The document discusses SOLID principles of object-oriented programming and provides an example of refactoring a user registration class to follow these principles. Specifically, it starts with a monolithic user registration class that handles all steps of registration. It then refactors the class to separate concerns by introducing new classes for user validation, persistence, email confirmation, and more. Dependencies are injected through the constructor rather than retrieved internally. This improves modularity and makes the code easier to change and extend.

1. SOLID In Practice
@jessicamauerhan | http://bit.ly/solid-oop

2. @jessicamauerhan
Software Engineer
@HelpScout
jessicamauerhan@gmail.com
jmauerhan.wordpress.com

3. @jessicamauerhan | http://bit.ly/solid-oop
What is SOLID?
● Five principles for Object
Oriented Programming
● Guidelines which can help
ensure system is easy to
maintain
● Primarily focused on
communication between
dependencies or
collaborators

4. Let ϕ(픁) be a property provable
about objects 픁 of type T.
Then ϕ(퐲) should be true for
objects 퐲 of type S where S is a
subtype of T

5. @jessicamauerhan | http://bit.ly/solid-oop
Class Rectangle
{
protected $length;
protected $height;
public function area()
{
return $this-length * $this-area();
}
public function setLength($length)
{
$this-length = $length;
}
public function setHeight($height)
{
$this-height = $height;
}
}
Class Square extends Rectangle
{
public function setLength($length)
{
$this-length = $length;
$this-height = $length;
}
public function setHeight($height)
{
$this-length = $height;
$this-height = $height;
}
}

6. http://bit.ly/solid-oop

7. @jessicamauerhan | http://bit.ly/solid-oop
S.O.L.I.D.
● Single Responsibility
● Open/Closed
● Liskov Substitution
● Interface Segregation
● Dependency Inversion

8. It's All About Change

9. @jessicamauerhan | http://bit.ly/solid-oop
User Registration
● Collect User Input
● Validate User Input
● Persist User Data
● Send Confirmation Email

10. Class UserRegistration
{
public function register(Request $request): Response
{
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
}
}

11. Class UserRegistration
{
public function register(Request $request): Response
{
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
}
private function emailIsRegistered(string $email): bool
{
$dsn = 'host='.$_ENV['DB_HOST'].' dbname='.$_ENV['DB_DB'].' password='.$_ENV['DB_PASS'].' user='.$_ENV['DB_USER'];
$db = pg_connect($dsn);
$result = pg_query($db, SELECT id FROM users WHERE (email='{$email}'));
$rows = pg_num_rows($result);
return $rows 0;
}
}

12. Class UserRegistration
{
public function register(Request $request): Response
{
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
//Validate user input
if ($this-emailIsRegistered($user-getEmail())) {
return new Response('register', ['error' = 'Your email address is already registered']);
}
}
private function emailIsRegistered(string $email): bool
{
$dsn = 'host='.$_ENV['DB_HOST'].' dbname='.$_ENV['DB_DB'].' password='.$_ENV['DB_PASS'].' user='.$_ENV['DB_USER'];
$db = pg_connect($dsn);
$result = pg_query($db, SELECT id FROM users WHERE (email='{$email}'));
$rows = pg_num_rows($result);
return $rows 0;
}
}

13. Class UserRegistration
{
public function register(Request $request): Response
{
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
//Validate user input
if ($this-emailIsRegistered($user-getEmail())) {
return new Response('register', ['error' = 'Your email address is already registered']);
}
//Persist User
$this-saveUser($user);
}
private function emailIsRegistered(string $email): bool { [...] }
private function saveUser(User $user)
{
$dsn = 'host='.$_ENV['DB_HOST'].' dbname='.$_ENV['DB_DB'].' password='.$_ENV['DB_PASS'].' user='.$_ENV['DB_USER'];
$db = pg_connect($dsn);
pg_query($db, INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}'));
}
}

