Observer

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Implementation

Several issues related to the implementation of the dependency mechanism are discussed in this section.

5. Making sure Subject state is self-consistent before notification. It's important to make sure Subject state is self-consistent before calling Notify, because observers query the subject for its current state in the course of updating their own state.

   This self-consistency rule is easy to violate unintentionally when Subject subclass operations call inherited operations. For example, the notification in the following code sequence is trigged when the subject is in an inconsistent state:

   void MySubject::Operation (int newValue) { BaseClassSubject::Operation(newValue); // trigger notification _myInstVar += newValue; // update subclass state (too late!) }

   You can avoid this pitfall by sending notifications from template methods in abstract Subject classes. Define primitive operation for subclasses to override, and make Notify the last operation in the template method, which will ensure that the object is self-consistent when subclasses override Subject operations.

   void Text::Cut (TextRange r) { ReplaceRange(r); // redefined in subclasses Notify(); }

   By the way, it's always a good idea to document which Subject operations trigger notifications.

Implementation of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor