Replace Hard-Coded Notifications with Observer

Refactorings · Chain Constructors · Compose Method · Encapsulate Classes with Factory · Encapsulate Composite with Builder · Extract Adapter · Extract Composite · Extract Parameter · Form Template Method · Inline Singleton · Introduce Null Object · Introduce Polymorphic Creation with Factory Method · Limit Instantiation with Singleton · Move Accumulation to Collecting Parameter · Move Accumulation to Visitor · Move Creation Knowledge to Factory · Move Embellishment to Decorator · Replace Conditional Dispatacher with Command · Replace Conditional Logic with Strategy · Replace Constructors with Creation Methods · Replace Hard-Coded Notifications with Observer · Replace Implicit Language with Interpreter · Replace Implicit Tree with Composite · Replace One/Many Distinctions with Composite · Replace State-Altering Conditionals with State · Replace Type Code with Class · Unify Interfaces · Unify Interfaces with Adapter