	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2002 Mediator & Type Object
	Previous Lecture Notes Index © 2002, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 02-May-02

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

Type Object, Ralph Johnson & Bobby Woolf in Pattern Languages of Program Design 3, Edited by Martin, Riehle, Buschmann, 1998, pp. 47-65


Doc 18, Mediator & Type Object 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 18, Mediator & Type Object 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 18, Mediator & Type Object Slide # 4

Motivating Example

Dialog Boxes
Objects

Interaction

How does this differ from a God Class?


Doc 18, Mediator & Type Object 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 18, Mediator & Type Object 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 18, Mediator & Type Object 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 18, Mediator & Type Object 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


Doc 18, Mediator & Type Object Slide # 9

Type Object

Intent
Decouples instances from their classes so those classes can be implemented as instances of a class

Allows new classes to be created dynamically at runtime

Lets a system provide its own type-checking rules

Also Known As

Power Type
Item Descriptor
Metaobject
Data Normalization


Doc 18, Mediator & Type Object Slide # 10

Motivation
Video Rental Store Inventory
Need to keep track of all the movies in the inventory

What

About individual movies
Multiple copies of a movie

Subclassing does not Work

What happens when new movies come out?

Instances of Videotape do not Work
Using one instance of Videotape class per movie

Need to track multiple copies of a movie

Using one instance of Videotape for each copy of a movie

Each copy contains a lot of duplicate information


Doc 18, Mediator & Type Object Slide # 11

Type Object Solution
Class Structure

Object Structure


Doc 18, Mediator & Type Object Slide # 12

Type Object Structure

TypeClass (Movie)

Is the class of TypeObject
Has a separate instance for each type of Object

TypeObject (SpiderMan, Monsoon Wedding)

Is instance of TypeClass
Represents a type of Object
Implements some of the behavior for TypeClass

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