SOLID PRINCIPLES

SOLID PRINCIPLES
LUCIANO QUEIROZ

SOLID
PRINCIPLES
Robert Martin aka Uncle Bob

SINGLE RESPONSIBILITY PRINCIPLE
“A CLASS SHOULD HAVE ONE,  
AND ONLY ONE, REASON TO
CHANGE.”
SOLID PRINCIPLES

<?php
class ConfirmationMailMailer
{
private $templating;
private $mailer;
public function __construct(
ITemplatingEngine $templating,
IMailer $mailer
) {
$this->templating = $templating;
$this->mailer = $mailer;
}
public function sendTo(User $user)
{
$message = $this->createMessageFor($user);
$this->sendMessage($message);
}
private function createMessageFor(User $user)
{
$subject = 'Confirm your email address';
$body = $this->templating
->render(
'confirmationMail.html.tpl', [
'confirmationCode' => $user->getConfirmationCode()
]
);
$message = new Message($subject, $body);
$message->setTo($user->getEmailAdress());
return $message;
}
private function sendMessage(IMessage $message)
{
$this->mailer->send($message);
}

SOLID PRINCIPLES
INITIAL SITUATION
ConfirmationMailMailer.php
IMailer.php
usesuses
ITemplatingEngine.php

SOLID PRINCIPLES
RESPONSIBILITIES
▸ Create Conﬁrmation Message
▸ Send Conﬁrmation Message to user

SINGLE RESPONSIBILITY PRINCIPLE
“RESPONSIBILITIES ARE
REASONS TO CHANGE”
SOLID PRINCIPLES

SOLID PRINCIPLES
SINGLE RESPONSIBILITY PRINCIPLE VIOLATIONS
▸ Many instance variables
▸ Many public methods
▸ Each method use variables from other instance
▸ Speciﬁc tasks are done by private methods

<?php
class ConfirmationMailFactory
{
private $templating;
public function __construct(ITemplatingEngine $templating)
{
$this->templating = $templating;
}
public function createMessageFor(User $user)
{
$subject = 'Confirm your email address';
$body = $this->templating
->render(
'confirmationMail.html.tpl', [
'confirmationCode' => $user->getConfirmationCode()
]
);
$message = new Message($subject, $body);
$message->setTo($user->getEmailAdress());
return $message;
}
}

<?php
class ConfirmationMailMailer
{
private $confirmationMailFactory;
private $mailer;
ConfirmationMailFactory $confirmationMailFactory,
IMailer $mailer
) {
$this->confirmationMailFactory = $confirmationMailFactory;
$this->mailer = $mailer;
}
public function sendTo(User $user)
{
$message = $this->confirmationMailFactory->createMessageFor($user);
$this->sendMessage($message);
}
private function sendMessage(IMessage $message)
{
$this->mailer->send($message);
}
}

SOLID PRINCIPLES
REFACTORED
ConfirmationMailMailer.php IMailer.phpuses
ConfirmationMailFactory.php
uses
ITemplatingEngine.phpuses

SOLID PRINCIPLES
SINGLE RESPONSIBILITY PRINCIPLE ADVANTAGES
▸ Smaller classes
▸ Classes easier to understand
▸ Classes easier to test
▸ Classes simpler to maintain

OPEN/CLOSED PRINCIPLE
“YOU SHOULD BE ABLE TO
EXTEND A CLASS`S BEHAVIOR,
WITHOUT MODIFYING IT.”
SOLID PRINCIPLES

<?php
class GenericEncoder
{
public function encodeToFormat($data, $format)
{
if ($format === 'json') {
$encoder = new JsonEncoder();
} elseif ($format === 'xml') {
$encoder = new XmlEncoder();
} else {
throw new InvalidArgumentException('Unknown format');
}
$data = $this->prepareData($data, $format);
return $encoder->encode($data);
}
private function prepareData($data, $format)
{
switch ($format) {
case 'json':
$data = $this->forceArrayKeys($data);
$data = $this->fixKeys($data);
break;
case 'xml':
$data = $this->fixAttributes($data);
break;
default:
throw new InvalidArgumentException(
'Format not supported'
);
}
return $data;
}
}

SOLID PRINCIPLES
INITIAL SITUATION
GenericEncoder.php
XmlEncoder.php
uses
uses
JsonEncoder.php

<?php
{
{
if (…) {
…
} elseif (…) {
…
} elseif ($format === 'yaml') {
$encoder = new YamlEncoder();
} else {
}
}
…
}

SOLID PRINCIPLES
NEW REQUIREMENT SITUATION
GenericEncoder.php
XmlEncoder.php
uses
uses
JsonEncoder.php
YamlEncoder.php
uses

OPEN/CLOSED PRINCIPLE
“A UNIT OF CODE CAN BE
CONSIDERED ‘OPEN’ FOR EXTENSION
WHEN ITS BEHAVIOR CAN BE EASILY
CHANGED WITHOUT MODIFYING IT”
SOLID PRINCIPLES

SOLID PRINCIPLES
OPEN/CLOSED PRINCIPLE VIOLATIONS
▸ Conditions to determine a strategy
▸ Conditions using the same variables or constants are
recurring inside the class or related classes
▸ Contains hard-coded references to other classes or class
names
▸ Objects are being created using the new operator

<?php
interface EncoderInterface
{
public function encode($data);
}
class JsonEncoder implements EncoderInterface
{
public function encode($data)
{
// TODO: Implement encode() method.
}
}
class XmlEncoder implements EncoderInterface
{
{
}
}
class YamlEncoder implements EncoderInterface
{
{
}
}

SOLID PRINCIPLES
INTRODUCING ENCODER INTERFACE
GenericEncoder.php
XmlEncoder.php
uses
JsonEncoder.phpYamlEncoder.php
implements
EncoderInterface.php
implements
implements

<?php
class EncoderFactory
{
public function createForFormat($format)
{
if ($format === 'json') {
return new JsonEncoder();
} elseif ($format === 'xml') {
return new XmlEncoder();
} elseif ($format === 'yaml') {
return new YamlEncoder();
}
}
}

<?php
{
private $encoderFactory;
public function __construct(EncoderFactory $encoderFactory)
{
$this->encoderFactory = $encoderFactory;
}
{
$encoder = $this->encoderFactory->createForFormat($format);
$data = $this->prepareData($data, $format);
}
}

ABSTRACT FACTORY
OPEN FOR EXTENSION

<?php
interface EncoderFactoryInterface
{
/**
* Create an encoder for the given format
*
* @param string $format
* @return EncoderInterface
*/
}
class EncoderFactory implements EncoderFactoryInterface
{
...
}
{
EncoderFactoryInterface $encoderFactory
) {
...
}
...
}

INTRODUCING DEPENDENCY INVERSION PRINCIPLE
SOLID PRINCIPLES
GenericEncoder.php
XmlEncoder.php
uses
JsonEncoder.phpYamlEncoder.php
implements
EncoderInterface.php
implements
implements
EncoderFactoryInterface.php
uses
creates

<?php
class EncoderFactory implements EncoderFactoryInterface
{
private $factories = [];
/**
* Register a callable that returns an instance of
* EncoderInterface for the given format.
*
* @param string $format
* @param callable $factory
*/
public function addEncoderFactory($format, callable $factory)
{
$this->factories[$format] = $factory;
}
{
$factory = $this->factories[$format];
$encoder = $factory;
return $encoder;
}
}

<?php
$encoderFactory = new EncoderFactory();
$encoderFactory->addEncoderFactory(
'xml',
function () {
return new XmlEncoder();
}
);
$encoderFactory->addEncoderFactory(
'json',
function () {
return new JsonEncoder();
}
);
$genericEncoder = new GenericEncoder($encoderFactory);
$data = ...
$jsonEncodedData = $genericEncoder->encode($data, 'json');

<?php
{
private function prepareData($data, $format)
{
switch ($format) {
case 'json':
$data = $this->forceArray($data);
break;
case 'xml':
$data = $this->fixAttributes($data);
break
default:
# code...
break;
}
return $data;
}
}

<?php
{
) {
$this->encoderFactory;
}
{
$data = $encoder->prepareData($data);
}
}

<?php
{
{
...
}
public function prepareData($data)
{
return $data;
}
}

<?php
{
{
$data = $this->prepareData($data);
return json_encode($data);
}
private function prepareData($data)
{
return $data;
}
}

<?php
interface EncoderInterface
{
public function encode($data);
}

<?php
{
) {
$this->encoderFactory;
}
{
}
}

LISKOV SUBSTITUTION PRINCIPLE
“DERIVED CLASSES MUST
BE SUBSTITUTABLE FOR
THEIR BASE CLASSES”
SOLID PRINCIPLES

SOLID PRINCIPLES
WE NEED TO UNDERSTAND
▸ Derived classes
▸ Being substitutable

<?php
/**
* A concrete class, all methods are implemented, but can be
* overridden by derived classes
*/
class ConcreteClass
{
public function implementedMethod()
{
}
}
/**
* An abstract class, some methods need to be implemented by
* by derived classes
*/
abstract class AbstractClass
{
abstract public function abstractMethod();
public function implementedMethod()
{
}
}
/**
* An interface, all methods need to be implemented by derived
* classes
*/
interface AnInterface
{
public function abstractMethod();
}

