export { Box };
`

Next, let's code our logic in a file called `systems.js`:

`javascript
const MoveBox = (entities, { input }) => {
//-- I'm choosing to update the game state (entities) directly for the sake of brevity and simplicity.
//-- There's nothing stopping you from treating the game state as immutable and returning a copy..
//-- Example: return { ...entities, t.id: { UPDATED COMPONENTS }};
//-- That said, it's probably worth considering performance implications in either case.

const { payload } = input.find(x => x.name === "onMouseDown") || {};

if (payload) {
const box1 = entities["box1"];

box1.x = payload.pageX;
box1.y = payload.pageY;
}

return entities;
};

export { MoveBox };
`

Finally let's bring it all together in our index.js:

`javascript
import React, { PureComponent } from "react";
import { GameEngine } from "react-game-engine";
import { Box } from "./renderers";
import { MoveBox } from "./systems"

export default class SimpleGame extends PureComponent {
render() {
return (
style={{ width: 800, height: 600, backgroundColor: "blue" }}
systems={[MoveBox]}
entities={{
//-- Notice that each entity has a unique id (required)
//-- and a renderer property (optional). If no renderer
//-- is supplied with the entity - it won't get displayed.
box1: { x: 200, y: 200, renderer: }
}}>


);
}
}
`

Build and run. Each entity is a "box". Every time you click on the screen, the first entity will move to the clicked coordinate.

If you're curious, our `GameEngine` component is a loose implementation of the Component-Entity-System pattern - we've written up a quick intro here.

GameEngine Properties

| Prop | Description | Default |
|---|---|---|
|systems|An array of functions to be called on every tick. |[]|
|entities|An object containing your game's initial entities. This can also be a Promise that resolves to an object containing your entities. This is useful when you need to asynchronously load a texture or other assets during the creation of your entities or level. |{} or Promise|
|renderer|A function that receives the entities and needs to render them on every tick. `(entities,screen) => { / DRAW ENTITIES / }` |DefaultRenderer|
|timer|An object that can be used to override the default timer behavior |new DefaultTimer()|
|running|A boolean that can be used to control whether the game loop is running or not |true|
|onEvent|A callback for being notified when events are dispatched |undefined|
|style|An object containing styles for the root container |undefined|
|className|A className for applying styles for the root container |undefined|
|children|React components that will be rendered after the entities |undefined|

GameEngine Methods

| Method | Description | Arg1, Arg2, ArgN |
|---|---|---|
|stop|Stop the game loop |NA|
|start|Start the game loop. |NA|
|swap|A method that can be called to update your game with new entities. Can be useful for level switching etc. You can also pass a Promise that resolves to an entities object into this method. |{} or Promise|
|dispatch|A method that can be called to fire events after the currenty frame completed. The event will be received by ALL the systems and any onEvent callbacks |event|

FAQ

$3

> This depends on your definition of production quality. You're not going to make a AAA title with RGE. You could however create some more basic games (doesn't mean they can't be fun games), or even jazz up your existing business applications with some interactive eye candy.

$3

> RGE does not come with an out-of-the-box physics engine. We felt that this would be an area where the game designers should be given greater liberty. There are lots of JS-based physics engines out there, each with their pros and cons. Check out Matter JS if you're stuck.

$3

> How you render your entities is up to you. You can use the stand web components (div, img, canvas) or using ThreeJS for 3D games.

Introduction

This package contains only two components:

- `GameLoop`
- `GameEngine`

Both are standalone components. The `GameLoop` is a subset of the `GameEngine` and gives you access to an `onUpdate` callback that fires every 16ms (or roughly 60 fps). On top of this, the `GameLoop` will supply a reference to the screen (via `Dimensions.get("window"))`, touch events for multiple fingers (start, end, press, long-press, move) and time + deltas. The `GameLoop` is useful for simple interactive scenes, and pretty much stays out of your way.

The `GameEngine` is more opinionated and is a react-friendly implementation of the Component-Entity-Systems pattern. It provides the same features out of the box as the `GameLoop` but also includes a crude event/signaling pipeline for communication between your game and your other React components. You probably want to use the `GameEngine` to implement slightly more complex games and interactive scenes.

Managing Complexity with Component Entity Systems

Typically, game developers have used OOP to implement complex game objects and scenes. Each game object is instantiated from a class, and polymorphism allows code re-use and behaviors to be extended through inheritance. As class hierarchies grow, it becomes increasingly difficult to create new types of game entities without duplicating code or seriously re-thinking the entire class hierarchy.

`
[GameEntity]
|
|
[Vehicle]
/ | \
/ | \
/ | \
/ | \
[Terrestrial] [Marine] [Airborne]
| | |
| | |
[Tank] [Boat] [Jet]
``
> How do we insert a new terrestrial and marine-based vehicle - say a Hovercraft - into the class hierarchy?

One way to address these problems is to favor composition over inheritance. With this approach, we break out the attributes and behaviours of our various game entities into decoupled, encapsulated and atomic components. This allows us to be really imaginative with the sorts of game entities we create because we can easily compose them with components from disparate domains and concerns.

Component entity systems are one way to organize your game entities in a composable manner. To start with, we take the common attributes (data) of our game entities and move them into siloed components. These don't have to be concrete classes, simple hash maps (or equivalent) and scalars will do - but this depends on the data you're storing.

- Position: { x: 0, y: 0 }
- Velocity: { x: 0, y: 0 }
- Acceleration: { x: 0, y: 0 }
- Mass: 1.0
- Health: 100
- Physics: Body b
- Controls: { jump: 'w', left: 'a', crouch: 's', right: 'd' }

> Examples of different types of components in a hypothetical programming language.

Your game entities will be reduced to lists/arrays of components and labeled with a unique identifier. An entity's components are by no means static - you're free to update components and even add or remove them on the fly. If our favourite Italian plumber ingests a mushroom, we simple double his velocity. If our character turns into a ghost - we remove his physics component and let him walk through walls.

- Player#1: [Position, Velocity, Health, Sprite, Physics, Controls]
- Enemy#1: [Position, Velocity, Health, Sprite, Physics, AI]
- Platform#1: [Position, Sprite, Physics]
- Platform#2: [Position, Sprite, Physics, Velocity] // <-- Moving platform!

> All entities are assigned a unique id.

Since our entities are simple data holders now, we must move all our game logic into our systems. At its core, a system is a function that processes related groups of components and is called on each iteration of the game loop. The system will extract entities that contain the necessary components it requires to run, update those entities as necessary, and wait for the next cycle. For example, we could code a "Gravity" component that calculates the force of gravity and applies it to all entities that have an acceleration AND velocity AND mass component. Entities that do not contain these components will not be affected by gravity.

- Gravity: (Acceleration, Velocity, Mass) => { // Update all matching entities // }
- Render: (Sprite, Position) => { }
- Movement: (Position, Velocity, Controls) => { }
- Damage: (Health) => { }
- Bot: (Position, Velocity, AI) => { }

> The logic in a system is inherently reusable because it can be applied to all entities that meet the system's criteria.

How exactly you choose to define your components, entities and systems is up to you. You'll probably find that coming up with well-defined components and systems will take some practice - but the general pattern is conducive to refactoring and the long term benefits will outweigh the learning curve.

$3

- Gamedev.net article
- Intro to Entity Systems
- Intro to CES from A-Frame

License

MIT License

Copyright (c) 2018 Boris Berak

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.