PackageFactory.Specification

Implementation of the Specification pattern for PHP

The specification pattern is a way to express business rules in a domain model using boolean logic. It is described in detail in the following document: https://www.martinfowler.com/apsupp/spec.pdf

Installation

composer require packagefactory/specification

Usage

Writing a Specification

Let's presume the following (very simplified) problem: You've got an application with a simple user registration workflow. Users can register freely, but have to verify their E-Mail address. If a user didn't verify their E-Mail address for a period of time, they shall be reminded (via E-Mail) that verification is still due.

How can this business rule be codified using the Specification pattern?

First, let's write a specification that checks if a given user has a verified E-Mail address:

use PackageFactory\Specification\Core\AbstractSpecification;
use Vendor\Project\Domain\User;

/**
 * The `@extends` annotation makes sure that static analysis tools like 
 * phpstan understand that this specification handles `User`-objects
 * only:
 * 
 * @extends AbstractSpecification<User>
 */
final class HasVerifiedEmailAddressSpecification extends AbstractSpecification
{
    public function isSatisfiedBy($user): bool
    {
        // In lieu of generics in PHP it is recommended to add a 
        // zero-cost assertion to ensure the type of the given value:
        assert($user instanceof User);

        return $user->emailAddress->isVerified;
    }
}

Then, let's write a specification that checks if a given user has been registered before a specific reference date:

use PackageFactory\Specification\Core\AbstractSpecification;
use Vendor\Project\Domain\User;

/**
 * @extends AbstractSpecification<User>
 */
final class HasBeenRegisteredBefore extends AbstractSpecification
{
    public function __construct(
        private readonly \DateTimeImmutable $referenceDate
    ) {
    }

    public function isSatisfiedBy($user): bool
    {
        assert($user instanceof User);

        return $user->registrationDate->getTimestamp() < $this->referenceDate->getTimestamp();
    }
}

We can now use the Specification API to combine both specifications and express our business rule:

// $twoWeeksAgo is a calculated \DateTimeImmutable
$needsReminderSpecification = (new HasBeenRegisteredBefore($twoWeeksAgo))
    ->andNot(new HasVerifiedEmailAddressSpecification());

$usersThatNeedReminder = $userRepository->findBySpecification($needsReminderSpecification);

foreach ($usersThatNeedReminder as $userThatNeedsReminder) {
    $notificationService->sendReminderTo($userThatNeedsReminder);
}

API

Each specification must implement PackageFactory\Specification\Core\SpecificationInterface. Usually, a custom specification should extend PackageFactory\Specification\Core\AbstractSpecification, which implements all methods of the SpecificationInterface except for isSatisfiedBy.

The SpecificationInterface covers the following methods:

Note on Generics: PHP does not have built-in Generics. However, there's static analysis tools like phpstan that do understand them. The SpecificationInterface comes with an annotation that allows you to specify the type of $candidate your specification is supposed to cover.

Your custom specification implementation should therefore name a concrete $candidate type like this:

/**
 * @extends AbstractSpecification<MyClass>
 */
final class MyCustomSpecification extends AbstractSpecification
{
    /**
     * @param MyClass $candidate
     * @return boolean
     */
    public function isSatisfiedBy($candidate): bool
    {
        // ...
    }
}

isSatisfiedBy

/**
 * @param C $candidate
 * @return boolean
 */
public function isSatisfiedBy($candidate): bool;

This method checks the given $candidate and returns true if it satisfies the specification and false if it doesn't.

In lieu of generics in PHP it is recommended to add a zero-cost assertion at the top of the implementation body to ensure the type of $candidate:

/**
 * @param MyClass $candidate
 * @return boolean
 */
public function isSatisfiedBy($candidate): bool;
{
    assert($candidate instanceof MyClass);

    // ...
}

For more on zero-cost assertions see: https://www.php.net/manual/en/function.assert.php

and

/**
 * @param SpecificationInterface<C> $other
 * @return SpecificationInterface<C>
 */
public function and(SpecificationInterface $other): SpecificationInterface;

The result of this method is a new specification that will be satisfied by a $candidate that satisfies both the calling specification and $other.

andNot

/**
 * @param SpecificationInterface<C> $other
 * @return SpecificationInterface<C>
 */
public function andNot(SpecificationInterface $other): SpecificationInterface;

The result of this method is a new specification that will be satisfied by a $candidate that satisfies the calling specification and does not satisfy $other.

or

/**
 * @param SpecificationInterface<C> $other
 * @return SpecificationInterface<C>
 */
public function or(SpecificationInterface $other): SpecificationInterface;

The result of this method is a new specification that will be satisfied by a $candidate that satisfies either the calling specification or $other (or both).

orNot

/**
 * @param SpecificationInterface<C> $other
 * @return SpecificationInterface<C>
 */
public function orNot(SpecificationInterface $other): SpecificationInterface;

The result of this method is a new specification that will be satisfied by a $candidate that either satisfies the calling specification or does not satisfy $other (or both).

not

/**
 * @return SpecificationInterface<C>
 */
public function not(): SpecificationInterface;

This method negates the calling specification. That means: the result is a specification that will be satisfied by a $candidate that does not satisfy the calling specification.

Contribution

We will gladly accept contributions. Please send us pull requests.

License

see LICENSE