Practically Applying Design Patterns - Part I and II


ANOOP MADHUSUDANAN

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Contents


Part I

 

Solution Architect: "But you can use patterns"

Dumb Developer: "Yes, But can I get it as an ActiveX control?"

Introduction

Introduction To This Article

This article is expected to

  • Introduce patterns to you in a simple, human readable (?) way
  • Train you how to really 'Apply' patterns (you can learn patterns easily, but to apply them to solve a problem, you need real design skills)
  • Provide you a fair idea regarding the contexts for applying the following patterns - Builder, Observer, Strategy and Decorator (well, they are few popular design patterns)
  • Demonstrate you how to apply the Observer pattern, to solve a design problem

In this entire article, you will go through the following steps

  1. You will model a very simple football game engine
  2. You will identify the design problems in your football game engine
  3. You will decide which patterns to use for solving your design problems
  4. You will then actually use the observer pattern, to solve one of your design problem.

As a prerequisite

  • You may need to get some grip on reading and understanding UML diagrams

Using The Code

  • The related zip file includes the code, UML designs (in Visio format) etc. After reading this article, you may download and extract the zip file - using a program like Winzip - to play with the source code.

An Overview Of Design Patterns

Even with out much knowledge about design patterns, designers and developers tend to reuse class relationships and object collaborations to simplify the design process. In short, "A Design pattern consists of various co-operating objects (classes, relationships etc)". They provide solutions for common design problems. More than anything else, they offer a consistent idiom for designers and programmers to speak about their design. For example, you can tell a friend that you used a 'Builder' pattern for addressing some design specifications in your project.

A consistent classification of patterns for common design problems are provided by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides [also known as the Gang of Four (GOF)]. The Gang of Four (GOF) patterns are generally considered the foundation for all other patterns.

The basic principle of using patterns is reusability. Once a problem is address some way, you are not really expected to re-invent the wheel if you properly understand the concept of pattern centric software engineering. Here are some important points to remember about design patterns.

  • A Design Pattern is not code. It is in fact an approach or a model that can be used to solve a problem.
  • Design Patterns are about design and interaction of objects and they provide reusable solutions for solving common design problems.
  • A Design Pattern is normally represented with the help of a UML diagram.

Some real hands on experience with patterns may provide you a better idea!!

Architecting Your (Simple) Football Engine

You are working with a popular computer game developing company, and they made you the Solution Architect of one of their major projects - a Soccer (Football) Game Engine (Nice, huh?). Now, you are leading the process of designing the entire Football game engine, and suddenly you have a lot of design considerations, straight away. Let us see

  • How you identify the entities in your game system,
  • How you identify the design problems, and
  • How you apply patterns to address your design specifications.

Identifying Entities

First of all, you need to identify the objects you use in your game engine. For this, you should visualize how the end user is going to use the system. Let us assume that the end user is going to operate the game in the following sequence (let us keep things simple).

  • Start the game
  • Select two teams
  • Add or remove players to/from a team
  • Pick a play ground
  • Start the game

Your system may have a number of PlayGrounds in it, a number of Teams etc. To list a few real world objects in the system, you have

  • Player who play the soccer
  • Team with various players in it
  • Ball which is handled by various players.
  • PlayGround where the match takes place.
  • Referee in the ground to control the game.

Also, you may need some logical objects in your game engine, like

  • Game which defines a football game, which constitutes teams, ball, referee, playground etc
  • GameEngine to simulate a number of games at a time.
  • TeamStrategy to decide a team's strategy while playing

So, here is a very abstract view of the system. The boxes represent classes in your system, and the connectors depicts 'has' relationships and their multiplicity. The arrow head represents the direction of reading. I.e, a GameEngine has (can simulate) Games. A Game has (consists of) three referees, one ball, two teams, and one ground. A team can have multiple players, and one strategy at a time.

 

Fig 1 - High level view

Identifying Design Problems

Now, you should decide

  • How these objects are structured
  • How they are created
  • Their behavior when they interact each other, to formulate the design specifications.

First of all, you have to write down a minimum description of your soccer engine, to identify the design problems. For example, here are few design problems related to some of the objects we identified earlier.

