Simulação dos grupos do Facebook utilizando Mediator

Colleague.php

<?php
class Colleague extends Hashed
{
    private $name;

    public function __construct($name)
    {
        $this->name = $name;
    }

    public function join(Group $group)
    {
        $group->add($this);
    }

    public function send(Message $message, Group $group)
    {
        $group->send($message, $this);
    }

    public function receive(Message $message, Colleague $from,
                            Group $group)
    {
        printf("Notificação para %s\n", $this->name);
        printf("\t %s publicou no grupo %s:\n", $from->name, $group->getName());
        printf("\t%s\n\n", $message->getContent());
    }
}

Group.php

<?php
class Group extends Hashed
{
    private $mediator;
    private $name;

    public function __construct(GroupMediator $mediator, $name)
    {
        $this->mediator = $mediator;
        $this->name = $name;
    }

    public function add(Colleague $colleague)
    {
        $this->mediator->add($colleague, $this);
    }

    public function getName()
    {
        return $this->name;
    }

    public function send(Message $message, Colleague $from)
    {
        $this->mediator->send($message, $from, $this);
    }
}

GroupMediator.php

<?php
class GroupMediator
{
    private $colleagues = array();

    public function add(Colleague $colleague, Group $group)
    {
        $groupHash = $group->getHash();

        if (!isset($this->colleagues[$groupHash])) {
            $this->colleagues[$groupHash] = array();
        }

        $this->colleagues[$groupHash][$colleague->getHash()] = $colleague;
    }

    public function send(Message $message, Colleague $from, Group $group)
    {
        $fromHash = $from->getHash();
        $groupHash = $group->getHash();

        if (!isset($this->colleagues[$groupHash][$fromHash])) {
            throw new RuntimeException(
                'Você precisa entrar no grupo para poder enviar mensagens');
        }

        foreach ($this->colleagues[$groupHash] as $hash => $colleague) {
            if ($hash !== $fromHash) {
                $colleague->receive($message, $from, $group);
            }
        }
    }
}

Hashed.php

<?php
class Hashed
{
    private $hash;

    public function getHash()
    {
        if ($this->hash === null) {
            $this->hash = spl_object_hash($this);
        }

        return $this->hash;
    }
}

Message.php

<?php
class Message
{
    private $content;

    public function __construct($content)
    {
        $this->content = $content;
    }

    public function getContent()
    {
        return $this->content;
    }
}