Implementing Domain-Driven Design

Use a Mediator to Publish Aggregate Internal State

To work around the problem of tight coupling between the model and its clients, you may choose to design Mediator [Gamma et al.] (aka Double-Dispatch and Callback) interfaces to which the Aggregate publishes its internal state. Clients would implement the Mediator interface, passing the implementer’s object reference to the Aggregate as a method argument. The Aggregate would then double-dispatch to that Mediator to publish the requested state, all without revealing its shape or structure. The trick is to not wed the Mediator’s interface to any sort of view specification, but to keep it focused on rendering Aggregate states of interest:

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public class BacklogItem ... ...