3. Introduction
• First conference where I’m a speaker
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4. Topics
• Introduction to Design Patterns
• Duck simulator
• Weather application
• Coffee bar
• Traveller
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5. Introduction to
Design Patterns
• Best-practise ‘design’ solution
• Lessons learned by other developers
• Based on the OO principles
• Language in-depended
• Puzzles for the creative mind of developer
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8. Introduction to
Design Patterns
OO Basics OO Principles
* Abstraction * Encapsulate what varies
* Encapsulation * Favour composition
* Polymorphism over inheritance
* Inheritance * Program to interfaces,
not implementations
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9. Introduction to
Design Patterns
• It’s all about creating flexible designs that
are maintainable and are able to deal with
changes!
• Don’t re-invent the wheel!
• Load the design patterns into your brain
and then design/build your application
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15. Duck simulator
First change
• Our ducks have to fly!
• No problem
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16. Duck simulator
First change
• Our ducks have to fly!
• No problem
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17. Duck simulator
First problem
• Rubber ducks don’t fly!
(and I don’t mean throwing)
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18. Duck simulator
First change
• This can't be the solution
(imagine what you have to do if you want to create a decoy duck)
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19. Duck simulator
First change
• This can't be the solution
(imagine what you have to do if you want to create a decoy duck)
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20. Duck simulator
Cleaner solution
• How about an interface?
• Only implementing the interface if the duck
is flyable
• Hmm that’s it, that’s the solution
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23. Duck simulator
Think again
• What happens if we have to change the fly
behaviour?
• Exactly, we have to change all instances!
• And we duplicate code
• Wouldn’t it be dreamy if we can change its
behaviour at runtime?
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24. Duck simulator
Think again
• Do you remember the OO basics and
principles?
• ‘Identify the aspects of your application that
vary and separate them from what stays
the same’
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29. Duck simulator
Strategy Design Pattern
• ‘The strategy pattern defines a family of
algorithms, encapsulates each one, and
makes them interchangeable. Strategy lets
the algorithm vary independently from
clients that use it.’
• Do you remember the OO basics and
principles?
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30. Duck simulator
Guru
• Now you’ll understand why you should
know about design patterns!
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33. Weather application
What we want
Displays
Pulls data
WheatherData
object
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34. Weather application
Requirements
• There are 3 get methods for temperature,
humidity and pressure
• measurementsChanged method will be
automatically called if the measurements are
changed
• We have to implement the 3 display elements and
they must be updated when measurements change
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38. Weather application
What’s wrong
• What if we want to add a new display?
• Exactly: we have to change the code
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39. Weather application
What’s wrong
• Remember the OO basics and principles!
• Not flexible
• Not extendable
• No encapsulation
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40. Weather application
Let’s look
• Investigate what’s going on
• What is the relationship between the weather
object and the display elements?
• Maybe we can apply a design pattern?
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41. Weather application
Let’s look
• The relationship looks like a newspaper
subscription
• If you want to receive the newspaper, you have to
subscribe yourself at the publisher
• The publisher will send you your newspaper every
time until you unsubscribe
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42. Weather application
So....
• The weather object is the publisher
• The display elements are the people who are
willing to receive the newspaper
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43. Weather application
And that is...
• Yes, that is a design pattern!
• Called ‘The Observer Pattern’
• Publisher == Weather object == Subject
• People == Display elements == Observers
• Subject notifies every observer to update
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44. Weather application
Formal description
• ‘The Observer Pattern defines a one-to-many
dependency between objects so that when one
object changes state, all of its dependents are
notified and updated automatically.’
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47. Weather application
New design principle
• Creating new observers won’t require a change in
the subject
• Changes to either the subject or an observer will
not affect the other
• Reusing a subject or an observer can
independently of each other
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49. Weather application
New design principle
OO Principles
* Encapsulate what varies
* Favour composition over inheritance
* Program to interfaces, not implementations
* Strive for loosely coupled designs between objects that interact
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38
60. Coffee bar
First attempt
• Euhm, this isn’t maintainable, this is a class
explosion!!
• Try again!!
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61. Coffee bar
OK, sorry
• Well that’s better, wouldn’t you agree?
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62. Coffee bar
OK, sorry
• Well that’s better, wouldn’t you agree?
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63. Coffee bar
Really?
• Did you really learn something?
• Do you remember the OO basics and
principles?
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64. Coffee bar
New design principle
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49
65. Coffee bar
New design principle
OO Principles
* Encapsulate what varies
* Favour composition over inheritance
* Program to interfaces, not implementations
* Strive for loosely coupled designs between objects that interact
* Classes should be open for extension, but closed for
modification
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49
66. Coffee bar
What we really want
• We’d like to extend a beverage with condiments
• We wouldn’t like to change the beverage
• We’d like to add new beverages and condiments
without changing the existing code
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50
67. Coffee bar
New design pattern
• The ‘Decorator Design Pattern’ saves our day
• ‘The decorator pattern attaches additional
responsibilities to an object dynamically.
Decorators provide a flexible alternative to
subclassing for extending functionality.’
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70. Coffee bar
Our new design
• This is really great!
• Now we can say:
One HouseBlend coffee with milk and mocha.
What is the price?
€ 1,95
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71. Coffee bar
How does it work?
• Wrap the houseBlend object with Milk object and
wrap that with a mocha object
• Ok, this is hard.
Show me the code man.
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72. Coffee bar
How does it work?
• Code sample
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55
74. Traveller
What we want
• An application which tells us if the
conditions are suitable enough to make the
trip
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75. Traveller
Wait a minute
For example:
• The traveller doesn’t want to make the trip
if the average temperature of the
destination is lower than his minimum
required temperature
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76. Traveller
First implementation
• What do you think?
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77. Traveller
First implementation
• What do you think?
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78. Traveller
First implementation
• Indeed, not very smart!
• Not possible to add requirements
• Requirements changes, so trip class has to
be changed
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81. Traveller
That’s nice
• We are able to create multiple
specifications without changing other
objects
• Readable, maintainable, simple and effective
• And it is a design pattern too!!
‘Hard-coded Specification Pattern’
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82. Traveller
But...
• The specification knows too much
• Not able to make logical specifications
• Does violate some OO principles
You know them by now, right?
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83. Traveller
What we really want
• A design which is flexible enough to
validate almost every object
• Validating against several conditions
• Making logical expressions
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86. Traveller
Wow, that’s heavy
• Code sample
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87. Traveller
This is ...
• ‘The Specification Pattern’ by Martin
Fowler (www.martinfowler.com)
• Flexible, maintainable
• Ready for complex validation
• Loosely coupled
• Most unknown design pattern
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88. Summary
• Best-practise ‘design’ solution
• Lessons learned by other developers
• Based on the OO principles
• Language in-depended
• Puzzles for the creative mind of developer
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91. Summary
OO Principles
OO Basics
* Encapsulate what varies
* Abstraction
* Favour composition over inheritance
* Encapsulation
* Program to interfaces, not implementations
* Polymorphism
* Strive for loosely coupled designs between objects that interact
* Inheritance
* Classes should be open for extension, but closed for
modification
Advanced PHP