14. Class UserRegistration
{
public function register(Request $request): Response
{
[...]
}
private function emailIsRegistered(string $email): bool
{
$dsn = 'host='.$_ENV['DB_HOST'].' dbname='.$_ENV['DB_DB'].' password='.$_ENV['DB_PASS'].' user='.$_ENV['DB_USER'];
$db = pg_connect($dsn);
$result = pg_query($db, SELECT id FROM users WHERE (email='{$email}'));
$rows = pg_num_rows($result);
return $rows 0;
}
private function saveUser(User $user)
{
$dsn = 'host='.$_ENV['DB_HOST'].' dbname='.$_ENV['DB_DB'].' password='.$_ENV['DB_PASS'].' user='.$_ENV['DB_USER'];
$db = pg_connect($dsn);
pg_query($db, INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}'));
}
}

15. Class UserRegistration
{
public function register(Request $request): Response
{
[...]
}
private function emailIsRegistered(string $email): bool
{
$rows = DB::query(SELECT count(id) FROM users WHERE (email='{$email}'));
return $rows 0;
}
private function saveUser(User $user)
{
DB::query(INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}'));
}
}

16. Class UserRegistration
{
public function register(Request $request): Response
{
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
//Validate user input
if ($this-emailIsRegistered($user-getEmail())) {
return new Response('register', ['error' = 'Your email address is already registered']);
}
//Persist User
$this-saveUser($user);
//Send Confirmation Email
$this-sendConfirmationEmail($user-getEmail());
return new Response('register', ['success' = 'You are registered! Please check your email!']);
}
private function sendConfirmationEmail(string $email)
{
$subject = Confirm Email;
$message = Please aclick here/a to confirm your email!;
$headers = From: mysite@email.com;
mail($email, $subject, $message, $headers);
}

17. Class UserRegistration
{
public function register(Request $request): Response {
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
//Validate user input
if ($this-emailIsRegistered($user-getEmail())) {
return new Response('register', ['error' = 'Your email address is already registered']);
}
//Persist User
$this-saveUser($user);
//Send Confirmation Email
$this-sendConfirmationEmail($user-getEmail());
return new Response('register', ['success' = 'You are registered! Please check your email!']);
}
private function sendConfirmationEmail(string $email) {
$subject = Confirm Email;
$message = Please aclick here/a to confirm your email!;
$headers = From: mysite@email.com;
mail($email, $subject, $message, $headers);
}
private function saveUser(User $user) {
DB::query(INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}'));
}
private function emailIsRegistered(string $email): bool {
$rows = DB::query(SELECT count(id) FROM users WHERE (email='{$email}'));
return $rows 0;
}
}

18. Class UserRegistration
{
public function register(Request $request): Response {
//Collect user input
if (!$request-has('email') || !$request-has('password')) {
return new Response('register', ['error' = 'Please provide an email and a password']);
}
$user = new User();
$user-email = $request-get('email');
$user-password = $request-get('password');
//Validate user input
if ($this-emailIsRegistered($user-getEmail())) {
return new Response('register', ['error' = 'Your email address is already registered']);
}
//Persist User
$this-saveUser($user);
//Send Confirmation Email
$this-sendConfirmationEmail($user-getEmail());
return new Response('register', ['success' = 'You are registered! Please check your email!']);
}
private function sendConfirmationEmail(string $email) {
$subject = Confirm Email;
$message = Please aclick here/a to confirm your email!;
$headers = From: mysite@email.com;
mail($email, $subject, $message, $headers);
}
private function saveUser(User $user) {
DB::query(INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}'));
}
private function emailIsRegistered(string $email): bool {
$rows = DB::query(SELECT count(id) FROM users WHERE (email='{$email}'));
return $rows 0;
}
}

19. @jessicamauerhan | http://bit.ly/solid-oop
Dependency Injection
● Collaborators are supplied to
class from outside - aka
injected into it
Service Location
● Collaborators are retrieved or
instantiated from inside the
class using them