“A DERIVED CLASS DOES NOT
HAVE AN IMPLEMENTATION
FOR ALL METHODS”
SOLID PRINCIPLES

<?php
interface FileInterface
{
public function rename($name);
public function changeOwner($user, $group);
}

<?php
class DropboxFile implements FileInterface
{
public function rename($name)
{
...
}
public function changeOwner($user, $group)
{
throw new BadMethodCallException(
"Not implemented for Dropbox files"
);
}
}

<?php
{
{
...
}
{
throw new BadMethodCallException(
"Not implemented for Dropbox files"
);
}
}
//ask the user to check if $file is an instance of DropboxFile
if (!($file instanceof DropboxFile)) {
$file->changeOwner(...);
}

<?php
{
...
{
//let's do nothing :)
}
}

INTERFACE SEGREGATION PRINCIPLE
“MAKE FINE-GRAINED
INTERFACES THAT ARE
CLIENT SPECIFIC”
SOLID PRINCIPLES

<?php
interface FileInterface
{
public function rename($name);
}
interface FileWithOwnerInterface extends FileInterface
{
public function changeOwner($user, $group);
}

<?php
{
{
...
}
}
class LocalFile implements FileWithOwnerInterface
{
{
...
}
{
...
}
}

SOLID PRINCIPLES
NEW HIERARCHY OF FILE CLASSES
DropboxFile.php
extends
LocalFile.php
implements
FileInterface.php
implements
FileWithOwnerInterface.php

“DIFFERENT SUBSTITUTES
RETURN THINGS OF
DIFFERENT TYPES”
SOLID PRINCIPLES

<?php
interface RouterInterface
{
public function getRoutes() : RouterCollection;
}
class SimpleRouter implements RouterInterface
{
public function getRoutes()
{
$routes = [];
//add Route objects to $routes
$routes[] = ...;
return $routes
}
}
class AdvancedRouter implements RouterInterface
{
public function getRoutes()
{
$routeCollection = new RouteCollection();
...
return $routeCollection;
}
}

<?php
interface RouterInterface
{
public function getRoutes() : RouterCollection;
}
class SimpleRouter implements RouterInterface
{
public function getRoutes() : RouteCollection
{
$routes = [];
//add Route objects to $routes2
$routes[] = ...;
return $routes
}
}
class AdvancedRouter implements RouterInterface
{
public function getRoutes() : RouteCollection
{
$routeCollection = new RouteCollection();
...
return $routeCollection;
}
}

“A DERIVED CLASS IS LESS
PERMISSIVE WITH REGARD
TO METHOD ARGUMENTS”
SOLID PRINCIPLES

<?php
interface MassMailerInterface
{
public function sendMail(
TransportInterface $transport,
MessageInterface $message,
RecipientsInterface $recipients
);
}

<?php
class SmtpMassMailer implements MassMailerInterface
{
TransportInterface $transport,
) {
if (!($transport instanceof SmtpTransport)) {
throw new InvalidArgumentException(
'SmtpMassMailer only works with SMTP'
);
}
}
}

“NOT EVERY KIND OF MAIL
TRANSPORT IS SUITABLE
FOR MASS MAILING”
SOLID PRINCIPLES

SOLID PRINCIPLES
NEW CLASS DIAGRAM
SmtpTransport.php
TransportInterface.php
extends
TransportWithMassMailSupportInterface.php
implements

<?php
interface MassMailerInterface
{
TransportWithMassMailSupportInterface $transport,
);
}

<?php
class SmtpMassMailer implements MassMailerInterface
{
TransportWithMassMailSupportInterface $transport,
) {
…
}
}

SOLID PRINCIPLES
A GOOD SUBSTITUTE
▸ Provides an implementation for all the methods of the
base class.
▸ Returns the type of things the base class prescribes.
▸ Doesn’t put extra constraints on arguments for methods.

DEPENDENCY INVERSION PRINCIPLE
“DEPEND ON ABSTRACTIONS,
NOT CONCRETIONS”
SOLID PRINCIPLES

SOLID PRINCIPLES
FIZZ BUZZ ASSIGNMENT
▸ Generate a list of integers, from 1 to n.
▸ Numbers that are divisible by 3 should be replace with
“Fizz”
▸ Numbers that are divisible by 5 should be replaced with
“Buzz”
▸ Numbers that are both divisible by 3 and by 5 should be
replaced with “FizzBuzz”

<?php
class FizzBuzz
{
public static function generateList($limit)
{
$list = [];
for ($number = 1; $number <= $limit; $number++) {
$list[] = self::generateElement($number);
}
return $list;
}
private static function generateElement($number)
{
if ($number % 3 === 0 && $number % 5 === 0) {
return 'FizzBuzz';
}
if ($number % 3 === 0) {
return 'Fizz';
}
if ($number % 5 === 0) {
return 'Buzz';
}
return $number;
}
}

SOLID PRINCIPLES
THEN…
▸ It should be possible to add another rule, without
modifying the FizzBuzz class.

