Copyright © 1995-2004 Active Frameworks Inc. - All Rights Reserved - V2.0More on Structural...

Copyright © 1995-2004 Active Frameworks Inc. - All Rights Reserved - V2.0 More on Structural Patterns - Page L7-1

PS95&96-MEF-L14-1Dr. M.E. Fayad

Creational

Paradigm Shift, Inc.Software Factory

Behavioral

Structural

Lesson 7:More on Structural Patterns

Lesson 7:More on Structural Patterns

Object-Object-OrientedOriented

DesignDesign PatternPatternss



Lesson ObjectivesLesson Objectives

Objectives

Present the following Patterns:

Proxy

Composite

Discuss in detail the structural patterns



Structural Patterns

Proxy Pattern

• Topics– Proxy Definition

– Structure

– Document Editor Example

– Proxy Properties

– Applicability

– Consequences

– ErrorProxy Example



Proxy Pattern: Definition

• A Proxy is a surrogate or a placeholder for another object to control access to the real objects.

• The Proxy pattern is applicable when there is a need for sophistication in a reference to an object. There is added function between a client and the intended subject object.

• Sometimes the Proxy pattern is known as an Ambassador pattern (when dealing with distributed objects).



Proxy Pattern: Structure

realSubject->Request();

Subject

Request()......

RealSubject

Request()

ClientClass

Proxy

Request()

Uses

realSubject

aClientObject

subject aSubjectProxy

realSubject aRealSubjectInstance Diagram



• The Subject defines the common interface for the RealSubject and Proxy.

• The Proxy can be used any place a RealSubject is expected.

• The RealSubject defines the real object that the Proxy represents.

Proxy Pattern: Structure (2)



Document Editor Example

if (image == 0) image=Loadimage(fileName)image->Draw()

GraphicItemDraw()GetEntent()Store()Load()

RealSubject

fileNameextent

DocumentEditor

ImageProxy

imageRepextent

Items

Image(creates)

Draw()GetEntent()Store()Load()

Draw()GetEntent()Store()Load()

if (image == 0) return extentreturn image->GetExtent()



• Maintains the ability to access the real object, while screening requests for direct manipulation of the object

• Provides an identical interface to the Subject so that it can be substituted for the Subject in any instance

• Controls access to the Subject, even managing it

• Has other responsibilities defined by the type of Proxy:

– Remote Proxies

– Virtual Proxies

– Protection Proxies

– Smart Reference Proxies

Proxy Pattern Properties



Remote Proxies

• Are responsible for encoding requests (with its accompanying arguments) to forward to its corresponding real object, which is found in another address space (Remote or Distributed Object).

• This Proxy is an “Ambassador” to a distributed object.

• This Proxy hides the fact from the user that the object is distributed.



Virtual Proxies

• Cache information about the real object, so that they only access the object when it is absolutely necessary

• This Proxy is used to access expensive objects, such as objects high in storage, access time, or another limit resource, created on demand.

• Example: ImageProxy



Protection Proxies

• Protect access to the original object

• This means that screen users to access the real object or only allow access to a limited public interface

• While the real object may have a private interface is then a subset of the real interface

• Example: KernelProxies in the Choices Operating System that provide a protected access to operating system objects



Smart Reference Proxy

• A Smart Reference Proxy is a replacement for a bare pointer

• A Smart Reference Proxy performs additional actions when an object is accessed:

– Count the number of references to the real object so that it can be automatically freed when there are no more references

– Create a transient representation for a persistent object that is loading the object into memory

– Copy or write, that is, just increment the reference count when asked to copy the Subject, only doing the actual copy when the client requests an operation that modifies the Subject

– Check for locking before forwarding an access request



Proxy Pattern: Participants & Collaborations

• Proxy

– See previous slides

• Subject

– defines the common interface for ConcreteSubject and Proxy so that Proxy can be used anywhere a ConcreteSubject is expected

• ConcreteSubject

– defines the real object that the Proxy represents

Collaborations

The Proxy forwards all requests to the ConcreteSubject. But before doing so, the Proxy performs additional actions depending on the kind of Proxy.

Collaborations

The Proxy forwards all requests to the ConcreteSubject. But before doing so, the Proxy performs additional actions depending on the kind of Proxy.