20. Class UserRegistration
{
public function register(Request $request): Response { [...] }
private function sendConfirmationEmail(string $email) { [...] }
private function saveUser(User $user)
{
$sql = INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}')
DB::query($sql);
}
private function emailIsRegistered(string $email): bool
{
$rows = DB::query(SELECT count(id) FROM users WHERE (email='{$email}'));
return $rows 0;
}
}

21. Class UserRegistration
{
private $db;
public function __construct(Database $db)
{
$this-db = $db;
}
public function register(Request $request): Response { [...] }
private function sendConfirmationEmail(string $email) { [...] }
private function saveUser(User $user)
{
$sql = INSERT INTO users(email, password) VALUES('{$user-email}', '{$user-password}')
$this-db-query($sql);
}
private function emailIsRegistered(string $email): bool
{
$rows = $this-db-query(SELECT count(id) FROM users WHERE (email='{$email}'));
return $rows 0;
}
}

22. Class UserRegistration
{
private $db;
private $mailer;
public function __construct(Database $db, Mailer $mailer)
{
$this-db = $db;
$this-mailer = $mailer;
}
public function register(Request $request): Response { [...] }
private function sendConfirmationEmail(string $email)
{
//Convert our email to Mandrill's email
$mandrillEmail = [
'to' = ['email' = $email-getTo()],
'from_email' = $email-getFrom(),
'subject' = $email-getSubject(),
'text' = $email-getMessage()
];
$this-mandrill-send($mandrillEmail);
}
private function saveUser(User $user) { [...] }
private function emailIsRegistered(string $email): bool { [...] }

23. Dependency Inversion

24. Single Responsibility Principle

25. Class UserRegistration
{
private $factory;
public function __construct(UserFactory $factory)
{
$this-factory = $factory;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
}
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}

26. interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
Class UserRegistration
{
private $factory, $validator;
public function __construct(UserFactory $factory, UserValidator $validator)
{
$this-factory = $factory;
$this-validator = $validator;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
}
}

27. Class UserRegistration
{
private $factory, $validator, $repository;
public function __construct(UserFactory $factory, UserValidator $validator,
UserRepository $repository)
{
$this-factory = $factory;
$this-validator = $validator;
$this-repository = $repository;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
try {
$this-repository-create($user);
return UserCreatedResponse($user);
} catch (UnableToCreateUser $exception) {
return UnableToCreateUserResponse($user);
}
}
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
interface UserRepository
{
/**
* @throws UnableToCreateUser
*/
public function create(User $user): bool;
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}

28. Class UserRegistration
{
private $factory, $validator, $repository, $emailer;
public function __construct(UserFactory $factory, UserValidator $validator,
UserRepository $repository, ConfirmationEmailer $emailer)
{
$this-factory = $factory;
$this-validator = $validator;
$this-repository = $repository;
$this-emailer = $emailer;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
try {
$this-repository-create($user);
$this-emailer-sendConfirmationEmail($user);
return UserCreatedResponse($user);
} catch (UnableToCreateUser $exception) {
return UnableToCreateUserResponse($user);
} catch (MailerException $mailerException) {
return UnableToSendConfirmationEmailResponse($user);
}
}
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
interface UserRepository
{
/**
* @throws UnableToCreateUser
*/
public function create(User $user): bool;
}
interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
interface UserRepository
{
/**
* @throws UnableToCreateUser
*/
public function create(User $user): bool;
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}

