Design Patterns - Mediator Pattern

Purpose

The purpose of the design pattern

This pattern provides an easy way to decouple many components working together. It is a good alternative to Observer IF you have a "central intelligence", like a controller (but not in the sense of the MVC).

All components (called Colleague) are only coupled to the Mediator interface and it is a good thing because in OOP, one good friend is better than many. This is the key-feature of this pattern.

UML

UML design pattern diagram

Code

Code snippets

Mediator

Mediator Interface which defines a getUser method signature.


namespace DesignPatterns\Behavioral\Mediator;

interface Mediator
{
    public function getUser(string $username): string;
}

Colleague

Abstract class Colleague which eliminates boilerplate code throughout any concrete subclasses which extend it. The setMediator method is final meaning it cannot be overridden by a sub class.


namespace DesignPatterns\Behavioral\Mediator;

abstract class Colleague
{
    protected Mediator $mediator;

    final public function setMediator(Mediator $mediator)
    {
        $this->mediator = $mediator;
    }
}

Ui extends Colleague and contains an outputUserInfo method. This method references the mediators getUser method.


namespace DesignPatterns\Behavioral\Mediator;

class Ui extends Colleague
{
    public function outputUserInfo(string $username)
    {
        echo $this->mediator->getUser($username);
    }
}

UserRepository

UserRepository extends Colleague and has a getUserName method.


namespace DesignPatterns\Behavioral\Mediator;

class UserRepository extends Colleague
{
    public function getUserName(string $user): string
    {
        return 'User: ' . $user;
    }
}

UserRepositoryUiMediator

The UserRepositoryUiMediator expects the UserRepository and Ui classes as construct parameters. This class proxies methods within those classes.


namespace DesignPatterns\Behavioral\Mediator;

class UserRepositoryUiMediator implements Mediator
{
    public function __construct(private UserRepository $userRepository, private Ui $ui)
    {
        $this->userRepository->setMediator($this);
        $this->ui->setMediator($this);
    }

    public function printInfoAbout(string $user)
    {
        $this->ui->outputUserInfo($user);
    }

    public function getUser(string $username): string
    {
        return $this->userRepository->getUserName($username);
    }
}

MediatorTest

MediatorTest example below. The mediator design pattern decouples components which all work together. A separation of concerns allows code to be reused and switched out easily.


namespace DesignPatterns\Tests\Mediator\Tests;

use DesignPatterns\Behavioral\Mediator\Ui;
use DesignPatterns\Behavioral\Mediator\UserRepository;
use DesignPatterns\Behavioral\Mediator\UserRepositoryUiMediator;
use PHPUnit\Framework\TestCase;

class MediatorTest extends TestCase
{
    public function testOutputHelloWorld()
    {
        $mediator = new UserRepositoryUiMediator(new UserRepository(), new Ui());

        $this->expectOutputString('User: Dominik');
        $mediator->printInfoAbout('Dominik');
    }
}