Participants



Proxy Pattern: Consequences

• A Proxy pattern introduces a level of indirection when accessing an object.

• A Remote Proxy can hide the fact that an object resides in a different address space.

• A Virtual Proxy can perform optimizations like creating an object on demand.

• Both Protected Proxies and Smart References allow additional housekeeping tasks when an object is accessed.



Proxy Pattern: Distinguishing Aspects

• The Proxy is a stand-in for a Subject when it is inconvenient or undesirable to access the Subject directly, as it may be on a remote machine, has restricted access, or is persistent.

• The Proxy maintains a reference to the Subject object and forwards all requests to the Concrete or RealSubject, but will perform additional actions before forwarding the request.

• The Proxy provides the same interface, or a subset, as the original object.

• The Proxy pattern models a one to one relationship and this relationship is a static relationship.



Proxy Pattern: Implementation Issues in C++

1. Overloading the class member access operator ->

2. Writing the forwarding operators manually, when the Proxy needs to know which operations are being called, or wants to perform additional actions before forwarding the request.



ErrorProxy Example:How Errors are Used in the System

• Errors can be reported and logged at any time while the system is running

• The information contained within an error is large

• A user may occasionally want to look at the error log, viewing the list of errors, and displaying some lightweight information about all the logged errors

• The user may decide to work with errors in the log and display or update the error information



All of the Proxy get

routines would check

for errorPtr==NULL

first & read if necessary.

The read routine would

return a new -- Real-

ErrorItem. Display &

Change would always

read, as they would need

all of the data.

ErrorProxy Diagram

ErrorClient ErrorItem

ErrId GetId();ErrDate GetDate();ErrStatus GetStatus();Display();Change();

uses

RealErrorItem


ErrId anId;ErrDate aDate;ErrStatus aStatus;

ProxyErrorItem


ErrId anId;ErrDate aDate;ErrStatus aStatus;RealErrorItem* errorPtr;Persistor* persistPtr;

if (errorPtr != NULL) return errorPtr->GetId();else return anId;

if (errorPtr != NULL) { errorPtr = new RealErrorItem; persistPtr->Read(anId, errorPtr);};errorPtr->Display();



Use of the ErrorProxy

• A Virtual Proxy “ProxyErrorItem” will be used to represent a “RealErrorItem” which spends most of its time out on DASD due to its size.

• ProxyErrorItem will contain some LightWeight information which will allow it to support several common method calls without having to Hydrate the RealError from the DASD unless it is really needed.

– Note: the good is to conserve system memory as much as possible.

• ProxyErrorItem is responsible for deleting the RealErrorItem it has created off DASD, as well as deleting the RealError out on DASD when the Proxy’s Destructor is called.



Scenario Diagram Walkthrough

DASD

Persistor

ErrorProxyItem

ErrorLogClient

ObjectReportingError

ErrorBuilder

ContainerOfProxyErrorItems

RealErrorItem

A1: BuildError(ErrorType)

A2: Write RealErrorItem to DASD

A3: WriteObjectToDASD

A4: Add ProxyErrorItem

B1: getNext()ErrorProxyItem

C1: getErrorId()

D1: Display()

D5: Display()

D2: Create RealErrorItem

D4: Hydrate RealErrorItem

D3: RealError(ErrorID, RealErrorItem)



Structural Patterns

Composite Pattern

• Topics– Composite Definition

– Structure

– Graphic Application Example

– Problems it Solves

– Benefits

– Related Patterns



Composite Pattern: Definition

• Compose objects into tree structures to represent part-whole or containment hierarchies

• Allows user to recursively compose complex and primitive objects

• Treat primitive and complex objects the same way

• Motivation: User can group simple objects into complex ones, then complex objects together with primitives to create even more complex objects



Composite Pattern: Example

A

CIRCLES

Composite

Circle2Circle1

SELCRIC

AComposite

Composite



Composite Pattern: Structure

for all g in children g.Operation();

Composite

Operation()Add(Component)Remove(Component)Iterator()

Component

Operation()Iterator()

Leaf

Operation()

Client

Children



Graphic Application Example:Type Hierarchy

Graphic

Primitive(User Defined)