29. Class UserRegistration
{
private $factory;
public function __construct(UserFactory $factory)
{
$this-factory = $factory;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
}
}
Class UserRegistration
{
private $factory, $validator;
public function __construct(UserFactory $factory, UserValidator $validator)
{
$this-factory = $factory;
$this-validator = $validator;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
}
}
Class UserRegistration
{
private $factory, $validator, $repository;
public function __construct(UserFactory $factory, UserValidator $validator,
UserRepository $repository)
{
$this-factory = $factory;
$this-validator = $validator;
$this-repository = $repository;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
try {
$this-repository-create($user);
return UserCreatedResponse($user);
} catch (UnableToCreateUser $exception) {
return UnableToCreateUserResponse($user);
}
}
}
Class UserRegistration
{
private $factory, $validator, $repository, $emailer;
public function __construct(UserFactory $factory, UserValidator $validator,
UserRepository $repository, ConfirmationEmailer $emailer)
{
$this-factory = $factory;
$this-validator = $validator;
$this-repository = $repository;
$this-emailer = $emailer;
}
public function register(Request $request): Response
{
try {
$user = $this-factory-createUser($request);
} catch (UnableToParseUser $exception) {
return new InvalidRequestResponse($exception-getErrors());
}
if ($this-validator-isValid($user) === false) {
$invalidProperties = $this-validator
-getInvalidProperties($user);
return new InvalidUserResponse($invalidProperties);
}
try {
$this-repository-create($user);
$this-emailer-sendConfirmationEmail($user);
return UserCreatedResponse($user);
} catch (UnableToCreateUser $exception) {
return UnableToCreateUserResponse($user);
} catch (MailerException $mailerException) {
return UnableToSendConfirmationEmailResponse($user);
}
}
}
interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createRegisteringUser(Request $request): User;
}
interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
interface UserRepository
{
/**
* @throws UnableToCreateUser
*/
public function create(User $user): bool;
}
interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}

30. interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createUser(Request $request): User;
}

31. interface UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createUser(Request $request): User;
}
class RegisterFormUserFactory implements UserFactory
{
/**
* @throws UnableToParseUser
*/
public function createUser(Request $request): User
{
if(!$request-has('email') || !$request-has('password')){
throw new UnableToParseUser();
}
$email = $request-get('email');
$password = $request-get('password');
return new User($email, $password);
}
}

32. interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}

33. interface UserValidator
{
public function isValid(User $user): bool;
public function getInvalidProperties(User $user):array;
}
class NewUserValidator implements UserValidator
{
/** @var UserRepository */
private $userRepository;
public function __construct(UserRepository $userRepository)
{
$this-userRepository = $userRepository;
}
public function isValid(User $user): bool
{
$emailExists = $this-userRepository-emailExists($user-getEmail());
if ($emailExists === true) {
return false;
}
return true;
}
}

34. interface UserRepository
{
/**
* @param User $user
* @return bool
*
* @throws UnableToCreateUser
*/
public function create(User $user): bool;
/**
* @param string $email
* @return bool
*
* @throws DataSourceError
*/
public function emailExists(string $email): bool;
}

35. class PdoUserRepository implements UserRepository
{
/** @var Pdo */
private $pdo;
public function __construct(Pdo $pdo) {
$this-pdo = $pdo;
}
public function create(User $user): bool {
try {
$statement = $this-pdo-prepare(INSERT INTO users(email, password) VALUES(:email, :password));
$statement-execute(['email' = $user-getEmail(), 'password' = $user-getPassword()]);
} catch (PDOException $PDOException) {
throw new UnableToCreateUser($user-getEmail());
}
return true;
}
public function emailExists(string $email): bool {
try {
$statement = $this-pdo-prepare(SELECT count(id) AS numRows FROM users WHERE email = :email);
$statement-execute([email = $email]);
$result = $statement-fetchColumn();
return ($result 0);
} catch (PDOException $PDOException) {
throw new DataSourceError();
}
}
}

36. interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}

37. interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}
class MandrillConfirmationEmailer implements ConfirmationEmailer
{
private $mandrill;
public function __construct(Mandrill $mandrill)
{
$this-mandrill = $mandrill;
}
public function send(User $user)
{
$message = Please aclick here/a to confirm your account.;
$from = app@example.com;
$subject = Confirm your account;
$email = [
'to' = [['EmailTemplate' = $user-getEmail()]],
'from_email' = $from,
'subject' = $subject,
'text' = $message
];
$this-mandrill-send($email);
}
}

38. interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}
class MandrillConfirmationEmailer implements ConfirmationEmailer
{
private $mandrill;
public function __construct(Mandrill $mandrill)
{
$this-mandrill = $mandrill;
}
public function send(User $user)
{
$message = Please aclick here/a to confirm your account.;
$from = app@example.com;
$subject = Confirm your account;
$email = [
'to' = [['EmailTemplate' = $user-getEmail()]],
'from_email' = $from,
'subject' = $subject,
'text' = $message
];
$this-mandrill-send($email);
}
}

39. interface ConfirmationEmailer
{
/**
* @throws MailerException
*/
public function send(string $to);
}
class MandrillConfirmationEmailer implements ConfirmationEmailer
{
private $mandrill;
public function __construct(Mandrill $mandrill)
{
$this-mandrill = $mandrill;
}
public function send(User $user)
{
$message = Please aclick here/a to confirm your account.;
$from = app@example.com;
$subject = Confirm your account;
$email = [
'to' = [['EmailTemplate' = $user-getEmail()]],
'from_email' = $from,
'subject' = $subject,
'text' = $message
];
$this-mandrill-send($email);
}
}

40. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}

41. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}
interface Mailer
{
public function send(Email $email);
}

42. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}
interface Mailer
{
public function send(Email $email);
}
class Email
{
private $to, $from, $subject, $message;
public function __construct(string $to, string $from,
string $subject, string $message) {
$this-to = $to;
$this-from = $from;
$this-subject = $subject;
$this-message = $message;
}
public function getTo(): string {
return $this-to;
}
public function getFrom(): string {
return $this-from;
}
public function getSubject(): string {
return $this-subject;
}
public function getMessage(): string {
return $this-message;
}
}

43. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}
interface Mailer
{
public function send(Email $email);
}
class Email
{
private $to, $from, $subject, $message;
public function __construct(string $to, string $from,
string $subject, string $message) {
$this-to = $to;
$this-from = $from;
$this-subject = $subject;
$this-message = $message;
}
public function getTo(): string {
return $this-to;
}
public function getFrom(): string {
return $this-from;
}
public function getSubject(): string {
return $this-subject;
}
public function getMessage(): string {
return $this-message;
}
}
class ConfirmationEmailService implements ConfirmationEmailer
{
private $mailer;
private $from = 'mysite@example.com';
private $subject = 'Confirm Email';
private $message = 'Please click here to confirm your email';
public function __construct(Mailer $mailer) {
$this-mailer = $mailer;
}
public function send(string $to) {
$email = new Email($to, $this-from,
$this-subject, $this-message);
$this-mailer-send($email);
}
}

44. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}
interface Mailer
{
public function send(Email $email);
}
class Email
{
private $to, $from, $subject, $message;
public function __construct(string $to, string $from,
string $subject, string $message) {
$this-to = $to;
$this-from = $from;
$this-subject = $subject;
$this-message = $message;
}
public function getTo(): string {
return $this-to;
}
public function getFrom(): string {
return $this-from;
}
public function getSubject(): string {
return $this-subject;
}
public function getMessage(): string {
return $this-message;
}
}
class ConfirmationEmailService implements ConfirmationEmailer
{
private $mailer;
private $from = 'mysite@example.com';
private $subject = 'Confirm Email';
private $message = 'Please click here to confirm your email';
public function __construct(Mailer $mailer) {
$this-mailer = $mailer;
}
public function send(string $to) {
$email = new Email($to, $this-from,
$this-subject, $this-message);
$this-mailer-send($email);
}
}
class MandrillMailer implements Mailer
{
private $mandrill;
public function __construct(Mandrill $mandrill) {
$this-mandrill = $mandrill;
}
public function send(Email $email) {
//Convert our email to Mandrill's email
$mandrillMail = ['to' = ['email' = $email-getTo()],
'from_email' = $email-getFrom(),
'subject' = $email-getSubject(),
'text' = $email-getMessage()];
$this-mandrill-send($mandrillEmail);
}
}

