6_Observer Design Pattern

 observer design pattern is a behavioral design pattern.
Behavioral design pattern deals with the interaction of object of different classes within the system.

Definition:

One Subject state is change, the observer get notified about the change.
In Simple words
When something happened, inform information to all people.

You subscribe the channel(observer)
The channel is subject
When the new video is upload(state changes), all subscribers are notified. 

Php Code Example

Step 1: Define observer Interface

<?php

interface Observer {

    public function update(string $message);

}

?>

Step 2: Define Subject Interface

<?php

interface Subject {

    public function attach(Observer $observer);

    public function detach(Observer $observer);

    public function notify(string $message);

}

?>

Step 3: Create Concrete Subject e.g New Publisher

<?php

class NewsPublisher implements Subject {

    private $observers = [];

    public function attach(Observer $observer) {

        $this->observers[] = $observer;

    }

    public function detach(Observer $observer) {

        $this->observers = array_filter(

            $this->observers,

            fn($obs) => $obs !== $observer

        );

    }

    public function notify(string $message) {

        foreach ($this->observers as $observer) {

            $observer->update($message);

        }

    }

    public function publishNews(string $news) {

        echo "📰 News: $news\n";

        $this->notify($news);

    }

}

?>

Step 4: Create Concrete Observers

These are the observer which will be notified when subject state change. 

<?php

class EmailSubscriber implements Observer {

    public function update(string $message) {

        echo "📧 Email sent: $message\n";

    }

}

class SmsSubscriber implements Observer {

    public function update(string $message) {

        echo "📱 SMS sent: $message\n";

    }

}

?>

Step 5: Run it.

<?php

$publisher = new NewsPublisher();

$email = new EmailSubscriber();

$sms = new SmsSubscriber();

$publisher->attach($email);

$publisher->attach($sms);

$publisher->publishNews("PHP 8.3 Released!");

?>

Output

<?php

📰 News: PHP 8.3 Released!

📧 Email sent: PHP 8.3 Released!

📱 SMS sent: PHP 8.3 Released!

?>

Real World Application Example of  Observer pattern.

1. Laravel Event/Listener follow Observer pattern
In Laravel when user register get wellcome email.

<?php

Event::listen(UserRegistered::class, SendWelcomeEmail::class);

Event::listen(UserRegistered::class, AddBonusPoints::class);

?>

2. Notification system
On Blog Post/ Order Places user get notify through channel
Email
SMS
Push Notification.

Each channel is observer listening to the same event of Blog Post/Order placement(Subject).

3. Stock market 
When product (Subject) price changes,  
All investers (Observers) get updated.

4. Laravel Model Observer
In Laravel when user register.

<?php

User::observe(UserObserver::class);

?>