	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2001 Mediator
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Contents of Doc 22, Mediator

References
Design Patterns: Elements of Resuable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison Wesley, 1995, pp. 273-282


Doc 22, Mediator Slide # 2

Mediator

A mediator is responsible for controlling and coordinating the interactions of a group of objects (not data structures)

Structure

Classes

Objects


Doc 22, Mediator Slide # 3

Participants
Mediator

Defines an interface for communicating with Colleague objects

ConcreteMediator

Implements cooperative behavior by coordinating Colleague objects

Knows and maintains its colleagues

Colleague classes

Each Colleague class knows its Mediator object

Each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague


Doc 22, Mediator Slide # 4

Motivating Example

Dialog Boxes
Objects

Interaction

How does this differ from a God Class?


Doc 22, Mediator Slide # 5

When to use the Mediator Pattern
When a set of objects communicate in a well-defined but complex ways

When reusing an object is difficult because it refers to and communicates with many other objects

When a behavior that's distributed between several classes should be customizable without a lot of subclassing


Doc 22, Mediator Slide # 6

Issues

How do Colleagues and Mediators Communicate?
1) Explicit methods in Mediator

class DialogDirector
   {
   private Button ok;
   private Button cancel;
   private ListBox courses;
   
   public void ListBoxItemSelected() { blah}
   
   public void ListBoxScrolled() { blah }   
   etc.
   }

2) Generic change method

class DialogDirector {
   private Button ok;
   private Button cancel;
   private ListBox courses;
   
   public void widgetChanged( Object changedWidget) { 
      if ( changedWidget == ok )         blah
      else if (  changedWidget == cancel )   more blah
      else if (  changedWidget == courses )   even more blah
   }
}


Doc 22, Mediator Slide # 7

3) Generic change method overloaded

class DialogDirector
   {
   private Button ok;
   private Button cancel;
   private ListBox courses;
   
   public void widgetChanged( Button changedWidget)
      { 
      if ( changedWidget == ok )
         blah
      else if (  changedWidget == cancel )
         more blah
      }
   
   public void widgetChanged( ListBox changedWidget)
      { 
      now find out how it changed and 
      respond properly
      }
   }


Doc 22, Mediator Slide # 8

Differences from Facade
Facade does not add any functionality, Mediator does

Subsystem components are not aware of Facade

Mediator's colleagues are aware of Mediator and interact with it

Copyright ©, All rights reserved.
2001 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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