ANU COMP2110 Software Design in 2004 Lecture 15 Slide 1

COMP2110 in 2004 Software DesignLecture 15: Software design patterns (2)

1 What are design patterns?

2 Where do they come from?

3 Why study them?

4 What are they again?

5 More thoughts:

• Abstraction

• Don’t reinvent the wheel

• Consequences

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 2

What are design patterns?

First, some examples of what they are not:

• adjustable templates like dress patterns

• as in “pattern matching”, like regexps

• repeated motifs like in art or music, e.g.• Islamic decorative art• harpsichord cadenza from the 5th

Brandenburg concerto• jazz riffs

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 3

So what are design patterns?

Examples of reusable, excellent object-oriented software design, together with

• the context in which each one applies

• the problem it tries to solve

• the forces at work

• the consequences of adopting it

• hints for implementation

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 4

What is the origin of design patterns?

• Started in architecture with the work of Christopher Alexander and his group at Berkeley.

• He asked: What makes great buildings great and awful buildings awful? When great buildings can be so different, what do they have in common?

• He looked very hard for examples and principles, working at a very subtle level

• Looked at different ways to solve the same problem

• Came up with a whole system for design

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 5

What does this have to do with software?

• In the early 90’ssome software people read Alexander’s work and saw a parallel

• The same problems occur again and again and can be solved the same way

• These examples of partial designs can be reused at the level of ideas, not of code

• Gamma, Helm, Johnson & Vlissides wrote the “Gang of Four” book

• An explosion of interest and activity since then

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 6

Why study design patterns?

• To learn some examples of great design that you can use

• To create a common vocabulary for talking about design

• To help you think about design at a higher level of abstraction

• To learn from them, so that you can apply the same principles to create great designs of your own

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 7

What is a design pattern again?

A solution to a problem in a context

Example: The Observer patternContext: Many objects, of varying number and different types, depend on another object and need to stay synchronised with it.Problem: How to manage that without having to change all the code every time a new type of observer comes along.Solution: See previous lecture, textbook, GoF book

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 8

Clock display using observer pattern

main driver loop:loop t = system.clock_time() for all zc in clocks do zcv.update_timer(t) this.suspend(interval)end loop

update_timer(time t) is zoned_time =t+zone_offset for all cd in displays do cd.redisplay() end update_timer

analogue redisplay() is dt = master.get_time h_angle = 360*hours(dt)/12 etc. etc.

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 9

More thoughts: Abstraction

• Abstraction is about choosing what details to leave out

• Just keep the important ones

• To understand and apply design patterns well, you have to learn to do this

• How hard or easy this is for you depends on your personality type

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 10

More thoughts: Don’t reinvent the wheel

Name: Wheel

Intent: Transport heavy loads long distances

Problem: They’re hard to carry, but it’s really important to move stuff like food around

Solution: Build vehicles with wheels, and smooth roads for them to run on

Consequences: Greatly increased capacity and speed. Need roads. Control of roads = power...

Implementation: Another time...

Alternatives: Carry it, drag it, use a boat or barge

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 11

More thoughts: Consequences

• Need to think in terms of cause and effect

• There is no absolute good and bad, only choices and their consequences

• In software we measure quality against requirements (and meta-requirements like extendability and reusability that often aren’t written down)

• When you make design decisions, look at the practical consequences

• Using a pattern isn’t always a good decision!

ANU COMP2110 Software Design in 2004 Lecture 15 Slide 12

More thoughts: Principles

• Prefer composition over inheritance

• Find what varies and encapsulate it

• Improve modifiability