Design Patterns Ref : Chapter 15 Bennett et al

useful groups of collaborating classes that provide a solution to commonly occuring problems. provide a means for capturing knowledge about problems and successful solutions.

vs Patterns More abstract that frameworks description of the way a type of problem can be solved, but not in themselves solutions, as they can’t be directly implemented. They are more primitive than frameworks Frameworks Frameworks (e.g..net framework, xna) Partially completed software systems that may be targeted at a specific type of application. Re-usable mini architectures. Can be customised by specialisation of classes, add-in operations and classes.

A Pattern Catalogue Lists commonly used design patterns using pattern templates Problem – describes the problem Context – describes how it occurs Forces – constraints or issues that must be addressed by solution Solution – describes solution approaches Code fragments – show how the solution can be implemented in code E.g. see

Example Whole-part pattern – specifically supports aggregation and composition relationships in UML (see page 334). Gang of Four (GOF) developed sets of design patterns and guidelines for their use. (Gamma Helm Johnston and Vlissides) Client Whole Service_1 Service_2 Part 2 Service_b1 Service_b2 Part 1 Service_a1 Service a_2

Scope of a pattern: class level (static- relationships) or object level (dynamic) Patterns are based on principles of good design: maximise encapsulation/ using composition – Maintainability – Extensibility – Restructuring – portability

3 Types of Design Pattern (GOF) Creational –concerned with how objects are constructed Structural- address issues concerned with the way classes and objects are organised. Behavioural- describe how objects communicate See c# examples on...

Example Creational Pattern:Singleton Problem: Ensure a class has only one instance and provide a global point of access to it. e.g. In water safety we only want one Water safety Authority object, which needs global access. Solution: Make the constructor private Have a get_instance method to either create the object or return the object reference. Class singleton _uniqueinstance -singletondata +get_instance +getdata -singleton

C# implementation class Singleton { private static Singleton _instance; // Constructor is 'protected‘ not public as usual protected Singleton() { } public static Singleton Instance() { if (_instance == null) { _instance = new Singleton(); } return _instance; } Note this is not thread safe.

Structural Pattern example: Composite Problem: to represent whole-part hierarchies so that both offer the same interface to client objects. Context : Both composite and component objects exist and should be treated in the same way. Solution : Combine inheritance and aggregation hierarchies. Compose objects into tree structures to represent part-whole hierarchies.

See figs 15.10, Bennett et al SoundClip +play() Advert Advert sequence MediaClipCollection[n] +play() +addclip() +=removeclip() +getchild() +changesequence() Media Clip _uniqueinstance -singletondata +play() VideoClip +play() * ordered For all m in MediaClipCollection m.play()

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Example Behavioural Pattern : state Problem: operations done differently on an object dependent on its state. Description: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Create additional classes, one for each state so that each holds a state-sepcific version of the operation. ‘pure state’ classes contain no attributes, just redefinitions of operations. This should only be used for objects with significant state- dependent behaviour, as there is a big code overhead with the extra classes.

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An object has complex behaviour which is dependent on its STATE The original object delegates reponsibility to the appropriate state object A changestate() method is sent to the parent who invokes a nextstate method.

Example Behavioural Pattern 2 : Command An object is used to encapsulate all the information needed to call a method at a later time:- the method name, the object that owns the method and values for the method parameters. – client instantiates the command object and provides the information required to call the method at a later time. – invoker decides when the method should be called. – receiver is an instance of the class that contains the method's code. makes it easier to construct general components that need to delegate, sequence or execute method calls at a time of their choosing without the need to know the owner of the method or the method parameters. Good for allowing undos.

How to use design Patterns NOT for the sake of it – there are many examples of badly used patterns. Analyse problem and context Ensure that you really understand patterns Find a pattern that addresses a similar problem, read and study it and determine whether it gives you a better solution Check it there is a simpler solution Check that the context of the pattern similar to your problem Make sure the consequences of using the pattern are acceptable. Examine sample code. Apply and refine.

A pattern.. (hillside.net) solves a problem: Patterns capture solutions, not just abstract principles or strategies. is a proven concept: Patterns capture solutions with a track record, not theories or speculation. describes a relationship: Patterns don't just describe modules, but describe deeper system structures and mechanisms. has a significant human component (minimize human intervention). All software serves human comfort or quality of life; the best patterns explicitly appeal to aesthetics and utility.

A pattern language defines a collection of patterns and the rules to combine them into an architectural style. Pattern languages describe software frameworks or families of related systems. The solution isn't obvious: Many problem-solving techniques (such as software design paradigms or methods) try to derive solutions from first principles. The best patterns generate a solution to a problem indirectly--a necessary approach for the most difficult problems of design.

References Chapter 15 Bennett et al explains patterns well and has some good examples. Hillside.net has lots of useful links, including a link to several pattern catalogues. Dofactory.com has lots of c# examplesand skeletal code.