MAKING FIZZBUZZ
OPEN FOR EXTENSION

<?php
class FizzBuzz
{
{
...
}
{
$fizzBuzzRule = new FizzBuzzRule();
if ($fizzBuzzRule->matches($number)) {
return $fizzBuzzRule->getReplacement();
}
$fizzRule = new FizzRule();
if ($fizzRule->matches($number)) {
return $fizzRule->getReplacement();
}
$buzzRule = new BuzzRule();
if ($buzzRule->matches($number)) {
return $buzzRule->getReplacement();
}
return $number;
}
}

<?php
interface RuleInterface
{
public function matches($number);
public function getReplacement();
}

<?php
class FizzBuzz
{
private $rules = [];
public function addRule(RuleInterface $rule) {
$this->rules[] = $rule;
}
{
...
}
{
foreach ($this->rules as $rule) {
if ($rule->matches($number)) {
return $rule->getReplacement();
}
}
return $number;
}
}

<?php
class FizzBuzzRule implements RuleInterface
{
...
}
class FizzRule implements RuleInterface
{
...
}
class BuzzRule implements RuleInterface
{
...
}

<?php
$fizzBuzz = new FizzBuzz();
$fizzBuzz->addRule(new FizzBuzzRule());
$fizzBuzz->addRule(new FizzRule());
$fizzBuzz->addRule(new BuzzRule());
...
$list = $fizzBuzz->generateList(100);

SOLID PRINCIPLES
HAVING CONCRETE DEPENDENCIES
FizzBuzz.php
FizzRule.php
BuzzRule.php
FizzBuzzRule.php

SOLID PRINCIPLES
HAVING ABSTRACT DEPENDENCIES
FizzBuzz.php RuleInterface.php
BuzzRule.phpFizzBuzzRule.php FizzRule.php

DEPENDENCY INVERSION PRINCIPLE VIOLATION
“MIXING DIFFERENT
LEVELS OF ABSTRACTION”
SOLID PRINCIPLES

<?php
use DoctrineDBALConnection;
class Authentication
{
private $connection;
public function __construct(Connection $connection)
{
$this->connection = $connection;
}
public function checkCredentials($username, $password)
{
$user = $this->connection->fetchAssoc(
'SELECT * FROM users WHERE username = ?',
[$username]
);
if ($user === null) {
throw new InvalidCredentialsException(
"User not found."
);
}
//validate password
...
}
}

SOLID PRINCIPLES
AUTHENTICATION CLASS DEPENDS ON CONNECTION
Authentication.php Connection.php

<?php
{
private $userRepository;
public function __construct(UserRepository $userRepository)
{
$this->userRepository = $userRepository;
}
public function checkCredentials($username, $password)
{
$user = $this->userRepository->ofUsername($username);
if ($user === null) {
throw new InvalidCredentialsException(
"User not found."
);
}
//validate password
...
}
}

<?php
class UserRepository
{
private $connection;
public function __construct(Connection $connection)
{
$this->connection = $connection;
}
public function ofUsername($username)
{
return $this->connection->fetchAssoc(
'SELECT * FROM users WHERE username = ?',
[$username]
);
}
}

SOLID PRINCIPLES
AUTHENTICATION CLASS DEPENDS ON USER REPOSITORY
Authentication.php UserRepository.php Connection.php

<?php
interface UserRepositoryInterface
{
public function ofUsername($username);
}

<?php
class DoctrineDbalUserRepository implements UserRepositoryInterface
{
...
}
class TextFileUserRepository implements UserRepositoryInterface
{
...
}
class InMemoryUserRepository implements UserRepositoryInterface
{
...
}
class MongoDbUserRepository implements UserRepositoryInterface
{
...
}

<?php
{
private $userRepository;
public function __construct(UserRepositoryInterface $userRepository)
{
$this->userRepository = $userRepository;
}
}

SOLID PRINCIPLES
AUTHENTICATION CLASS DEPENDS ON USER REPOSITORY INTERFACE
Authentication.php UserRepositoryInterface.php
TextFileUserRepository.php DoctrineDbalUserRepository.php Connection.php

SOLID PRINCIPLES

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