# Build a Space Game Part 1: Introduction  ## Pre-Lecture Quiz [Pre-lecture quiz](https://ff-quizzes.netlify.app/web/quiz/29) ### Inheritance and Composition in game development In earlier lessons, the design architecture of the apps you built wasn't a major concern because the projects were relatively small. However, as your applications grow in size and complexity, architectural decisions become increasingly important. There are two main approaches to building larger applications in JavaScript: *composition* and *inheritance*. Both have their advantages and disadvantages, but let's explore them in the context of game development. ✅ One of the most famous programming books ever written focuses on [design patterns](https://en.wikipedia.org/wiki/Design_Patterns). In a game, you have `game objects`, which are entities that exist on the screen. These objects have a location within a cartesian coordinate system, defined by an `x` and `y` coordinate. As you develop a game, you'll notice that all your game objects share certain common properties, which are typical for any game you create. These include: - **location-based** Most, if not all, game elements are tied to a specific location. This means they have an `x` and `y` coordinate. - **movable** These objects can move to a new location. Examples include heroes, monsters, or NPCs (non-player characters), but not static objects like trees. - **self-destructing** These objects exist for a limited time before they are marked for deletion. This is often represented by a `dead` or `destroyed` boolean that signals to the game engine that the object should no longer be rendered. - **cool-down** 'Cool-down' is a common property for short-lived objects. For example, a piece of text or a graphical effect like an explosion that should only appear for a few milliseconds. ✅ Think about a game like Pac-Man. Can you identify the four object types listed above in this game? ### Expressing behavior The properties described above represent behaviors that game objects can have. So how do we encode these behaviors? We can express them as methods associated with either classes or objects. **Classes** One approach is to use `classes` along with `inheritance` to add specific behaviors to a class. ✅ Inheritance is an important concept to understand. Learn more from [MDN's article about inheritance](https://developer.mozilla.org/docs/Web/JavaScript/Inheritance_and_the_prototype_chain). In code, a game object might look like this: ```javascript //set up the class GameObject class GameObject { constructor(x, y, type) { this.x = x; this.y = y; this.type = type; } } //this class will extend the GameObject's inherent class properties class Movable extends GameObject { constructor(x,y, type) { super(x,y, type) } //this movable object can be moved on the screen moveTo(x, y) { this.x = x; this.y = y; } } //this is a specific class that extends the Movable class, so it can take advantage of all the properties that it inherits class Hero extends Movable { constructor(x,y) { super(x,y, 'Hero') } } //this class, on the other hand, only inherits the GameObject properties class Tree extends GameObject { constructor(x,y) { super(x,y, 'Tree') } } //a hero can move... const hero = new Hero(); hero.moveTo(5,5); //but a tree cannot const tree = new Tree(); ``` ✅ Take a few minutes to imagine how a Pac-Man character (like Inky, Pinky, or Blinky) could be implemented in JavaScript. **Composition** An alternative way to handle object inheritance is through *Composition*. In this approach, objects express their behaviors like this: ```javascript //create a constant gameObject const gameObject = { x: 0, y: 0, type: '' }; //...and a constant movable const movable = { moveTo(x, y) { this.x = x; this.y = y; } } //then the constant movableObject is composed of the gameObject and movable constants const movableObject = {...gameObject, ...movable}; //then create a function to create a new Hero who inherits the movableObject properties function createHero(x, y) { return { ...movableObject, x, y, type: 'Hero' } } //...and a static object that inherits only the gameObject properties function createStatic(x, y, type) { return { ...gameObject x, y, type } } //create the hero and move it const hero = createHero(10,10); hero.moveTo(5,5); //and create a static tree which only stands around const tree = createStatic(0,0, 'Tree'); ``` **Which pattern should I use?** The choice between these patterns is up to you. JavaScript supports both paradigms. -- Another common pattern in game development addresses the challenge of managing the game's user experience and performance. ## Pub/sub pattern ✅ Pub/Sub stands for 'publish-subscribe' This pattern is based on the idea that different parts of your application shouldn't directly depend on one another. Why is this beneficial? It makes it easier to understand the overall structure of your application when its components are decoupled. It also simplifies making changes to behavior when needed. Here's how this is achieved: - **message**: A message is typically a text string, sometimes accompanied by an optional payload (data that provides additional context about the message). For example, a typical message in a game might be `KEY_PRESSED_ENTER`. - **publisher**: This component *publishes* a message and sends it to all subscribers. - **subscriber**: This component *listens* for specific messages and performs an action in response, such as firing a laser. Although the implementation is relatively small, this pattern is incredibly powerful. Here's an example of how it can be implemented: ```javascript //set up an EventEmitter class that contains listeners class EventEmitter { constructor() { this.listeners = {}; } //when a message is received, let the listener to handle its payload on(message, listener) { if (!this.listeners[message]) { this.listeners[message] = []; } this.listeners[message].push(listener); } //when a message is sent, send it to a listener with some payload emit(message, payload = null) { if (this.listeners[message]) { this.listeners[message].forEach(l => l(message, payload)) } } } ``` To use the code above, we can create a simple implementation: ```javascript //set up a message structure const Messages = { HERO_MOVE_LEFT: 'HERO_MOVE_LEFT' }; //invoke the eventEmitter you set up above const eventEmitter = new EventEmitter(); //set up a hero const hero = createHero(0,0); //let the eventEmitter know to watch for messages pertaining to the hero moving left, and act on it eventEmitter.on(Messages.HERO_MOVE_LEFT, () => { hero.move(5,0); }); //set up the window to listen for the keyup event, specifically if the left arrow is hit, emit a message to move the hero left window.addEventListener('keyup', (evt) => { if (evt.key === 'ArrowLeft') { eventEmitter.emit(Messages.HERO_MOVE_LEFT) } }); ``` In this example, we connect a keyboard event, `ArrowLeft`, to send the `HERO_MOVE_LEFT` message. We then listen for that message and move the `hero` accordingly. The advantage of this pattern is that the event listener and the hero are completely independent of each other. You could remap the `ArrowLeft` key to the `A` key, or even make the `ArrowLeft` key trigger a completely different action by modifying the eventEmitter's `on` function: ```javascript eventEmitter.on(Messages.HERO_MOVE_LEFT, () => { hero.move(5,0); }); ``` As your game becomes more complex, this pattern remains consistent in its simplicity, helping to keep your code clean. It's highly recommended to adopt this approach. --- ## 🚀 Challenge Consider how the pub-sub pattern could improve a game. Which components should emit events, and how should the game respond to them? Use this opportunity to get creative and brainstorm a new game concept, thinking about how its parts might interact. ## Post-Lecture Quiz [Post-lecture quiz](https://ff-quizzes.netlify.app/web/quiz/30) ## Review & Self Study Learn more about Pub/Sub by [reading about it](https://docs.microsoft.com/azure/architecture/patterns/publisher-subscriber/?WT.mc_id=academic-77807-sagibbon). ## Assignment [Mock up a game](assignment.md) --- **Disclaimer**: This document has been translated using the AI translation service [Co-op Translator](https://github.com/Azure/co-op-translator). While we strive for accuracy, please note that automated translations may contain errors or inaccuracies. The original document in its native language should be regarded as the authoritative source. For critical information, professional human translation is recommended. We are not responsible for any misunderstandings or misinterpretations resulting from the use of this translation.