Welcome! In this lesson, we'll dissect two main software design patterns: the Facade and Adapter patterns. We aim to unravel how these patterns ensure backward compatibility while adding new features. Backward compatibility means that new updates don't break existing systems, allowing for new functionalities without affecting the current code. Think of the Facade and Adapter patterns as cassette-shaped tape adapters for CD players, bridging the new and the old.

Design patterns are recognized solutions to frequent problems in software design and are time-tested methods resulting from the craftsmanship of seasoned developers. Among many design patterns, we're focusing on the Facade and Adapter patterns today. The Facade pattern provides a simplified interface to a complex subsystem, whereas the Adapter pattern enables classes with incompatible interfaces to work together. Let's delve deeper to unravel their use cases.

The Facade pattern simplifies complex processes by providing a higher-level interface. Consider an online shopping application. When a user places an order, it triggers many operations. By using the Facade pattern, we can create an OrderFacade class to simplify these operations:

TypeScript
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1// Component interfaces and implementations
2class Order {
3    create(): void {
4        console.log("Order created.");
5    }
6}
7
8class Product {
9    checkAvailability(): void {
10        console.log("Product availability checked.");
11    }
12}
13
14class Payment {
15    processPayment(): void {
16        console.log("Payment processed.");
17    }
18}
19
20class Delivery {
21    arrangeDelivery(): void {
22        console.log("Delivery arranged.");
23    }
24}
25
26// With Facade
27class OrderFacade {
28    private order: Order;
29    private product: Product;
30    private payment: Payment;
31    private delivery: Delivery;
32
33    constructor() {
34        this.order = new Order();
35        this.product = new Product();
36        this.payment = new Payment();
37        this.delivery = new Delivery();
38    }
39
40    public placeOrder(): void {
41        this.order.create();
42        this.product.checkAvailability();
43        this.payment.processPayment();
44        this.delivery.arrangeDelivery();
45    }
46}
47
48let orderFacade = new OrderFacade();
49orderFacade.placeOrder();

Here, assume the Order, Product, Payment, and Delivery are already correctly implemented elsewhere, with all the necessary methods.

The Facade pattern, as demonstrated in the online shopping application example, ensures backward compatibility by consolidating complex subsystem interactions (ordering, payment, delivery) behind a simple OrderFacade interface. This allows the underlying subsystems to evolve independently (e.g., changing the payment process or delivery options) without necessitating changes to the client code, thereby preserving the interface's constancy over time. Additionally, it makes the code more decoupled, allowing all order steps to be updated independently.

The Adapter pattern is our bridge for making otherwise incompatible interfaces work together, similar to how a travel adapter allows devices from one country to be used in the electrical outlets of another. For a more streamlined example, imagine a simple scenario where we have a legacy MusicPlayer designed to play MP3 files alone, and we're looking to support more formats like WAV without changing its interface.

TypeScript
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1class MusicPlayer {
2    play(file: string): void {
3        if (file.endsWith(".mp3")) {
4            console.log(`Playing ${file} as mp3.`);
5        } else {
6            console.log("File format not supported.");
7        }
8    }
9}
10
11class MusicPlayerAdapter {
12    private player: MusicPlayer;
13
14    constructor(player: MusicPlayer) {
15        this.player = player;
16    }
17
18    play(file: string): void {
19        if (file.endsWith(".wav")) {
20            // Convert WAV file playback request into MP3 format request
21            let convertedFile = file.replace(".wav", ".mp3");
22            console.log(`Converting ${file} to ${convertedFile} ...`);
23            this.player.play(convertedFile);
24        } else {
25            this.player.play(file);
26        }
27    }
28}
29
30// Existing music player
31let legacyPlayer = new MusicPlayer();
32legacyPlayer.play("song.mp3");  // Directly supported
33
34// Adapter-enhanced player
35let adapterPlayer = new MusicPlayerAdapter(legacyPlayer);
36adapterPlayer.play("song.wav");  // Supported through adapter

In this example, the MusicPlayerAdapter wraps the MusicPlayer, allowing it to play WAV files by converting them to the MP3 format it supports. This demonstrates the Adapter pattern's core idea: facilitating backward compatibility by enabling a new feature (WAV support) without altering the original music player's code. It's a seamless way to extend functionality while preserving the old system's integrity.

Great work! We've covered two powerful design patterns: Facade and Adapter. Both serve specific needs to ensure backward compatibility when adding features to existing software. You now understand their functions, usage, and their role in ensuring backward compatibility in software development. Ready your programming hats! In our upcoming practical exercises, we'll work hands-on with these patterns!