  • Ball
    • When the position of a ball changes, all the players and the referee should be notified straight away.
  • Team and TeamStrategy
    • When the game is in progress, the end user can change the strategy of his team (E.g., From Attack to Defend)
  • Player
    • A player in a team should have additional responsibilities, like Forward, Defender etc, that can be assigned during the runtime.
  • PlayGround
    • Each ground constitutes of gallery, ground surface, audience, etc - and each ground has a different appearance.

So now, let us see how to identify the patterns, to address these design problems.

Identifying Patterns To Use

Have a look at the design problems you identified above (yes, do it once more). Now, let us see how to address these problems using design patterns.

1: Addressing the design problems related with the 'Ball'

First of all, take the specifications related to the ball. You need to design a framework such that when the state (position) of the ball is changed, all the players and the referee are notified regarding the new state (position) of the ball. Now, let us generalize the problem

Specific Design Problem: "When the position of a ball changes, all the players and the referee should be notified straight away."

Problem Generalized: "When a subject (in this case, the ball) changes, all its dependents (in this case, the players) are notified and updated automatically."

Once you have such a design problem, you refer the GOF patterns - and suddenly you may find out that you can apply the 'Observer' pattern to solve the problem.

Observer Pattern: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

In this case, we used this pattern because we need to notify all the players, when the position of the ball is changed.

2: Addressing the design problems related with 'Team' And 'TeamStrategy'

Next, we have to address the specifications related to the team and team strategy. As we discussed earlier, when the game is in progress, the end user can change the strategy of his team (E.g., From Attack to Defend). This clearly means that we need to separate the Team's Strategy from the Team that uses it.

Specific Design Problem: "When the game is in progress, the end user can change the strategy of his team (E.g., From Attack to Defend)"

Problem Generalized: "We need to let the algorithm (TeamStrategy) vary independently from clients (in this case, the Team) that use it."

Then, you can chose the 'Strategy' pattern to address the above design problem.

Strategy Pattern: Define a family of algorithms, encapsulate each one, and make them interchangeable.  Strategy lets the algorithm vary independently from clients that use it.

3: Addressing the design problems related with 'Player'

Now, let us address the design specifications related to the player. From our problem definition, it is clear that we need to assign responsibilities (like forward, defender etc) to each player during run time. At this point, you can think about sub classing (i.e, inheritance) - by creating a player class, and then inheriting classes like Forward, Defender etc from the base class. But the disadvantage is that, when you do sub classing, you cannot separate the responsibility of an object from its implementation.

I.e, In our case, sub classing is not the suitable method, because we need to separate the responsibilities like 'Forward', 'Midfielder', 'Defender' etc from the Player implementation. Because, a player can be a 'Forward' one time, and some other time, the same player can be a 'Midfielder'.

Specific Design Problem: "A player in a team should have additional responsibilities, like Forward, Defender etc, that can be assigned during the runtime."

Problem Generalized: "We need to attach additional responsibilities (like Forward, Midfielder etc) to the object (In this case, the Player) dynamically, with out using sub classing"

Then, you can chose the 'Decorator' pattern to address the above design problem.

Decorator Pattern: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to sub classing for extending functionality

4: Addressing the design problems related with 'PlayGround'

If you take a look at the specifications of Ground, we see that a ground's appearance is decided by various sub units like gallery, surface of the ground, audience etc. The appearance of the ground may vary, according to these sub units. Hence, we need to construct the ground in such a way that, the construction of the ground can create different representations of the ground. I.e, a ground in Italy may have different gallery structure and surface when compared to a ground in England. But, the game engine may create both these grounds by calling the same set of functions.

Specific Design Problem: "Each ground constitutes of gallery, ground surface, audience, etc - and each ground has a different appearance."

Problem Generalized: "We need to separate the construction of an object (ground) from its representation (the appearance of the ground) and we need to use the same construction process to create different representations."

Builder Pattern: Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Now, you can chose the 'Builder' pattern to address the above design problem.


Part II

 

Solution Architect: "I asked you to learn about patterns"

Dumb Developer: "Yes, now I can develop a football engine using patterns"

Solution Architect: "Huh? What do you mean? !@@#!"

Applying Observer Pattern

In this section, we will have a closer look at the observer pattern, and then we will apply the pattern to solve our first design problem. If you can remember, our first design problem was,

  • "When the position of a ball changes, all the players should be notified straight away."

Understanding the Observer Pattern

The UML class diagram of the observer pattern is shown below.

Fig 2 - Observer Pattern

The participants of the pattern are detailed below.

