Tiny Web Metaverse Client core concept

This document describes the Tiny Web Metaverse Client core concept needed for Client or Addons development.

Client overview

Client is the software (Web page) that end-users directly operate in the Tiny Web Metaverse framework, and it mainly plays the following roles.

• 3D graphics rendering using WebGL
• VR/AR processing using WebXR, such as positional tracking
• Network synchronization of object states with remote clients using WebSockets via State server
• Audio and video communication with remote clients using WebRTC via Stream server
• Input handling from input devices such as mouse, keyboard, touchscreen, VR headset, and so on

Client core

Client provides the minimum and basic functions for the virtual 3D space. Advanced functions are implemented extensively by framework users.

For example, Client provides a way to define an avatar, but setting the model and other settings of the avatarthe is user's responsibility.

Additionally, Client detects input from input devices and shapes it into a form that is easy to process, but the users implement how to process the input.

For example, the users implement how to move the avatar with which button when keyboard input is received.

Moreover, Client does not involve in the 2D UI that is overlapped on the 3D Canvas. The implementation of the 2D UI is the responsibility of the user.

Hereafter, The "Client that provides basic functions" will be referred to as "Client core" when emphasizing it

Addons

Framework users need to implement application-specific processing outside of the Client core. However, many of these processes are highly reusable.

For example, the operation of an avatar using a keyboard is a similar process in many applications. Such processes are published as Addons.

By selectively importing Addons, framework users do not need to implement highly reusable processes themselves.

If you want to fine-tune the functionality of an Addon, copy the source code and edit it for use.

If you want to add an addon to Addons, please send a Pull Request.

Core third-party libraries

Three.js

Tiny Web Metaverse Client uses the JavaScript 3D graphics library Three.js to manage objects in 3D space, render 3D space using WebGL, and process VR and AR using WebXR.

Three.js knowledge is essential for developing the Client core, Addon, and user apps. If you are new to Three.js, please learn the basics from the documentation.

The following explanations assume that the reader has a basic understanding of Three.js.

bitECS

Tiny Web Metaverse Client uses the JavaScript ECS library bitECS.

Similarly to Three.js, basic knowledge of bitECS is required to develop a Client, Addon, or user app. If you are not familiar with bitECS, I recommend learning the basics from the documentation.

From here on, this document assumes that the reader has basic knowledge of bitECS.

ECS architecture

Tiny Web Metaverse Client adopts ECS (Entity Component System) architecture.

Entity Component System (ECS) is a software architectural pattern commonly used in game development for representing game world objects. It decomposes game objects into three distinct parts: entities, components, and systems.

• Entities: Entities represent the unique objects in the game world. They serve as mere identifiers and don't hold any data or behavior. Each entity is assigned a unique identifier, allowing systems to reference and operate on them.
• Components: Components are data containers that hold the attributes and properties of entities. They are essentially data structures that encapsulate the characteristics of an entity, such as position, velocity, health, sprite, and other relevant information.
• Systems: Systems are the functional units that act upon entities and components. They represent the behavior and logic of the game, processing data from components and modifying entities accordingly. Systems operate on groups of entities that possess specific components.

ECS in Tiny Web Metaverse

Entity

Use addEntity() of bitECS to create a new entity.

import { addEntity, IWorld } from "bitecs";

const somewhere = (world: IWorld): void => {
  const eid = addEntity(world);
  ...
};

The essence of an entity is an integer, called an Entity ID. It is often abbreviated as eid in Tiny Web Metaverse.

When an entity is deleted by the removeEntity() of bitECS, the Entity ID is returned to a pool managed by bitECS. This ID can be reused later.

In Tiny Web Metaverse, we assume that the pool is large enough that Entity IDs are not immediately reused, for simplicity.

Component

Use defineComponent() of bitECS to define a component and addComponent() to assign a component to an entity.

// src/components/foo.ts

import { defineComponent } from "bitecs";

export const FooComponent = defineComponent();

// Add a component to an entity

import { IWorld } from "bitecs";
import { FooComponent } from "../components/foo";

const somewhere = (world: IWorld, eid: number): void => {
  addComponent(world, FooComponent, eid);
  ...
};

You can define component with component data definition.

// src/components/foo.ts
import { defineComponent, Types } from "bitecs";

export const FooComponent = defineComponent({
  data: Types.f32
});

// Add a component to an entity and
// initialize the component data

import { IWorld } from "bitecs";
import { FooComponent } from "../components/foo";

const somewhere = (world: IWorld, eid: number): void => {
  addComponent(world, FooComponent, eid);
  FooComponent.data[eid] = 0.0;
  ...
};

bitECS doesn't support non-numeric data types natively. If you want to use non-numeric data type, use Component Proxy style. (Our Component Proxy style is not exactly same as the bitECS one but inspired by it.)We use static get method to reuse Proxy instance to maintain high performance iteration.

