Clean Architecture & Design Patterns in Practice · Leçon

Modèles observateur et stratégie

Implémentez l'observateur pour gérer les dépendances et la stratégie pour utiliser des algorithmes interchangeables.

Modèles observateur et stratégie est une leçon Clean Architecture & Design Patterns in Practice gratuite sur CoddyKit. Ceci est la leçon 1 sur 4. Tu peux lire la leçon complète ci-dessous gratuitement — puis la pratiquer en direct dans le navigateur avec un éditeur de code intégré et un tuteur IA 24/7. Elle fait partie du parcours d'apprentissage Clean Architecture & Design Patterns in Practice, et ta progression se synchronise sur le web et l'application CoddyKit. Le cours Clean Architecture & Design Patterns in Practice comprend 4 leçons au total.

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Behavioral Patterns: Actions & Interactions

Welcome to the world of Behavioral Design Patterns! These patterns focus on how objects interact and communicate, helping distribute responsibilities effectively.

They describe common communication patterns between objects, making your system more flexible and manageable.

Observer Pattern: Notifying Changes

First up is the Observer Pattern. Imagine you subscribe to a newsletter – when a new edition is out, you get notified! This pattern works similarly.

It defines a one-to-many dependency between objects.
When one object (the Subject) changes state, all its dependents (Observers) are notified and updated automatically.

It's great for event handling and distributed systems.

Observer Components: The Subject

The Subject (also known as Observable) is the object being watched. It has two main jobs:

Maintain a list of its observers.
Provide methods to attach (add) or detach (remove) observers.
Notify all attached observers when its state changes.

Think of it as the news publisher.

Observer Components: The Observer

The Observer is any object that wants to be notified of changes in the Subject. It defines an update() method.

When the Subject notifies, it calls each Observer's update() method.
This method allows the Observer to react to the change, perhaps by fetching new data or updating its display.

These are your newsletter subscribers!

Observer Pattern: Publisher & Subscribers

Let's see the Observer pattern in action with a simple news publisher and readers. The NewsPublisher is our Subject, and NewsReaders are our Observers.

Try running the code to see how readers get updates!

import java.util.ArrayList;
import java.util.List;

// Observer Interface
interface Subscriber {
    void update(String news);
}

// Subject Class
class NewsPublisher {
    private List<Subscriber> subscribers = new ArrayList<>();
    private String latestNews;

    public void addSubscriber(Subscriber s) {
        subscribers.add(s);
    }

    public void removeSubscriber(Subscriber s) {
        subscribers.remove(s);
    }

    public void publishNews(String news) {
        this.latestNews = news;
        notifySubscribers();
    }

    private void notifySubscribers() {
        for (Subscriber s : subscribers) {
            s.update(latestNews);
        }
    }
}

// Concrete Observer
class NewsReader implements Subscriber {
    private String name;

    public NewsReader(String name) {
        this.name = name;
    }

    @Override
    public void update(String news) {
        System.out.println(name + " received: " + news);
    }
}

public class Main {
    public static void main(String[] args) {
        NewsPublisher publisher = new NewsPublisher();

        NewsReader reader1 = new NewsReader("Alice");
        NewsReader reader2 = new NewsReader("Bob");

        publisher.addSubscriber(reader1);
        publisher.addSubscriber(reader2);

        publisher.publishNews("Breaking: New programming course released!");

        System.out.println("\nAlice unsubscribes...");
        publisher.removeSubscriber(reader1);
        publisher.publishNews("Update: Course enrollment extended!");
    }
}

Strategy Pattern: Choosing Algorithms

Next, we have the Strategy Pattern. This pattern allows you to define a family of algorithms, encapsulate each one, and make them interchangeable.

It lets the algorithm vary independently from clients that use it. Think of choosing different payment methods (credit card, PayPal, bank transfer) at checkout.

Strategy Components: The Strategy

The Strategy is an interface or abstract class that declares a common operation for all supported algorithms.

Each specific algorithm (e.g., CreditCardPayment, PayPalPayment) implements this interface.
This ensures they all have the same method signature, making them interchangeable.

Strategy Components: The Context

The Context is the class that holds a reference to a Strategy object. It defines an interface for clients to interact with.

The Context does not know which concrete strategy it is using.
It delegates the execution of the algorithm to its current Strategy object.

This keeps the Context simple and flexible, as it doesn't need to change when new strategies are added.

Strategy Pattern: Payment Methods

Here's a practical example of the Strategy pattern. We have different payment strategies (Credit Card, PayPal) and a ShoppingCart as our Context.

Notice how the ShoppingCart can easily switch between payment methods without changing its own core logic.

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;
    public CreditCardPayment(String card) { this.cardNumber = card; }
    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using Credit Card " + cardNumber);
    }
}

class PayPalPayment implements PaymentStrategy {
    private String email;
    public PayPalPayment(String email) { this.email = email; }
    @Override
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using PayPal account: " + email);
    }
}

class ShoppingCart {
    private PaymentStrategy strategy;
    public void setPaymentStrategy(PaymentStrategy strategy) {
        this.strategy = strategy;
    }
    public void checkout(int amount) {
        if (strategy == null) {
            System.out.println("Please set a payment strategy first.");
            return;
        }
        strategy.pay(amount);
    }
}

public class Main {
    public static void main(String[] args) {
        ShoppingCart cart = new ShoppingCart();

        // Pay with Credit Card
        cart.setPaymentStrategy(new CreditCardPayment("1234-5678-9012-3456"));
        cart.checkout(100);

        // Pay with PayPal
        cart.setPaymentStrategy(new PayPalPayment("user@example.com"));
        cart.checkout(50);
    }
}

Quick Check: Observer or Strategy?

Consider a stock trading application. It needs to calculate trading fees using different methods (e.g., a flat fee, a percentage of trade value, or a tiered system) based on the user's account type. The application should be able to easily switch between these methods.

Recap: Notifying & Swapping

Great job! You've explored two powerful behavioral design patterns:

Observer Pattern: Manages one-to-many dependencies, where a Subject notifies multiple Observers of state changes. It promotes loose coupling between components.
Strategy Pattern: Allows you to define a family of algorithms, encapsulate each one, and make them interchangeable. It helps in swapping algorithms at runtime, promoting flexibility and clean code.

These patterns help make your code more modular, flexible, and easier to maintain!

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