March 200692.3913 Ron McFadyen1 Design Patterns In software engineering, a design pattern is a...

March 2006 92.3913 Ron McFadyen 1

Design Patterns

In software engineering, a design pattern is a generally repeatable solution to a commonly-occurring problem in software design.

A design pattern isn't a finished design that can be transformed directly into code; it is a description or template for how to solve a problem that can be used in many different situations.


Design Patterns

Patterns originated as an architectural concept by Christopher Alexander. •1977: A Pattern Language: Towns, Buildings, Construction is a book on architecture - by Christopher Alexander, Sara Ishikawa and Murray Silverstein

In 1994: •Design Patterns: Elements of Reusable Object-Oriented Software -GoF•First Pattern Languages of Programs conference

Some online resourses:•en.wikipedia.org/wiki/Design_pattern_(computer_science)•www.dofactory.com


Design Patterns

Design patterns are composed of several sections

Format used by the Gang of Four.

•Pattern Name and Classification•Intent•Also Known As•Motivation•Applicability•Structure•Participants

•Collaboration•Consequences•Implementation •Sample Code•Known Uses•Related Pattern

Of particular interest are the Structure, Participants, and Collaboration sections.


Also known as publish subscribe

The essence of this pattern is that one or more objects (called observers or listeners) are registered (or register themselves) to observe an event which may be raised by the observed object (the subject). The object which may raise an event maintains a collection of the observers.

Observer Pattern

http://en.wikipedia.org/wiki/Image:Observer-pattern-uml.jpg


Observer Pattern

The object that is responsible for the event is given the responsibility of monitoring for the event – this object is the subject.

Objects that are interested in the event must register with the subject as interested parties – as observers.

The subject will notify its observers when the event occurs.

Two interfaces are used: a subscriber interface and a subject interface.


Observer Pattern

Observer: objects that want to be notified of a certain event. An observer must have an update method whereby it is notified of an event.

Subject: the object that triggers the event. It must implement:

attach (observer) - add an observer to its list of observers

detach (observer) - remove an observer from …

notify () - goes through its list of observers calling each observer’s update method

As needed - additional methods to allow an observer to get additional information


Interfaces

«interface»

Subject

attach()

detach()

notify()

«interface»

Observer

update()


Generic pattern

«interface»

Subject

attach()

detach()

notify()

«interface»

Observer

update()

ConcreteSubject

attach()

detach()

notify()

…

ConcreteObserver

update()

…

*


Weather Station Example

«interface»

Subject

attach()

detach()

notify()

«interface»

Observer

update()

WeatherDataattach()detach()notify()

CurrentConditionsDisplayupdate()display()

*

StatisticsDisplayupdate()display()

ForecastDisplayupdate()display()

«interface»

DisplayElement

display()


Weather Station Example

«interface»

Subject

attach()

detach()

notify()

«interface»

Observer

update()

WeatherDataattach()detach()notify()

CurrentConditionsDisplayupdate()display()

*

StatisticsDisplayupdate()display()

ForecastDisplayupdate()display()

«interface»

DisplayElement

display()

ObserverConcrete observer

Concrete subject

observersubject

Concrete observer

Concrete observer


Object diagram

What collection of objects exist? How are they related?


Behaviour

What is the typical behaviour expressed with sequence diagrams?


Decorator pattern

This design pattern allows new/additional behavior to be added to an existing method of an object dynamically

Decorator works by wrapping a new "decorator" object around the original object, which is typically achieved by passing the original object as a parameter to the constructor of the decorator, with the decorator implementing the new functionality. The interface of the original object needs to be maintained by the decorator.

Decorators are alternatives to subclassing. Subclassing adds behaviour at compile time whereas decorators provide new behaviour at runtime.


Decorator

1 componentcomponent

decoratorConcrete component

Concrete decoratorA

Concrete decoratorB

operation()

operation()

operation() operation()

operation()

Client


Decorator textbook example

1 componentBeverage

Condiment DecoratorHouseBlend

cost()

getDescription

getDescription()DarkRoast

Decaf

cost()

cost()

cost()

Espressocost()

Milk Mocha Soy Whip cost()

getDescription()

cost()

getDescription()

cost()

getDescription()

cost()

getDescription()


Decorator textbook example

Beverage

Condiment DecoratorHouseBlend DarkRoast

DecafEspresso

Milk Mocha Soy Whip

DecoratorDecorator

Concrete decorator

Concrete component

Component

Concrete decorator

Concrete decorator

Concrete decorator

Concrete component

Concrete component

Concrete component


Object diagram

What collection of objects exist? How are they related?


Behaviour

What is the typical behaviour expressed with sequence diagrams?


Singleton pattern

Singleton is designed to restrict instantiation of a class to one (or a few) objects.

Useful when exactly one object is needed to coordinate actions across the system.

Singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist.


Singleton pattern

If an instance already exists, it simply returns a reference to that object. To make sure that the object cannot be instantiated any other way, the constructor is made either private or protected.

• eager instantiation• lazy instantiation - no resources until needed

The singleton pattern must be carefully constructed in multi-threaded applications.


Singleton pattern

Structure?

Behaviour?

March 200692.3913 Ron McFadyen1 Design Patterns In software engineering, a design pattern is a...

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