Introduction To Value Objects

Transcript

1. ValueObjects the virtue of immutability and encapsulation is here!
2. Example lengths of different units
4. 5 m 500 cm 196.851 inch 16.404 ft
5. 5 m 500 cm 196.851 inch 16.404 ft
6. 5 m > 499 cm ? 5 > 499 ? 5 m > 4.99 m ?
7. 5 m - 499 cm = ? 5 - 499 = ? 5 m - 4.99 m = ?
8. the unit is equally important as the value to become correct results
9. we should know at any time what the unit of a length is
10. carry the unit together with the value
11. Example making invalid state impossible to represent
13. user input: 4,99 correct to: 4.99 m
14. user input: -4.99 invalid length, negative value.
15. user input: five meters invalid length, string given.
16. make invalid state impossible to represent
17. Example identity
19. €10 = €5 + €5
20. ValueObjects are immutable
21. Recap • Encapsulate data that belongs together • Make invalid state impossible to represent • Values are immutable
22. implementation in php
23. final class Length { private $amount; private $unit; public function __construct($amount, Unit $unit) { $this->amount = ( float) $amount; $this->unit = $unit; } }
24. final class Length { // … public function __construct($amount, Unit $unit) { $this->assertZeroOrMore($amount); $this->amount = ( float) $amount; $this->unit = $unit; } private function assertZeroOrMore($amount) { if ($amount < 0) { throw new LengthShouldBeZeroOrMore (); } } }
25. final class Unit { private $unit; public function __construct($unit) { $this->assertKnownUnit($unit); $this->unit = ( string) $unit; } public function assertKnownUnit($unit) { if (!in_array((string) $unit, static::KNOWN_UNITS)) { throw new UnknownUnit($unit); } } }
27. we have valid, encapsulated data, that belongs together. it is immutable too.
28. final class Length { // … public function isSameUnit(Length $otherLength) { return $this->unit->equals($otherLength->unit); } }
29. final class Unit { // … public function equals(Unit $otherUnit) { return $this->unit == $otherUnit->unit; } }
30. final class Length { // … private function assertSameUnit( Length $otherLength) { if (!$this->isSameUnit($otherLength)) { throw new CanNotCompareDifferentUnits (); } } public function greaterThan(Length $otherLength) { $this->assertSameUnit($otherLength); return $this->amount > $otherLength->amount; } }
31. final class Length { // … public function add(Length $otherLength) { $this->assertSameUnit($otherLength); return new static ( $this->amount + $otherLength->amount, $this->unit ); } }
33. we have encapsulated methods working on our data, and kept immutability.
34. final class Length { // … public function __toString() { return $this->amount . ' ' . $this->unit->__toString(); } }
35. final class Length { // … public static function fromString($string) { if (!preg_match('/^(\d+(?:[.,]?\d+?)?)\s(\w*)$/ ', $foo, $matches)) { throw new InvalidStringRepresentation( ); } return new static ( $matches[1], $matches[2] ); } }
37. persistence in string form or normalized form (e.g. everything in SI units)
38. some operations are external
39. $lengthInCm = $lengthConvertor->convert($lengthInMeter, ' cm');
40. unit testing ValueObjects is easy
42. class StackTest extends PHPUnit_Framework_TestCase { function it_adds_two_lengths_of_the_same_unit() { $length1 = new Length(10, new Unit('m')); $length2 = new Length(5, new Unit('m')); $expected = new Length(15, new Unit('m')); $this->assertEquals( $expected, $length1->add($length2) ); } }
43. class StackTest extends PHPUnit_Framework_TestCase { // … /** * @expectedException CanNotCompareDifferentUnits */ function it_throws_when_adding_lengths_of_different_unit() { $length1 = new Length(10, new Unit('m')); $length2 = new Length(5, new Unit('cm')); $length1->add($length2); } // etcetera }
44. other examples
46. other examples • money • telephone number • bank account • gender • temperature • DateTime? • interval • ...
47. read also • http://verraes.net/2011/04/fowler-money-pattern-in-php/ • http://verraes.net/2013/02/casting-value-objects/ • http://verraes.net/2013/11/value-objects-and-user-interfaces/ • https://leanpub.com/principles-of-package-design • google “primitive obsession”
48. thanks