design patterns illustrated-2015-03

Design Patterns illustrated

Herman Peeren, March 17, 2015(Design Patterns illustrations: Nelleke Verhoeff, 2010)

in this presentation I also used some UML-diagramsfrom these handy UML-reference cards:http://www.mcdonaldland.info/2007/11/28/40/

http://www.mcdonaldland.info/2007/11/28/40

Design Patterns ● recipes against common (OO-) programming problems ● code reuse: no need to reinvent the wheel ● common language ● GOF: 23 “classical” patterns

classic, The Book

四人帮

The one constant in software development:

CHANGE!The one constant in software development:

CHANGE!The one constant in software development:

I knew it ...

Ideal: code as modular black boxes

Single responsibility principle

Open for extension, closed for modification

Liskov subsitution principle

Interface segregation

Dependency inversion

Avoid: tight coupling!

It might get you into trouble...

Code smells!

Beware of:

some code smells: ► duplicate code ► long method ► large class ► combinatorial explosion ► conditional complexity ► switch statements ► indecent exposure

Code often starts simple

But “grows”:

► more methods in a class

► longer methods

► more if then but if then while x>0 or and etc.

► more classes, that depends on other classes

and becomes more complex

Example: a shopping cart

► implement putting products in the cart

► start with some pay() method

► if paypal then..., if iDEAL then...

► let’s extract a payment interface

► with several implementations (paypal, iDEAL, etc)

► mutually exclusive choices

each choice is a “strategy” (hurray, our first pattern!)

Encapsulate what varies

Strategy pattern

For instance: different payment possibilities at checkout

Replace Conditional Logic with Strategy if ($income <= 10000) { return $income*0.365; } else if ($income <= 30000) { return ($income-10000)*0.2+35600; } else //etc (...) return ($income-60000)*0.02+105600; } // note: mutual exclusive grouping

if ($income <= 100000) { $strategy = new InsuranceStrategyLow($income); } else if ($income <= 300000) { $strategy = new InsuranceStrategyMedium($income); } else //etc (...) $strategy = new InsuranceStrategyVeryHigh($income); } return $strategy->calculateInsurance();

http://wiki.jetbrains.net/intellij/Replace_conditional_logic_with_strategy_pattern

http://wiki.jetbrains.net/intellij/Replace_conditional_logic_with_strategy_pattern

StrategyWhen something can be done in several ways, make those ways interchangeable.

POSSI-BILITIES

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State example: states of a document

► a draft doesn’t need a depublish() method

► a published document doesn’t need a publish() method

or states of a shopping-cart

► while shopping you don’t need a pay() method

► but you need methods to put product in or out the cart

► when paid you don’t need a pay() method

► nor put_product_in_cart()

StateLet an object show other methods after a change of internal state (as if it changes it’s class).

in a.....hick......different state, ....hick....I behave differently....hick.....

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Bridge example: payment and payment providers

BridgeDecouple an abstraction from its implementation.

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Create those objects

Factory method example: different kinds of customers...

...with different kinds of invoices

Factory MethodProvide an interface for the creation of objects.Allow subclasses to “decide” which class to instantiate.

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Abstract factory example: a Gold customer, cart, invoice, etc.

Abstract FactoryPovide an interface for creating families of related or dependent objects. A factory for factories.

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Building (complex) objects

Example: DI-container

Other example: documentbuilder (for tree-like structures):

/$domtree = new DOMDocument(‘1.0’, ‘UTF-8’);

/* create the root element of the xml tree */$xmlRoot = $domtree->createElement(“xml”);/* append it to the document created */$xmlRoot = $domtree->appendChild($xmlRoot);

$currentTrack = $domtree->createElement(“track”);$currentTrack = $xmlRoot->appendChild($currentTrack);// etc...

BuilderSeperate the construction process (how) of a complex object from the concrete representations (what).

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Encapsulate what doesn’t vary...

Template MethodThe skeleton of an algorithm is fixed, but parts can be filled in differently.

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Add behaviour

► but respect the Open-Closed principle

Decorator

In PHP you can use __call to copy parent methods:

public function __call($method, $args) { return call_user_func_array( array($this->decoratedInstance, $method), $args );}

N.B.: Magic functions are magic... but come at a cost!

DecoratorAdd extra functionallity (at runtime), while keeping the interface the same. Matroushka’s...

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Move Accumulation to Visitor

VisitorMake a kind of plugin-possibility for methods: in that way methods can be added in runtime.

printservice!

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CompositeCreate a tree structure for part-whole hierarchies. A node is also a (part of a) tree. Recursive:

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Interfacing

Unify interfaces with Adapter:For instance: different payment gateways (PayPal, iDEAL, Hipay, Moneybookers, etc.)

Instead of different interfaces refactor to

one preferred interfaceand write adapters for the others

Adapter (= Wrapper)Adapt an interface to an expected interface.

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ProxyProvide a surrogate or placeholder for another object to control access to it.

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FacadeProvide a general (simpler) interface for a set of interfaces.

lookssimple

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Decoupling

ObserverNotify “subscribers” of changes.

WHO?

ME

ME

MENO

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MediatorLayer in between: communication via one object.

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MementoSave and restore the internal state of an object.

ME

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Sequence of actions

Chain of ResponsibilityDefine a method of passing a request among a chain of objects.

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A command is an object to execute 1 method

Decoupling (Symfony2) Forms from Entities:http://verraes.net/2013/04/decoupling-symfony2-forms-from-entities/

Chain of Command: Chain of Responsability with Commands

Replace Conditional Dispatcher with Commandif ($actionName == NEW_WORKSHOP) { //do a lot} else if ($actionName == ALL_WORKSHOPS) { // do a lot of other things } // and many more elseif-statements

NewWorkshopHandler, AllWorkshopsHandler, etc.

http://verraes.net/2013/04/decoupling-symfony2-forms-from-entities/

CommandEncapsulate a command request in an object.

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LIGHTOFF

LIGHTON

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Leftovers

FlyweightUse one instance of a class to provide many “virtual” instances.

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PHP: SPL iterators

► http://www.php.net/manual/en/class.iterator.php ► http://www.php.net/manual/en/spl.iterators.php

Stefan Froelich: ► http://www.sitepoint.com/using-spl-iterators-1/ ► http://www.sitepoint.com/using-spl-iterators-2/

Anthony Ferrara video: ► http://blog.ircmaxell.com/2013/01/todays-programming-with-anthony-vi-

deo.html

http://www.php.net/manual/en/class.iterator.php

http://www.php.net/manual/en/spl.iterators.php

http://www.sitepoint.com/using-spl-iterators-1/

http://www.sitepoint.com/using-spl-iterators-2/

http://blog.ircmaxell.com/2013/01/todays-programming-with-anthony-video.html

http://blog.ircmaxell.com/2013/01/todays-programming-with-anthony-video.html

IteratorEnable sequential access to collection elements, without showing the underlying data-structures (array, list, records, etc)

nextnext

next

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Replace implicit language with Interpreter:

search-methods including combinations: ► belowPriceAvoidingAColor( ) ► byColorAndBelowPrice( ) ► byColorSizeAndBelowPrice( )

interpretable expression:$productSpec = new AndSpec( new BelowPriceSpec(9.00), new NotSpec(newColorSpec(WHITE)) );

“You don’t need an Interpreter for complex languages or for really simple ones.” (Joshua Kerievsky)

InterpreterDomain -> (little) language -> grammar -> objects (DSL)

he means:do this, do that,

and after finishing it, go there!

HÉ!HÉ!

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Javascript:var Person = function() { // bladibla };var Customer = function(name) { this.name = name;};Customer.prototype = new Person();

Prototype in PHP: ► adding properties is easy ► adding behaviour is less obvious, but... ► CLOSURES can help here, with some (dirty) tricks

this.name

Customer.prototype

PrototypeMake variations on copies of a basic-object.