// src/components/foo.ts
import { defineComponent } from "bitecs";
import { Foo } from "foo-lib";
import { NULL_EID } from "../common";

export const FooComponent = defineComponent();

export class FooProxy {
  private static instance: FooProxy = new FooProxy();
  private eid: number;
  private map: Map<number, Foo>;

  private constructor() {
    this.eid = NULL_EID;
    this.map = new Map();
  }

  static get(eid: number): Foo {
    FooProxy.instance.eid = eid;
    return FooProxy.instance;
  }

  allocate(foo: Foo): void {
    this.map.set(this.eid, foo);
  }

  free(): void {
    this.map.delete(this.eid);
  }

  get foo(): Foo {
    return this.map.get(this.eid)!;
  }
}

When you add a component that has proxy to an entity you also need to get proxy and initialize the component data.

import { IWorld } from "bitecs";
import { Foo } from "foo-lib";
import { FooComponent, FooProxy } from "../components/foo";

const somewhereToAdd = (world: IWorld, eid: number): void => {
  addComponent(world, FooComponent, eid);
  const proxy = FooProxy.get(eid);
  proxy.allocate(new Foo());
  ...
};

TODO: A mechanism is needed to ensure that a component and its data managed by proxy have the same lifetime. Currently, this requires manual oversight, which is prone to errors.

Note that only a single proxy instance exists for each component. If you need to access components for multiple entities, obtain a new proxy after each entity's operation is complete.

// Bad
const proxy1 = FooProxy.get(eid1);
const proxy2 = FooProxy.get(eid2);

// This operation is wrong, proxy1 internally refers to eid2's component
proxy1.foo.operation();

proxy2.foo.operation();

// Good
const proxy1 = FooProxy.get(eid1);
proxy1.foo.operation();
const proxy2 = FooProxy.get(eid2);
proxy2.foo.operation();

TODO: This limitation can be an error prone because static type checking can't detect the problem and it requires manual oversight.

System

A system in Tiny Web Metaverse Client is just a function that takes IWorld of bitECS.

import { IWorld } from "bitecs";

export const fooSystem = (world: IWorld): void => {
  ...
};

Systems registered to App that is explained later are invoked once an animation frame.

We highly recommend to use query of bitECS to access specific components.

import { defineQuery, IWorld } from "bitecs";
import { FooComponent, FooProxy } from "../components/foo";

const fooQuery = defineQuery([FooComponent]);

export const fooSystem = (world: IWorld): void => {
  fooQuery(world).forEach(eid => {
    const foo = FooProxy.get(eid).foo;
    foo.operation();
  });
};

Remove entities

Removing an entity is a tricky part in Tiny Web Metaverse Client because we use Component Proxy style described above. Just removing an entity removeEntity() of bitECS doesn't release proxy and component data. It can cause memory leak and also may cause a problem when an entity is recycled.

To resolve this problem, a special flow has been introduced.

First, write a system for a component that has proxy to release the component data when the component is removed from an entity.

// src/systems/clear_foo.ts

import {
  defineQuery,
  exitQuery,
  IWorld
} from "bitecs";
import {
  FooComponent,
  FooProxy
} from "../components/foo";

const exitFooQuery = exitQuery(defineQuery([FooComponent]));

export const clearFooSystem = (world: IWorld): void => {
  exitFooQuery(world).forEach(eid => {
    const proxy = FooProxy.get(eid);
    const foo = proxy.foo;
    foo.close();
    proxy.free();
  });
};

When you remove the component from an entity, just use removeComponent() of bitECS. The component data will be released when the releasing system is called next time.

import { IWorld, removeComponent } from "bitecs";
import { FooComponent } from "../components/foo";

const somewhereToRemove = (world: IWorld, eid: number): void => {
  removeComponent(world, FooComponent, eid);
  ...
};

If you want to remove an entity, use built-in removeComponentsAndThenEntity() utility function. It immediately removes all the components associated with an entity, and then remove the entity after several animation frames. It allows systems to release component data in that interval.

import { IWorld } from "bitecs";
import { removeComponentsAndThenEntity } from "@tiny-web-metaverse/client/src";

const somewhereToRemove = (world: IWorld, eid: number): void => {
  removeComponentsAndThenEntity(world, eid);
  ...
};

App

App in Client manages systems and calls registered systems once an animation frame for each.

Framework user creates an App instance with canvas and roomId in their user applications. roomId is an identifier for room. Only clients in the same room can see and communicate each other. In the constructor built-in entities and systems are created or registered.

App.start() starts an application. This is an example of a minimal user application. (But nothing is rendered because no entity has been created to which Three.js objects are assigned. How to assign Three.js objects is explained later.)

import { App } from "@tiny-web-metaverse/client/src";

const roomId = '1234';
const canvas = document.createElement('canvas');

const app = new App({ canvas, roomId });
document.body.appendChild(canvas);

app.start();

registerSystem()

App.registerSystem() is a method for registering a system to App. A registered system is called in animation loop at specified timing.

The method takes system and orderPriority as arguments. system is a function that takes IWorld of bitECS. orderPriority is an integer.

Registered systems are called in the animation loop in the order of the orderPriority numbers. Note that the order in which systems with the same orderPriority value are called is not specified.

System order

Systems are generally expected to be executed in the following order.

