RDF.GetResource("http://purl.org/dc/elements/1.1/description");
var target = RDF.GetLiteral("One of the thirty or so Egyptian obelisks");
datasource.Assert(source, predicate, target, true);

The Assert call adds a new triple to the RDF datasource. When this happens, any templates observing the datasource will be notified via the RDF observer's onAssert method.

The second situation when notifications are made is when a datasource is being loaded or reloaded. Actually, internally, this isn't any different than the other notifications, but it is worth discussing separately. When the RDF parser loads RDF/XML, it starts with a new empty datasource, and as the parser parses the input data, it calls the datasource's Assert function to add each found triple. In effect, this isn't any different than adding the same set of triples yourself using the Assert method.

When reloading a datasource, you might think that the RDF parser removes all the existing data, loads the new data, and adds it to the datasource. Or, you might think that it creates a fresh datasource with the new data. Actually, the parser does something smarter. When reloading a datasource, it keeps the existing RDF triples intact, and only modifies the datasource based on what has changed. When parsing, any triples that already exist are not added again. If a triple does not exist yet, it will be added. Any triples that don't exist in the new data but were there before are removed. This means that the observer will be called only for the triples that differ between the new and old version of the data. If the reloaded datasource hasn't changed, the builder won't receive any notifications. This saves a lot of extra work.

The RDF observer also has two methods onBeginUpdateBatch and onEndUpdateBatch. These are called when performing a lot of operations on a datasource. When changing the datasource, the changes are surrounded by begin and end batch calls. Then, rather than notify on every change, the datasource will send one notification when the changes are finished. The template builder then rebuilds the template completely when done. This is useful when making a large amount of changes to avoid having to keep recalculating parts of the template that might change again quickly.

We'll look at some specifics of how the template builder handles changes next.