The true value of objects 

Stijn Vannieuwenhuyse @stijnvnh 

ONE Agency - Ausy Belgium Team lead DDD, CQRS, event sourcing 

The true value of objects 

what are Objects? 

represent domain concepts encapsulate data expose behaviour 

SOLID 

Domain Driven Design 

Service Objects 

Entities 

Value Objects 

class Money { private $amount; private $currency; public function __construct($amount, $currency) { $this->amount = $amount; $this->currency = $currency; } public function getAmount() { return $this->amount; } public function getCurrency() { return $this->currency; } } 

$a = new Money(5.00, "EUR"); $b = new Money(5.00, "EUR"); $c = new Money(4.23, "EUR"); $d = new Money(5.00, "USD"); $a === $b; //false $a == $b; //true $a == $c; //false $a == $d; //false 

but ... Why? 

Model domain concepts 

$amsterdam = new City( "Amsterdam", "Netherlands", 4.8951679, 52.3702157 ); class City { private $latitude; private $longitude; //other fields public function __construct( $name, $country, $latitude, $longitude ) { $this->latitude = $latitude; $this->longitude = $longitude; //other assignments } //getters } 

class Coordinate { private $latitude; private $longitude; public function __construct($latitude, $longitude) { $this->latitude = $latitude; $this->longitude = $longitude; } //getters } 

$coordinate = new Coordinate(52.3702157, 4.8951679); $amsterdam = new City("Amsterdam", "Netherlands", $coordinate); class City { /** * @var Coordinate */ private $coordinate; //other fields public function __construct( $name, $country, Coordinate $coordinate ) { $this->coordinate = $coordinate; //other assignments } //getters } 

Avoid bugs 

$amsterdam = new City( "Amsterdam", "Netherlands", 4.8951679, 52.3702157 ); class City { public function __construct( $name, $country, $latitude, $longitude ) { //... } } 

Achieve single responsability 

class HexColor { private $hexColor; public function __construct($hexColor) { if(!preg_match('/^#?[a-f0-9]{6}$/i', $hexColor)) { throw new \InvalidArgumentException(); } $this->hexColor = $hexColor; } public function blend(HexColor $otherHexColor) { $newHexColor = // color blending...; return new HexColor($newHexColor); } } 

PHP BUILT IN 

$a = new DateTime("2015-06-26"); $b = $a->modify("+1 day"); $a == new DateTime("2015-06-26"); //false 

class User { /** * @var DateTime */ private $registrationDate; public function __construct(DateTime $registrationDate) { $this->registrationDate = $registrationDate; } public function getRegistrationDate() { return $this->registrationDate; } } $user = new User(new DateTime("2015-06-26")); if($user->getRegistrationDate()->modify("+1 year") == new DateTime("today")) { //send anniversary gift } $user->getRegistrationDate() == new DateTime("2015-06-26"); //false $user->getRegistrationDate() == new DateTime("2016-06-26"); //true 

CAUTION 

Value objects should be immutable 

class User { /** * @var DateTimeImmutable */ private $registrationDate; public function __construct(DateTimeImmutable $registrationDate) { $this->registrationDate = $registrationDate; } public function getRegistrationDate() { return $this->registrationDate; } } $user = new User(new DateTimeImmutable("2015-06-26")); if($user->getRegistrationDate()->modify("+1 year") == new DateTimeImmutable("today")) { //send anniversary gift } $user->getRegistrationDate() == new DateTimeImmutable("2015-06-26"); //true 

class Money { //fields //constructor //getters public function add(Money $other) { if($this->currency != $other->currency) { throw new CurrenciesDontMatch(); } return new Money($this->amount + $other->amount, $this->currency); } } 

ENDLESS POSSIBILITIES 

class DateRange { private $start; private $end; public function __construct( DateTimeImmutable $start, DateTimeImmutable $end ) { if($start > $end) { throw new InvalidArgumentException(); } $this->start = $start; $this->end = $end; } //getters } $a = new DateRange(new DateTimeImmutable("today"), new DateTimeImmutable("tomorrow")); $b = new DateRange(new DateTimeImmutable("today"), new DateTimeImmutable("tomorrow")); $c = new DateRange(new DateTimeImmutable("yesterday"), new DateTimeImmutable("today")); $a == $b; //still true $a < $c; //will it work? 

