CS 635: Advanced Object-Oriented Design & Programming

Doc 26, Facade and Mediator Slide # 2

Facade

Review of Wirfs-Brock (CRC Design Process)

• Exploratory Phase
  
  

Who is on the team?

What are their tasks, responsibilities?

Who works with whom?

• Analysis Phase
  
  

Who's related to whom?

Finding sub teams

Putting it all together

Doc 26, Facade and Mediator Slide # 3

Exploratory Phase

• Who is on the team?
  
  

What are the goals of the system?

What must the system accomplish?

What objects are required to model the system and accomplish the goals?

• What are their tasks, responsibilities?
  
  

What does each object have to know in order to accomplish each goal it is involved with?

What steps toward accomplishing each goal is it responsible for?

• Who works with whom?
  
  

With whom will each object collaborate in order to accomplish each of its responsibilities?

What is the nature of the objects' collaboration

Doc 26, Facade and Mediator Slide # 4

Analysis Phase

• Who's related to whom?
  
  

Determine which classes are related via inheritance

Finding abstract classes

Determine class contracts

• Finding sub teams
  
  

Divide responsibilities into subsystems

Designing interfaces of subsystems and classes

• Putting it all together
  
  

Construct protocols for each class

Produce a design specification for each class and subsystem

Write a design specification for each contract

Doc 26, Facade and Mediator Slide # 5

Subsystems

There is no conceptual difference between the responsibilities of a class and a subsystem of classes

The difference between a class and subsystem of classes is a matter of scale

A subsystem should be a good abstraction

There should be as little communication between different subsystems as possible

Doc 26, Facade and Mediator Slide # 6

The Facade Pattern - Basic Idea

Clients interface with the Facade class to deal with the subsystem

Consequences of Facade Pattern

It promotes weak coupling between the subsystems and its clients

It does not prevent clients from using subsystems class, should it?

Doc 26, Facade and Mediator Slide # 7

Mediator

Structure

Doc 26, Facade and Mediator Slide # 8

Participants

Defines an interface for communicating with Colleague objects

Implements cooperative behavior by coordinating Colleague objects

Knows and maintains its colleagues

Each Colleague class knows its Mediator object

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

Doc 26, Facade and Mediator Slide # 9

Motivating Example

Doc 26, Facade and Mediator Slide # 10

When to use the Mediator Pattern

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 26, Facade and Mediator Slide # 11

Issues

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

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 26, Facade and Mediator Slide # 12

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 26, Facade and Mediator Slide # 13

Subsystem components are not aware of Facade

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

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