Mediator Pattern

Also Known As

TL;DR

Explanation

The Mediator pattern reduces chaotic many-to-many dependencies between objects by routing all communication through a central mediator. Instead of objects knowing about each other, they only know about the mediator. This reduces coupling from O(n²) to O(n). In PHP, the Command Bus (used in CQRS) is a form of mediator: commands are sent to the bus and routed to handlers, with no direct coupling between the sender and handler. Event dispatchers also act as mediators. Avoid letting the mediator become a God Object — it should route, not contain, business logic.

Common Misconception

Why It Matters

Common Mistakes

• The mediator becoming a god class that knows too much — distribute logic back to components and keep the mediator as a router.
• Using a mediator for components that have a simple, stable relationship — adds unnecessary complexity.
• Not distinguishing mediator (coordinates peers) from observer (notifies subscribers) — they solve different problems.
• Tightly coupling mediator to concrete component types — use interfaces so components can be swapped.

Code Examples

// Direct coupling between components — every component knows every other:
class Chat {
    public function send(User $from, User $to, string $msg): void {
        $to->receive($msg, $from); // Direct coupling — use a ChatMediator
    }
}

// Without mediator: components know about each other
// With mediator: all communication goes through one hub

interface Mediator {
    public function notify(object $sender, string $event): void;
}

class DialogMediator implements Mediator {
    public function __construct(
        private Checkbox $checkbox,
        private Input    $input,
        private Button   $submitBtn,
    ) {}

    public function notify(object $sender, string $event): void {
        if ($sender === $this->checkbox && $event === 'check') {
            $this->input->setEnabled($this->checkbox->isChecked());
            $this->submitBtn->setEnabled($this->checkbox->isChecked());
        }
    }
}

References

• ↗ https://refactoring.guru/design-patterns/mediator

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