  • Subject

    This class provides an interface for attaching and detaching observers. Subject class also holds a private list of observers. Functions in Subject class are

    • Attach - To add a new observer to the list of observers observing the subject
    • Detach - To remove an observer from the list of observers observing the subject
    • Notify- To notify each observer by calling the Update function in the observer, when a change occurs.

     

  • ConcreteSubject

    This class provides the state of interest to observers. It also sends a notification to all observers, by calling the Notify function in its super class (i.e, in the Subject class). Functions in ConcreteSubject class are

    • GetState - Returns the state of the subject

     

  • Observer

    This class defines an updating interface for all observers, to receive update notification from the subject. The Observer class is used as an abstract class to implement concrete observers

    • Update - This function is an abstract function, and concrete observers will over ride this function

     

  • ConcreteObserver

    This class maintains a reference with the subject, to receive the state of the subject when a notification is received.

    • Update - This is the overridden function in the concrete class. When this function is called by the subject, the ConcreteObserver calls the GetState function of the subject to update the information it have about the subject's state.

Adapting the Observer Pattern

Now, let us see how this pattern can be adapted to solve our specific problem. This will give you a better idea.

Fig 3 - Solving Our First Design Problem

 

When we call the SetBallPosition function of the ball to set the new position, it inturn calls the Notify function defined in the Ball class. The Notify function iterates all observers in the list, and invokes the Update function in each of them. When the Update function is invoked, the observers will obtain the new state position of the ball, by calling the GetBallPosition function in the Foot ball class.

The participants are detailed below.

Ball (Subject)

The implementation of Ball class is shown below.

' Subject : The Ball Class


Public Class Ball

'A private list of observers

Private observers As new System.Collections.ArrayList

'Routine to attach an observer

Public Sub AttachObserver(ByVal obj As IObserver)
observers.Add(obj)
End Sub

'Routine to remove an observer

Public Sub DetachObserver(ByVal obj As IObserver)
observers.Remove(obj)
End Sub

'Routine to notify all observers

Public Sub NotifyObservers()
Dim o As IObserver
For Each o In observers
o.Update()
Next
End Sub

End Class ' END CLASS DEFINITION Ball
FootBall (ConcreteSubject)

The implementation of FootBall class is shown below.

' ConcreteSubject : The FootBall Class


Public Class FootBall
Inherits Ball

'State: The position of the ball

Private myPosition As Position

'This function will be called by observers to get current position

Public Function GetBallPosition() As Position
Return myPosition
End Function

'Some external client will call this to set the ball's position

Public Function SetBallPosition(ByVal p As Position)
myPosition = p
'Once the position is updated, we have to notify observers

NotifyObservers()
End Function

'Remarks: This can also be implemented as a get/set property


End Class ' END CLASS DEFINITION FootBall

IObserver (Observer)

The implementation of IObserver class is shown below. This class provides interface specifications for creating Concrete Observers.

' Observer: The IObserver Class


'This class is an abstract (MustInherit) class

Public MustInherit Class IObserver

'This method is a mustoverride method

Public MustOverride Sub Update()


End Class ' END CLASS DEFINITION IObserver

Player (ConcreteObserver)

The implementation of Player class is shown below. Player is inherited from IObserver class

' ConcreteObserver: The Player Class


'Player inherits from IObserver, and overrides Update method

Public Class Player
Inherits IObserver

'This variable holds the current state(position) of the ball

Private ballPosition As Position

'A variable to store the name of the player

Private myName As String

'This is a pointer to the ball in the system

Private ball As FootBall

'Update() is called from Notify function, in Ball class

Public Overrides Sub Update ()
ballPosition = ball.GetBallPosition()
System.Console.WriteLine("Player {0} say that the ball is at {1},{2},{3} ", _
        myName, ballPosition.X, ballPosition.Y, ballPosition.Z)
End Sub

'A constructor which allows creating a reference to a ball

Public Sub New(ByRef b As FootBall, ByVal playerName As String)
ball = b
myName = playerName
End Sub

End Class ' END CLASS DEFINITION Player
Referee (ConcreteObserver)

The implementation of Referee class is shown below. Referee is also inherited from IObserver class

