16.55. Change Class

The existing ChangeRequest subclasses already contain the information to accurately and completely describe the operation. Reusing these classes means we don't need a duplicate class structure or come up with a generic event class.

The requests have all the state required for an event, plus they often will have more. For example, the DeleteBranchRequest has the actual location of the branch that was deleted (and in this way is not much different than a more generic event), but the CreateNodeRequest has the actual location of the created node along with the properties of that node. Additionally, the RemovePropertyRequest has the actual location of the node along with the name of the property that was removed. In many cases, these requests have all the information a more general event class might have but then hopefully enough information for many observers to use directly without having to read the graph to decide what actually changed.

The requests that make up a Changes instance can actually be replayed. Consider the case of a cache that is backed by a JcrRepositorySource, which might use an observer to keep the cache in sync. As the cache is notified of Changes, the cache can replay the changes against its source.