class DateRange { private $start; private $end; //constructor //getters /** * @return bool */ public function isLessThan(DateRange $other) { return $this->start < $other->start; } } $a = new DateRange(new DateTimeImmutable("today"), new DateTimeImmutable("tomorrow")); $b = new DateRange(new DateTimeImmutable("today"), new DateTimeImmutable("tomorrow")); $c = new DateRange(new DateTimeImmutable("yesterday"), new DateTimeImmutable("today")); $a->isLessThen($b); //false $a->isLessThan($c); //true 

class DateRange { private $start; private $end; //constructor //getters //comparation public function __toString() { return $this->start->format("U") . " " . $this->end->format("U"); } public static function fromString($string) { if(!preg_match("/^(\S+) (\S+)$/", string, $matches)) { throw new InvalidArgumentException(); } return new DateRange( new DateTimeImmutable($matches[1]), new DateTimeImmutable($matches[2]) ) } } 

class DateRange { //... /** * @return DateRange[] */ public static function summarize(array $dateRanges) { //some magic to combine overlapping dateranges return $dateRanges; } } 

class PersonName { private $firstName; private $nickName; private $lastName; public function __construct($firstName, $lastName, $nickName = '') { $this->firstName = $firstName; $this->lastName = $lastName; $this->nickName = $nickName; } //getters public function getDisplayShortName() { return $this->nickName ?: $this->firstName; } public function getDisplayFullName() { return $this->getDisplayShortName() . " " . $this->lastName; } } 

$string = " FooBar "; trim($string); //FooBar substr($string, 0, 2); //Fo strlen($string); // 2 strtolower($string); //fo //... 

class String { public function __construct($string) { /*...*/ } public function length() { /*...*/ } public function substring($start, $length) { /*...*/ } public function toLowercase() { /*...*/ } public function trim() { /*...*/ } } $string = new String("foobar"); $string ->trim() ->substring(0, 2) ->toLowercase(); //fo 

$weeklyTimeSlot = new WeeklyTimeSlot( WeekDay::friday(), new TimeSlot( new Time(16, 45) new Time(17, 30) ) ); /** @var duration Duration */ $duration = $weeklyTimeSlot->getDuration(); 

class SchoolYear { private $startYear; public function __construct($startYear) { if(!is_int($startYear)) { throw new InvalidArgumentException(); } $this->startYear = $startYear; } public function getStartYear() { /* ... */ } public function getEndYear() { /* ... */ } public function __toString() { /* ... */ } //2014-2015 } 

class SchoolYear { private $startYear; public function __construct($startYear) { /* ... */ } public static function current() { /* ... */ } public static function containingDate(DateTimeImmutable $date) { /* ... */ } public static function fromString($string) { /* ... */ } public function next() { /* ... */ } public function previous() { /* ... */ } public function getStartYear() { /* ... */ } public function getEndYear() { /* ... */ } public function __toString() { /* ... */ } public function equals(SchoolYear $schoolYear) { /* ... */ } public function getStartDate() { /* ... */ } public function getEndDate() { /* ... */ } public function getDateRange() { /* ... */ } public function getCalendarWeeks() { /* ... */ } public function startsInWeekend() { /* ... */ } public function isActive() { /* ... */ } public function isPassed() { /* ... */ } public function containsCalendarWeek(CalendarWeek $calendarWeek) { /* ... */ } public function containsDate(DateTimeImmutable $date) { /* ... */ } public function containsDateRange(DateRange $date) { /* ... */ } } 

But ... 

what about database storage? 

class Talk { //... /** * @ORM\Column(type="datetime") */ private $startTime; /** * @ORM\Column(type="datetime") */ private $endTime; public function __construct(/*...*/, TimeSlot $slot) { $this->startTime = $slot->getStartTime(); $this->endTime = $slot->getEndTime(); } /** * @return TimeSlot */ public function getTimeSlot() { return new TimeSlot($this->startTime, $this->endTime); } } 

/** * @ORM\Embeddable */ class TimeSlot { /** * @ORM\Column(type="datetime") */ private $startTime; /** * @ORM\Column(type="datetime") */ private $endTime; //... } 

class Talk { //... /** * @ORM\Embedded(class="TimeSlot") */ private $timeSlot public function __construct(/*...*/, TimeSlot $slot) { $this->timeSlot = $timeSlot; } /** * @return TimeSlot */ public function getTimeSlot() { return $this->timeSlot; } //... } 

what about memory usage? 

$cars = []; for($i = 0; $i <= 1000000; $i++) { $cars[] = new Car("Audi A6", "VIN-NR-2015" . $i, new HexColor("FF0000")); } 

$cars = []; for($i = 0; $i <= 1000000; $i++) { $cars[] = new Car("Audi A6", "VIN-NR-2015" . $i, HexColor::define("FF0000")); } class HexColor { private $hexColor; private static $hexColors = []; private function __construct($hexColor) { /*...*/ } public static function define($hexColor) { if(!array_key_exists($hexColor, self::$hexColors)) { self::$hexColors[$hexColor] = new self($hexColor); } return self::$hexColors[$hexColor]; } } 

Value object or entity? 

User 

Username 

Product-id 

Date 

Money 

Blog Post 

Blog Post Category 

VALUE OBJECTS represent domain values encapsulate data expose behaviour 

VALUE OBJECTS dry readability immutable 

QUESTIONS? 

Thank you Stijn Vannieuwenhuyse - @stijnvnh https://joind.in/14229 

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