45. interface ConfirmationEmailer
{
public function send(string $to); /** @throws MailerException */
}
interface Mailer
{
public function send(Email $email);
}
class Email
{
private $to, $from, $subject, $message;
public function __construct(string $to, string $from,
string $subject, string $message) {
$this-to = $to;
$this-from = $from;
$this-subject = $subject;
$this-message = $message;
}
public function getTo(): string {
return $this-to;
}
public function getFrom(): string {
return $this-from;
}
public function getSubject(): string {
return $this-subject;
}
public function getMessage(): string {
return $this-message;
}
}
class ConfirmationEmailService implements ConfirmationEmailer
{
private $mailer;
private $from = 'mysite@example.com';
private $subject = 'Confirm Email';
private $message = 'Please click here to confirm your email';
public function __construct(Mailer $mailer) {
$this-mailer = $mailer;
}
public function send(string $to) {
$email = new Email($to, $this-from,
$this-subject, $this-message);
$this-mailer-send($email);
}
}
class SendGridMailer implements Mailer
{
private $sendgrid;
public function __construct(SendGrid $sendgrid) {
$this-sedngrid = $sendgrid;
}
public function send(Email $email) {
//Convert our email to SendGrid's email
$sgEmail = new SendGridEmail($email-getTo(),
$email-getFrom(),
$email-getSubject(),
$email-getMessage());
$this-sendgrid-queue($sgEmail);
}
}

46. class AccountOverdueListener
{
private $mailer;
public function __construct(Mailer $mailer)
{
$this-mailer = $mailer;
}
public function handle(OverdueEvent $event)
{
$account = $event-getAccount();
$email = new OverdueEmail($account-getEmailAddress(), $account-getAmountDue());
$this-mailer-send($email);
}
}

47. class AccountOverdueListener
{
private $mailer;
private $texter;
public function __construct(Mailer $mailer, Texter $texter)
{
$this-mailer = $mailer;
$this-texter = $texter;
}
public function handle(OverdueEvent $event)
{
$account = $event-getAccount();
$notifications = $account-getEnabledNotifications();
foreach ($notifications AS $notificationType) {
if ($notificationType === 'email') {
$email = new OverdueEmail($account-getEmailAddress(), $account-getAmountDue());
$this-mailer-send($email);
} elseif ($notificationType === 'text') {
$text = new OverdueText($account-getPhoneNumber(), $account-getAmountDue());
$this-texter-send($text);
}
}
}
}

48. Open/Closed Principle

49. class AccountOverdueEmailListener
{
public function handle(Mailer $mailer, OverdueEvent $event)
{
$account = $event-getAccount();
$email = new OverdueEmail($account-getEmailAddress(), $account-getAmountDue());
$mailer-send($email);
}
}
class AccountOverdueTextListener
{
public function handle(Texter $texter, OverdueEvent $event)
{
$account = $event-getAccount();
$text = new OverdueText($account-getPhoneNumber(), $account-getAmountDue());
$texter-send($text);
}
}

50. E-Commerce App

51. Class PercentageDiscount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item)
{
return ($this-discount / 100) * $item-getCost();
}
}

52. Class PercentageDiscount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item)
{
return ($this-discount / 100) * $item-getCost();
}
}
Class BuyItemsToGetDiscount extends PercentageDiscount
{
private $type;
private $countItemsRequired = 2;
public function setEligibleType(string $type)
{
$this-type = $type;
}
public function calculateAmountOff(Item $item)
{
if ($this-hasEnoughItems() $item-isType($this-type)) {
return ($this-discount / 100) * $item-getCost();
}
return 'Error: Not enough items';
}
public function hasEnoughItems(array $items): bool
{
$found = 0;
foreach ($items AS $item) {
if ($item-isType($this-type)) {
$found++;
}
if ($found == $this-countItemsRequired) {
return true;
}
}
return false;
}
}

