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Cam Quach Joel Derstine

The Mediator design pattern facilitates loose coupling between objects by coordinating their interactions through a central mediator class. This approach enhances the reusability of object behavior and simplifies communication protocols within an application. For instance, in an air traffic control scenario, the mediator acts as a control tower, managing the complexities of communication among multiple aircraft without direct interactions. By abstracting the interactions, the Mediator pattern enables flexibility and maintainability in system design.

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Cam Quach Joel Derstine

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  1. Mediator: Object Behavioral Cam Quach Joel Derstine

  2. Intent • Mediatoris an object to coordinate state changes between other objects • Mediator promotes loose coupling by preventing objects from referring to each other explicitly • Mediator allows designers to vary their interactions independently

  3. Problem • Goal in OO design is to distribute behavior among objects • Distribution will increase reusability  # of objects  interconnections  reusability  reusability

  4. Solutions mediator Mediator Colleague ConcreteMediator ConcreteColleague1 ConcreteColleague2 • Structure • Mediator provides an interface for communicating with Colleague objects • ConcreteMediatorimplements a cooperative behavior by coordinating the Colleague objects • Colleaguessend and receive requests from a Mediator object

  5. A Conceptual Example: The Problem • Multiple aircraft in the air space of an airport need to communicate with each other about changes in (some of) their properties (speed, altitude, direction). • Where do the complexities reside if they tried to do this by communicating with each other directly? Duell, “Non-software examples of software design patterns”, Object Magazine, July 1997

  6. A Conceptual Example: The Problem Some complexities? • Who do they need to contact • When (or how often) do they need to contact • What do they need to tell them I’m landing! ? Not yet! I’m still taking off!! Duell, “Non-software examples of software design patterns”, Object Magazine, July 1997

  7. A Conceptual Example: A Solution A Control Tower (the “mediator”)! Manages the complexity of communications: • Who to contact (observer object?) • When to contact • What to say ? Duell, “Non-software examples of software design patterns”, Object Magazine, July 1997

  8. A Technical Example: The Problem Some issues/questions: • How should the widgets on this dialog box communicate? • What are the dependencies? • Each widget knows how to do its job. But should it know how it fits into the larger picture? GOF, “Design Patterns”, 1995

  9. A Technical Example: A Solution Create a FontDialogDirector Mediator! • Knows the widgets in the dialog box • Serves as the intermediary (widgets only know about the mediator). • Coordinates their interaction DialogDirector ShowDialog() CreateWidgets() WidgetChanged(Widget) director Widget Changed() FontDialogDirector CreateWidgets() WidgetChanged(Widget) ListBox GetSelection() list EntryField SetText() field GOF, “Design Patterns”, 1995

  10. Consequences Mediator • Limits subclassing • Decouples colleagues • Simplifies object protocols • Abstracts how objects cooperate • Centralizes control

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