' ConcreteObserver : The Referee Clas


Public Class Referee
Inherits IObserver

'This variable holds the current state(position) of the ball

Private ballPosition As Position

'This is a pointer to the ball in the system

Private ball As FootBall

'A variable to store the name of the referee

Private myName As String

'Update() is called from Notify function in Ball class

Public Overrides Sub Update()
ballPosition = ball.GetBallPosition()
System.Console.WriteLine("Referee {0} say that the ball is at {1},{2},{3} ", _
            myName, ballPosition.X, ballPosition.Y, ballPosition.Z)
End Sub

'A constructor which allows creating a reference to a ball

Public Sub New(ByRef b As FootBall, ByVal refereeName As String)
myName = refereeName
ball = b
End Sub

End Class ' END CLASS DEFINITION Referee
Position Class

Also, we have a position class, to hold the position of the ball.

'Position: This is a data structure to hold the position of the ball


Public Class Position

Public X As Integer
Public Y As Integer
Public Z As Integer

'This is the constructor


Public Sub New(Optional ByVal x As Integer = 0, _
Optional ByVal y As Integer = 0, _
Optional ByVal z As Integer = 0)

Me.X = x
Me.Y = y
Me.Z = Z
End Sub

End Class ' END CLASS DEFINITION Position


Putting It All Together

Now, let us create a ball and few observers. We will also attach these observers to the ball, so that they are notified automatically when the position of the ball changes. The code is pretty self explanatory.

'Let us create a ball and few observers

Public Class GameEngine

Public Shared Sub Main()

'Create our ball (i.e, the ConcreteSubject)

Dim ball As New FootBall()

'Create few players (i.e, ConcreteObservers)

Dim Owen As New Player(ball, "Owen")
Dim Ronaldo As New Player(ball, "Ronaldo")
Dim Rivaldo As New Player(ball, "Rivaldo")

'Create few referees (i.e, ConcreteObservers)

Dim Mike As New Referee(ball, "Mike")
Dim John As New Referee(ball, "John")


'Attach the observers with the ball

ball.AttachObserver(Owen)
ball.AttachObserver(Ronaldo)
ball.AttachObserver(Rivaldo)
ball.AttachObserver(Mike)
ball.AttachObserver(John)

System.Console.WriteLine("After attaching the observers...")
'Update the position of the ball. 

'At this point, all the observers should be notified automatically

ball.SetBallPosition(New Position())

'Just write a blank line

System.Console.WriteLine()


'Remove some observers

ball.DetachObserver(Owen)
ball.DetachObserver(John)


System.Console.WriteLine("After detaching Owen and John...")

'Updating the position of ball again

'At this point, all the observers should be notified automatically

ball.SetBallPosition(New Position(10, 10, 30))

'Press any key to continue..

System.Console.Read()


End Sub

End Class
Running the project

After running the project, you'll get the output as

Conclusion

Patterns can be classified

  • With respect to purpose.
  • With respect to scope.

With respect to purpose, patterns are classified to Creational, Structural and Behavioral. For example,

  • The Observer pattern we just learned is a behavioral pattern (because it help us model the behavior and interactions of objects)
  • The Builder pattern is a creational pattern (because it details how an object can be created in a particular way) and so on.

Here is the complete classification diagram.

I hope this article

  • May help you to understand how to use design patterns.
  • May help you some way to apply patterns in your projects
  • May help you to give a brief talk about patterns to your friends :)

History

  • "History may make you realize that life is just a play"
  • Nov 07,2005 - Prepared this article for publishing

5 comments:

  1. Pretty good posting on usage of patterns. Thanks.

    ReplyDelete
  2. Thank you Nitin. A second part of this article is available at http://amazedsaint.blogspot.com/2008/01/practically-applying-design-patterns.html if you are interested.

    ReplyDelete
  3. Anoop,
    Good and understandable posting.

    Anoop where r u now,ust ,i tried to reach u by ust mail ,do u remeber me,we joined same day i nust
    shaju

    ReplyDelete
  4. Thanks,
    I found this was a interesting and valuable articles.
    I would give you 5/5 Marks.

    ReplyDelete
  5. Thanks for the informative article. You really cover the whole software process.

    The Strategy pattern is definitely the one to use for designing the different strategies in the TeamStrategy class. Could you also use the Decorator Pattern? Where the concrete component would be AttackStrategy, DefendStrategy and the Decorators would be AggressiveStrategy and PassiveStrategy.

    Thanks

    ReplyDelete

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