53. Class PercentageDiscount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item): float
{
return ($this-discount / 100) * $item-getCost();
}
}
Class BuyItemsToGetDiscount extends PercentageDiscount
{
private $type;
private $countItemsRequired = 2;
public function setEligibleType(string $type)
{
$this-type = $type;
}
public function calculateAmountOff(Item $item): float
{
if ($this-hasEnoughItems() $item-isType($this-type)) {
return ($this-discount / 100) * $item-getCost();
}
throw new NotEnoughItems();
}
public function hasEnoughItems(array $items): bool
{
$found = 0;
foreach ($items AS $item) {
if ($item-isType($this-type)) {
$found++;
}
if ($found == $this-countItemsRequired) {
return true;
}
}
return false;
}
}

54. Liskov Substitution Principle

55. Class PercentageDiscount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item): float
{
return ($this-discount / 100) * $item-getCost();
}
}
Class BuyItemsToGetDiscount extends PercentageDiscount
{
private $type;
private $countItemsRequired = 2;
public function setEligibleType(string $type)
{
$this-type = $type;
}
public function calculateAmountOff(Item $item): float
{
if ($this-hasEnoughItems() $item-isType($this-type)) {
return ($this-discount / 100) * $item-getCost();
}
throw new NotEnoughItems();
}
public function hasEnoughItems(array $items): bool
{
$found = 0;
foreach ($items AS $item) {
if ($item-isType($this-type)) {
$found++;
}
if ($found == $this-countItemsRequired) {
return true;
}
}
return false;
}
}

56. Class PercentageDiscount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item): float
{
return ($this-discount / 100) * $item-getCost();
}
}
Class BuyItemsToGetDiscount extends PercentageDiscount
{
private $type;
private $countItemsRequired = 2;
public function __construct($discount, $type)
{
$this-type = $type;
parent::__construct($discount);
}
public function calculateAmountOff(Item $item)
{
if ($this-hasEnoughItems() $item-isType($this-type)) {
return ($this-discount / 100) * $item-getCost();
}
return 0;
}
public function hasEnoughItems(array $items): bool
{
$found = 0;
foreach ($items AS $item) {
if ($item-isType($this-type)) {
$found++;
}
if ($found == $this-countItemsRequired) {
return true;
}
}
return false;
}

57. Interface Discount
{
/** @throws DiscountException */
public function calculateAmountOff(Item $item): float;
}
Class DiscountException extends Exception
{
}
Class PercentageDiscount implements Discount
{
private $discount;
public function __construct(int $discount)
{
$this-discount = $discount;
}
public function calculateAmountOff(Item $item): float
{
return ($this-discount / 100) * $item-getCost();
}
}
Class BuyItemsToGetDiscount extends PercentageDiscount
{
private $type;
private $countItemsRequired = 2;
public function __construct($discount, $type)
{
$this-type = $type;
parent::__construct($discount);
}
public function calculateAmountOff(Item $item)
{
if ($this-hasEnoughItems() $item-isType($this-type)) {
return ($this-discount / 100) * $item-getCost();
}
throw new DiscountException ();
}
public function hasEnoughItems(array $items): bool
{
$found = 0;
foreach ($items AS $item) {
if ($item-isType($this-type)) {
$found++;
}
if ($found == $this-countItemsRequired) {
return true;
}
}
return false;
}

58. Shopping Cart

59. Interface Item
{
public function getName(): string;
public function getPrice(): float;
public function getShippingWeight(): float;
public function getShippingCost(): float;
}

60. Interface Item
{
public function getName(): string;
public function getPrice(): float;
public function getShippingWeight(): float;
public function getShippingCost(): float;
}
class Cart
{
private $items;
private $total;
public function addItem(Item $item)
{
$this-items[] = $item;
}
public function calculateTotal()
{
$subtotal = 0;
$shippingTotal = 0;
foreach ($this-items AS $item) {
$subtotal += $item-getPrice();
$shippingTotal += $item-getShippingCost();
}
return $subtotal + $shippingTotal;
}
}

