Observer

Intent

Defines a subscription mechanism that notifies multiple objects about events in the object they observe.

Problem

Customers keep polling the store for a new product, or the store spams everyone.

Solution

The publisher keeps a list of subscribers and notifies them when something happens. Events, signals and pub/sub are all Observer.

Real-world analogy

Structure

Hover or tap a numbered role to highlight it in the diagram.

1. 1Publisher — Publisher issues events and keeps the subscription list.
2. 2Publisher — When an event happens, it loops over subscribers and calls update().
3. 3Subscriber — Subscriber declares the notification interface, usually a single update method.
4. 4ConcreteSubscribers — Concrete Subscribers react to notifications.
5. 5Client — Client creates publishers and subscribers and registers subscriptions.

Try it

Code example

final class Document implements SplSubject
{
    private SplObjectStorage $observers;
    public string $lastSaved = '';

    public function __construct() { $this->observers = new SplObjectStorage(); }

    public function attach(SplObserver $observer): void { $this->observers[$observer] = null; }
    public function detach(SplObserver $observer): void { unset($this->observers[$observer]); }
    public function notify(): void
    {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }

    public function save(string $name): void
    {
        $this->lastSaved = $name;
        $this->notify();
    }
}

final class AuditLog implements SplObserver
{
    public function update(SplSubject $subject): void { echo "Saved {$subject->lastSaved}\n"; }
}

Applicability

How to implement

1. Split business logic into a publisher and subscribers
2. Declare the subscriber interface (often a callback type)
3. Add subscribe / unsubscribe to the publisher
4. Notify subscribers when the event happens
5. Subscribers register with publishers

Pros and cons

Relations with other patterns

Check yourself