Chapter 7 Structural Patterns: Decorator

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Chapter 7 -Structural Patterns Decorator
CIS 476/566 Software Architecture and Design
Patterns

• Dr. Brahim Medjahed
• brahim_at_umich.edu

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Design Patterns in GoF The Big Picture
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What Are Structural Patterns?

• Structural Patterns
• Deal with the composition of classes or objects
• Two categories
• Object structural patterns
• Describe ways to assemble objects (e.g.,
  Decorator, Proxy)
• Class structural patterns
• Use inheritance to compose classes (e.g.,
  Adapter)

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Decorator Pattern - Intent

• Attach additional responsibilities to an object
  dynamically.
• Provide a flexible alternative to sub-classing
  for extending functionality.
• Also Known As (AKA)
• Wrapper

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Motivating Example BestCafé

• BestCafé coffee shop with a large beverage
  offering
• Example inspired from Head First Design
  Patterns by Bert Bates, Elisabeth Freeman, Eric
  Freeman, Kathy Sierra
• Offers four (4) beverages
• House Blend
• Dark Roast
• Decaf
• Espresso

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BestCafé Initial Design
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Additional Requirements

• Customers can also select condiments
• Milk
• Soy
• Mocha
• Topped with
• Whipped cream
• Each one has its own charge

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Solution 1 Sub-classing
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Solution 1 - Problems

• Explosion of classes!
• Maintenance nightmare
• What happens when price of milk goes up?
• Change code in all relevant subclasses
• What happens when new topping called caramel is
  added?
• Much more classes

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Solution 2 Instance Variables
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Solution 2 Class Diagram
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Solution 2 - Problems

• What if the price of condiment changes?
• Change the code?
• Adding new condiments
• Change the super class
• We may have new beverages for which set of
  condiments are not good.
• What if customer wants double mocha?

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Solution 3 Decorate Objects

• Take a DarkRoast object
• Decorate it with Mocha object
• Decorate it with Whip object
• Call the cost()
• Rely on delegation to add condiments cost.

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Solution 3 Decorate Objects (contd)

• We start with DarkRoast object
• The customer wants Mocha.
• Create Mocha and wrap it with DarkRoast

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Solution 3 Decorate Objects (contd)

• The customer wants Whipped Cream
• Create a WhippedCream object and wrap it with
  Mocha

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Solution 3 Decorate Objects (contd)
Total Cost 1.29
0.10
.99
0.20
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Decorator Pattern - Structure
Each component can be used on its own or wrapped
by a decorator
Each decorator HAS-A (wraps) a component. It has
an instance variable that refers to that component
Objects we are going to dynamically add behavior
to.
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Decorator Pattern - Participants

• Component
• Defines the interface for objects that can have
  responsibilities added to them dynamically
• ConcreteComponent
• Defines an object to which additional
  responsibilities can be attached
• Decorator
• Maintains a reference to a component object and
  defines an interface that conforms to Components
  interface
• ConcreteDecorator
• Adds responsibilities to the component

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BestCafé UML Class Diagram
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Decorator Pattern - Applicability

• Add responsibilities to objects dynamically and
  transparently
• Responsibilities that can be withdrawn
• When extension by sub-classing is impractical
• Explosion of subclasses
• Class definition hidden or unavailable

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Decorator Pattern - Collaborations

• Decorator forwards requests to its Component
  object.
• It may optionally perform additional operations
  before and after forwarding the request

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Implementation - Example

• Implement UML class diagram in slide 17
• Operation() in ConcreteComponent displays Im
  concrete component
• Operation() in ConcreteDecoratorA displays Im
  concrete Decorator A
• Operation() in ConcreteDecoratorB displays Im
  concrete Decorator B

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Implementation Example (contd)

•   // MainApp test application   class
  MainApp      static void Main()          //
  Create ConcreteComponent and two Decorators
        ConcreteComponent c new
  ConcreteComponent()      ConcreteDecoratorA d1
  new ConcreteDecoratorA()      ConcreteDecorato
  rB d2 new ConcreteDecoratorB()      // Link
  decorators       d1.SetComponent(c)      d2.Set
  Component(d1)      d2.Operation()       

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Implementation Example (contd)

•   // "Component"   abstract class
  Component      public abstract void
  Operation()    // "ConcreteComponent"
    class ConcreteComponent Component      pu
  blic override void Operation()          Console
  .WriteLine(Im Concrete Component()")      

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Implementation Example (contd)

• // "Decorator"   abstract class Decorator
  Component      protected Component
  comp    public void SetComponent(Component
  comp)          this.comp comp        pub
  lic override void Operation()          componen
  t.Operation()      

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Implementation Example (contd)

•   // "ConcreteDecoratorA"   class
  ConcreteDecoratorA Decorator      public
  override void Operation()          base.Operati
  on()      Console.WriteLine(Im Concrete
  Decorator A")      
• // "ConcreteDecoratorB"   class
  ConcreteDecoratorB Decorator      public
  override void Operation()          base.Operati
  on()      Console.WriteLine(Im Concrete
  Decorator B")      

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Exercise 1

• Implementation of BestCafé
• Cost() returns the price of the corresponding
  beverage/condiment
• Prices hard-coded in the methods
• Main application
• Create a Dark Roast decorated with double mocha
  then Whipped Cream

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Exercise 1 - Implementation

• To be done in class

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Exercise 2

• Window abstract class
• Has one abstract method draw()
• SimpleWindow is a subclass of Window
• draw() displays Simple Window
• Two possible concrete decorators
• VerticalScrollBarDecorator
• draw() displays with vertical scroll bar
• HorizontalScrollBarDecorator
• draw() displays with horizontal scroll bar
• Main Application
• simple window with horizontal scroll bar with
  vertical scroll bar

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Exercise 2 UML Class Diagram

• To be done in class

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Exercise 2 Implementation

• To be done in class