Picture(Composite)

User Defined Data Iterator



Graphic Application Example

for all g in graphics g.Draw();

Picture

Draw()Add(Graphic g)Remove(Graphic)Iterator()

Component

Operation()Iterator()

Text

Draw()

graphics

Line

Draw()

add g to list ofgraphics

Rectangle

Draw()



Composite Pattern: What it Solves

Use the Composite pattern when

• You want to represent part-whole or containment hierarchies of objects

• You are adding a new component with no need to distinguish the primitive components from the complex

• The number of Composite layers is dynamic



Composite Pattern: Participants

• Component (Graphic )

– declares the interface common to all objects in the composition

– declares an interface for enumerating its child Components

• Leaf (Rectangle, Text, Line, ..)

– represents leaf objects in the composition

– defines behavior for the primitive objects in the composition

• Composite (Picture)

– defines behavior for components

– stores child Components

– defines and implements services for enumerating its children

• Client

– manipulates the composition through the interface declared by Component



Composite Pattern: Collaborations

Clients use the Component class interface to interact with objects in the Composite structure

– If the recipient is a Leaf, then the request is handled directly

– If the recipient is a Composite, then the Composite usually forwards requests to its child Components



Composite Pattern: Benefits

• Composite pattern makes the client simple

– Clients can treat Composite structures and individual objects uniformly

– Clients normally don’t care whether they are dealing with a leaf or a Composite component

– This simplifies the client code

• Composite pattern makes it easier to add new kinds of components



Composite Pattern: Related Patterns

• Decorator is often used with Composite

• Iterator offers flexible ways to traverse Composite

• Visitor localizes operations and behaviors that would otherwise be distributed across Composite and Leaf classes

• Flyweight can be used to keep components from having to know about their parents, and thus will let components be shared



Structural Patterns

Discussion & Conclusions

• Topics

– Adapter versus Bridge versus Facade

– Composite versus Decorator versus Proxy

– Key Insights



Adapter & Bridge: Common Attributes

• Both have class and object forms

• Both use multiple inheritance in their class form to couple independent classes

• Both promote flexibility in their object form by providing a level of indirection to another object

• Both forward requests to the object from an interface other than its own



Adapter Pattern• Focus is on resolving incompatibilities

between two existing interfaces• Doesn’t focus on how those interfaces

are implemented• Doesn’t consider how those interfaces

might evolve independently• Makes things work after they are re-

designed

The Differences Between an Adapter & a Bridge

Bridge Pattern

• Focus is on bridging an abstraction and its (potentially numerous) implementations

• Provides a stable interface to clients

• Accommodates new implementations as the system evolves

• Makes things work before they are redesigned

AdapterFacade

Makes two existing interfaces Defines a new interfacework together

AdapterFacade

Makes two existing interfaces Defines a new interfacework together



Composite & Decorator: Common Attributes

• Both have similar structure diagrams

• Both rely on recursive composition to organize an open-ended number of objects

• Both lead to the kind of design in which you can build applications just by plugging objects together without defining any new classes



Difference Between Composite & Decorator

• Both have different intents

• Decorator is designed to let you add attributes and behaviors to objects without resorting or subclassing

• Composite focuses on structuring classes so that many related objects can be treated uniformly, and multiple objects can be treated as one

What are the common attributes and the differences between the Decorator and Proxy Patterns?

What are the common attributes and the differences between the Decorator and Proxy Patterns?



Discussion QuestionsDiscussion Questions

1. Define the different types of the Proxy pattern and list their responsibilities.

2. Mark (T) for true or (F) for false

( ) 1. Sometimes the Proxy pattern is known as an Ambassador when dealing with distributed objects.

( ) 2. Iterator offers flexible ways to traverse Composites.

( ) 3. Use Composite when you want to represent part-whole or containment hierarchies of objects.

( ) 4. Composite pattern makes the client simple. Why?

( ) 5. Decorator is designed to let you add attributes or behaviors to objects without resorting or subclassing.

( ) 6. An adapter makes two existing interfaces work together as opposed to defining an entire new one.

( ) 7. Decorator & Proxy describe how to provide a level indirection to an object, but they intended for different purposes.

Date post:	21-Dec-2015
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