COPY-SERVICE

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SingletonEnsure a class only has one instance, and provide a global point of access to it.

Oh, I’m soloooooooonly

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SingletonEnsure a class only has one instance, and provide a global point of access to it. Did anybody say GLOBAL???


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“Every advantage

has its disadvantages”

(free to Johan Cruyff, Dutch Football Pattern Designer and Ajax-fan...)

Résumé

Classic pattern categories

creational, structural and behavioral patterns:

► creational: object instantiation

► structural: larger structures of classes or objects

► behavioral: interaction and distribution of responsibility

Other categorisations

Loek Bergman (dev. from Rotterdam):

► transformational

► transportational

► translational

Anthony Ferrara:

► Shim : not necessary (for PHP)

► decompositional: breaking objects apart

► compositional: making things simpler by assembling

http://loekbergman.nl/InsideArchitecture

http://blog.ircmaxell.com/2013/09/beyond-design-patterns.html

http://loekbergman.nl/InsideArchitecture

http://blog.ircmaxell.com/2013/09/beyond-design-patterns.html

Creational design patterns

► Factory Method: Allow subclasses to “decide” which class to instantiate.

► Abstract Factory: Encapsulate a set of analo- gous factories that produce families of objects.

► Builder: Encapsulate the construction of complex objects from their representation; so, the same building process can create various representations by specifying only type and content.

► Singleton: Ensure that only a single instance of a class exists and provide a single method for gaining access to it.

► Prototype: Create an initialized instance for cloning or copying.

Factory MethodProvide an interface for the creation of objects.Allow subclasses to “decide” which class to instantiate.

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Abstract FactoryPovide an interface for creating families of related or dependent objects. A factory for factories.

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SingletonEnsure a class only has one instance, and provide a global point of access to it. Did anybody say GLOBAL???


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Structural design patterns ● Adapter: Adapt an interface to an expected interface.

● Bridge: Decouple an interface from its implementation.

● Composite: Create a tree structure for part-whole hierarchies.

● Decorator: Extend functionality dynamically.

● Facade: Simplify usage by defining a high-level interface.

● Flyweight: Support fine-grained objects efficiently by sharing.

● Proxy: Represent an object with another object for access control.

BridgeDecouple an abstraction from its implementation.

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Behavioral design patterns ● Chain of Responsibility: Define a method of passing a request among a chain of objects. ● Command: Encapsulate a command request in an object. ● Interpreter: Allow inclusion of language elements in an appli-cation. ● Iterator: Enable sequential access to collection elements. ● Mediator: Define simplified communication between classes. ● Memento: Save and restore the internal state of an object. ● Observer: Define a scheme for notifying objects of changes to another object. ● State: Alter the behavior of an object when its state changes. ● Strategy: Encapsulate an algorithm inside a class. ● Template Method: Allow subclasses to redefine the steps of an algorithm. ● Visitor: Define a new operation on a class without changing it.

InterpreterDomain -> (little) language -> grammar -> objects (DSL)

he means:do this, do that,

and after finishing it, go there!

HÉ!HÉ!

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IteratorEnable sequential access to collection elements, without showing the underlying data-structures (array, list, records, etc)

nextnext

next

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StateLet an object show other methods after a change of internal state (as if it changes it’s class).

in a.....hick......different state, ....hick....I behave differently....hick.....

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VisitorMake a kind of plugin-possibility for methods: in that way methods can be added in runtime.

printservice!

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Some booksGOF: 23 “classical” patterns:

classic, The Book

fun!

good start

Dec. 2013Simple

Febr. 2013Selection

PHP-examples

PHP and Design Patterns

Fowler: architectural patterns for enterprise applications

Fowler: also known from refactoring

Kerievsky: refactoring to patterns

PEAA & Refactoring

Resign Patterns: Ailments of Unsuitable Project-Disoriented Software

Questions?

Contact info:Herman [email protected]

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