61. Interface Item
{
public function getName(): string;
public function getPrice(): float;
public function getShippingWeight(): float;
public function getShippingCost(): float;
public function getWorkerAssigned(): Worker;
public function assignWorker(Worker $worker);
public function getServiceScheduledDate(): DateTime;
public function getServiceCompletedDate(): DateTime;
public function isCompleted(): bool;
}
Class Cart { [...] }
Class ServiceManager
{
private $services;
public function addService(Item $item)
{
$this-services[] = $item;
}
public function getCompletedServices()
{
$completed = [];
foreach ($this-services AS $service) {
if ($service-isCompleted()) {
$completed[] = $service;
}
}
return $completed;
}
}

62. Interface Segregation

63. Interface Item
{
public function getName(): string;
public function getPrice(): float;
public function getShippingWeight(): float;
public function getShippingCost(): float;
public function getWorkerAssigned(): Worker;
public function assignWorker(Worker $worker);
public function getServiceScheduledDate(): DateTime;
public function getServiceCompletedDate(): DateTime;
public function isCompleted(): bool;
}
Interface Buyable
{
public function getName(): string;
public function getPrice(): float;
}
Interface Product
{
public function getShippingWeight(): float;
public function getShippingCost(): float;
}
Interface Service
{
public function getWorkerAssigned(): Worker;
public function assignWorker(Worker $worker);
public function getServiceScheduledDate(): DateTime;
public function getServiceCompletedDate(): DateTime;
public function isCompleted(): bool;
}

64. Interface Buyable
{
public function getName(): string;
public function getPrice(): float;
}
Interface Product
{
public function getShippingWeight(): float;
public function getShippingCost(): float;
}
Interface Service
{
public function getWorkerAssigned(): Worker;
public function assignWorker(Worker $worker);
public function getServiceScheduledDate(): DateTime;
public function getServiceCompletedDate(): DateTime;
public function isCompleted(): bool;
}
Class BuyableItem implements Buyable
{
protected $name, $price;
public function __construct(string $name,
float $price)
{
$this-name = $name;
$this-price = $price;
}
public function getName(): string
{
return $this-name;
}
public function getPrice(): float
{
return $this-price;
}
}

65. Interface Buyable
{
public function getName(): string;
public function getPrice(): float;
}
Interface Product
{
public function getShippingWeight(): float;
public function getShippingCost(): float;
}
Interface Service
{
public function getWorkerAssigned(): Worker;
public function assignWorker(Worker $worker);
public function getServiceScheduledDate(): DateTime;
public function getServiceCompletedDate(): DateTime;
public function isCompleted(): bool;
}
Class BuyableItem implements Buyable { [...] }
Class BuyableProduct extends BuyableItem implements
Product
{
protected $shippingCost, $shippingWeight;
public function __construct(string $name,
float $price,
float $shippingCost,
float $shippingWeight)
{
$this-shippingCost = $shippingCost;
$this-shippingWeight = $shippingWeight;
parent::__construct($name, $price);
}
public function getShippingCost(): float {
return $this-getShippingCost();
}
public function getShippingWeight(): float {
return $this-shippingWeight;
}
}

66. S.O.L.I.D.
● Single Responsibility - Registration Class delegating to Collaborators
● Open/Closed - Adding new Notifiers without always editing Listener
● Liskov Substitution - Ensured Discount subtypes could be used as Discount
● Interface Segregation - Items use Buyable, Product and Service interfaces
● Dependency Inversion - Registration Controller, Emailer, Shopping Cart,
Notification Factory - all use interfaces, not implementations

67. @jessicamauerhan | http://bit.ly/solid-oop
SLD: Apply Often
Preemptively
● Single Responsibility
● Liskov Substitution
● Dependency Inversion
OI: Apply As Needed
When Changes Occur
● Open/Closed
● Interface Segregation

68. @jessicamauerhan | http://bit.ly/solid-oop
Thank You!
SOLID in Practice
http://bit.ly/solid-oop
Feedback Questions?
Welcome Encouraged!
@jessicamauerhan
jessicamauerhan@gmail.com
jmauerhan.wordpress.com