The true value of objects

Although the latest PHP versions provide us with enough types and keywords to enable object oriented development, the language itself is not truly object oriented. In pure object oriented languages, like Java, almost everything is an object. Even primitives have their object equivalent. Concepts that first look like simple values, can in fact be modelled as objects. This enables us to add a lot of behaviour to them. These so called Value Objects make our code more readable, elegant, maintainable and dry. We will explore the possibilities and advantages of these Value Objects together, guided by some real world code samples.

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1. The true value of
   objects
   

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2. Stijn Vannieuwenhuyse
   @stijnvnh
   

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

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4. The true value of
   objects
   

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5. what are
   Objects?
   

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6. represent domain concepts
   encapsulate data
   expose behaviour
   

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7. SOLID
   

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8. Domain
   Driven
   Design
   

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9. Service
   Objects
   

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10. Entities
    

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11. Value
    Objects
    

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12. 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;
    }
    }
    

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13. $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
    

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14. but ...
    Why?
    

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15. Model domain
    concepts
    

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16. $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
    }
    

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

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18. $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
    }
    

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19. Avoid bugs
    

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

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21. Achieve single
    responsability
    

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22. 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);
    }
    }
    

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23. PHP
    BUILT IN
    

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

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25. 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
    

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26. CAUTION
    

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27. Value objects should
    be immutable
    

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28. 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
    

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29. 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);
    }
    }
    

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30. ENDLESS POSSIBILITIES
    

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31. 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?
    

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32. 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
    

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33. 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])
    )
    }
    }
    

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

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35. 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;
    }
    }
    

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

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37. 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
    

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

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39. 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
    }
    

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40. 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) { /* ... */ }
    }
    

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41. But ...
    

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42. what about database
    storage?
    

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43. 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);
    }
    }
    

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44. /**
    * @ORM\Embeddable
    */
    class TimeSlot {
    /**
    * @ORM\Column(type="datetime")
    */
    private $startTime;
    /**
    * @ORM\Column(type="datetime")
    */
    private $endTime;
    //...
    }
    

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

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46. what about memory usage?
    

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

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48. $cars = [];
    for($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];
    }
    }
    

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49. Value object
    or entity?
    

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50. User
    

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51. Username
    

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52. Product-id
    

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53. Date
    

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54. Money
    

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55. Blog Post
    

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56. Blog Post
    Category
    

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57. VALUE OBJECTS
    represent domain values
    encapsulate data
    expose behaviour
    

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58. VALUE OBJECTS
    dry
    readability
    immutable
    

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59. QUESTIONS?
    

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

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