Domain Modeling Value Objects

Value Objects

A small simple object, like money or a date range, whose equality isn't based on identity.
-- Martin Fowler

Creating a Value Object is quite easy you only need to use the Trait IsValueObject this will add the HasAttributes, HasInvariants and the HasEquality Traits. In addition, you could use the ValueObject interface to expose the values and equals methods.

The following example illustrates the implementation of these components.

use ComplexHeart\Contracts\Domain\Model\ValueObject;
use ComplexHeart\Domain\Model\Traits\IsValueObject;

/**
 * Class Color
 * @method string value()
 */
final class Color implements ValueObject 
{
    use IsValueObject;
    
    private string $value;
 
    public function __construct(string $value) 
    {
        $this->initialize(['value' => $value]);
    }
    
    protected function invariantValueMustBeHexadecimal(): bool 
    {
        return preg_match('/^#(?:[0-9a-fA-F]{3}){1,2}$/', $this->value) === 1;
    }
    
    public function __toString(): string 
    {
        return $this->value();
    }
}

$red = new Color('#ff0000');
$red->equals(new Color('#00ff00')); // false
$red->value(); // #ff0000
$magenta = new Color('ff00ff'); // Exception InvariantViolation: Value must be hexadecimal.