Mediator Pattern
CSE776 Design Pattern. Talks
Yunding Li
Tianhang Zhang
Cunyang Shui
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Motivation
• ln order to have a good object oriented design we have to create lots
of classes interacting one with each other. If certain principles are not
applied the final framework will end in a total mess where each
object relies on many other objects in order to run. In order to avoid
tight coupled frameworks, we need a mechanism to facilitate the
interaction between objects in a manner in that objects are not aware
of the existence of other objects.
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Motivation
• Programming scales
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Motivation
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Motivation
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Mediator
• FontDialogBox:
Structure.Related Patterns.Implementation
Will Zhang
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Structure
Mediator (DialogDirector)
-defines an interface for
communicating with Colleague objects.
ConcreteMediator(FontDialogDirector)
-implements cooperative behavior by
coordinating Colleague objects.
-knows and maintains its colleagues.
Colleague classes (ListBox, EntryField)
-each Colleague class knows its
Mediator object.
-each colleague communicates with its
mediator whenever it would have
otherwise communicated with another
colleague.
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Structure
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Consequences
1. Decouples colleagues. Mediator promotes loose coupling among
colleagues.
2. Simplifies object protocols. Mediator replaces many-to-many
interactions with one-to-many interactions.
3. Integrate objects to work together as a system.
4. Centralizes control. The Mediator pattern trades complexity of
interaction for complexity in the mediator. Because a mediator
encapsulate protocols, it can become more complex than any
individual colleague. This can make the mediator itself a monolith
that’s hard to maintain.
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Related Patterns
Facade
Mediator
Adapter
Observer
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Related Patterns – Mediator vs. Facade
Client
Facade
Med
iator
Mediator
Facade
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Related Patterns – Mediator vs. Adaptor
Incompatibility
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Related Patterns – Mediator vs. Observer
• Observers send out messages unselectively, while Mediator
can target related receivers.
• Easier to make reusable Observers and Subjects than to
make reusable Mediators.
• On the other hand, Mediator can leverage Observer for
dynamically registering colleagues and communicating with
them.
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Implementation
• Colleague-Mediator communication
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Implementation
• When communicating
with the mediator, a
colleague passes
itself as an argument,
allowing the mediator
to identify the sender.
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Implementation
• sending notifications to the mediator whenever they change state. The
mediator responds by propagating the effects of the change to other
colleagues.
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Control Tower Example
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