• Time: Get elapsed and delta time.
• EventHandling: Handling async events detected while ideling.
• Setup: Set up any resource at the beginning of an animation loop.
• BeforeMatricesUpdate: Update transforms (position/rotation/scale).
• MatricesUpdate: App updates scene graph matrices. See "Matrices update" section for the details.
• BeforeRender: Operate anything that use updated matrices and that don't need transform update. Or operate anything that should be done right before rendering.
• Render: App renders the scene with Three.js WebGLRenderer.render()
• AfterRender: Operate anything that should be done right after rendering.
• PostProcess: Apply post-processing visual effects.
• TearDown: Operate anything that should be done at the end of an animation loop, for example clearing event components.

We highly recommend to use predefined SystemOrder corresponsing to them to specify systems execution order. The values are just integers so

export const SystemOrder = Object.freeze({
  Time: 0,
  EventHandling: 100,
  Setup: 200,
  BeforeMatricesUpdate: 300,
  MatricesUpdate: 400,
  BeforeRender: 500,
  Render: 600,
  AfterRender: 700,
  PostProcess: 800,
  TearDown: 900
});

This is an example.

import { App, SystemOrder } from "@tiny-web-metaverse/client/src";
import { barSystem } from "./systems/bar";
import { fooSystem } from "./systems/foo";

const roomId = '1234';
const canvas = document.createElement('canvas');

const app = new App({ canvas, roomId });
document.body.appendChild(canvas);

app.registerSystem(fooSystem, SystemOrder.BeforeMatricesUpdate);
// "+ 1" is to ensure that barSystem runs after fooSystem
app.registerSystem(barSystem, SystemOrder.BeforeMatricesUpdate + 1);

app.start();

getSystemOrderPriority()

App.getSystemOrderPriority() returns the order priority of a registered system. This function is useful to ensure to execure a system before or after certain systems including built-in systems.

import { interactSystem } from "@tiny-web-metaverse/client/src";

...

const priority = app.getSystemOrderPriority(interactSystem);
app.registerSystem(fooSystem, priority + 1);

Coroutine

Systems execution order is predictable in Tiny Web Metaverse Client. It makes easier to control systems and improves the simplicity and maintainability.

Async/Await must not be used in systems to keep this policy. Instead, consider to use Coroutine approach with JavaScript generator function and yield*. Some async/await operations may not be avoidable, for example calling async functions in a third-party library. In that case, use built-in toGenerator() utility function that allows to handle an async function as a generator function.

This is an example.

// src/systems/load_foo.ts

import {
  addComponent,
  defineQuery,
  enterQuery,
  exitQuery,
  IWorld,
  removeComponent
} from "bitecs";
import { loadFooAsync } from "foo-lib";
import { toGenerator } from "@tiny-web-metaverse/client/src";
import {
  FooComponent,
  FooLoader,
  FooLoaderProxy,
  FooProxy
} from "../components/foo";

function* load(world: IWorld, eid: number): Generator {
  const url = FooLoaderProxy.get(eid).url;
  const foo = yield* toGenerator(loadFooAsync(url));
  addComponent(world, FooComponent, eid);
  FooProxy.get(eid).allocate(foo);
}

const loaderQuery = defineQuery([FooLoader]);
const enterLoaderQuery = enterQuery(loaderQuery);
const exitLoaderQuery = exitQuery(loaderQuery);

const generators = new Map<number, Generator>();

export const loadFooSystem = (world: IWorld): void => {
  enterLoaderQuery(world).forEach(eid => {
    generators.set(eid, load(world, eid));
  });

  loaderQuery(world).forEach(eid => {
    let done = false;
    try {
      if (generators.get(eid).next().done === true) {
        done = true;
      }
    } catch (error) {
      console.error(error);
      done = true;
    }
    if (done) {
      removeComponent(world, FooLoader, eid);
    }
  });

  exitLoaderQuery(world).forEach(eid => {
    generators.delete(eid);
    FooLoader.get(eid).free();
  });
};

Note that any state can change while waiting for the completion of a generator function. You must not use any data fetched before a generator function starts, after a generator function has completed.

// Bad
const data: number = BarProxy.get(eid).bar.data;
yield* generatorFunction();
operate(data);

// Good
yield* generatorFunction();
const data: number = BarProxy.get(eid).bar.data;
operate(data);

Three.js stuffs

There are some limitations and restrictions for Three.js operations to simplify and optimize.

EntityObject3D

If you want to assign Three.js Object3Ds to an entity, use built-in EntityObject3D component and its proxy. EntityObject3DProxy.allocate() allocates a new Three.js Group, called EntityRootGroup, as root.

You can access root via EntityObject3DProxy.root. You can control the transform of an entity's Object3D immediately after assigning EntityObject3D.

import { addComponent, IWorld } from "bitecs";
import {
  EntityObject3D,
  EntityObject3DProxy
} from "@tiny-web-metaverse/client/src";

const setupEntityObject3D = (world: IWorld, eid: number): void => {
  addComponent(world, EntityObject3D, eid);
  EntityObject3DProxy.get(eid).allocate();
  const root = EntityObject3DProxy.get(eid).root;
  root.position.set(0.0, 0.0, -2.0);
};

Call built-in addObject3D() utility function to add your Three.js Object3D (eg: Mesh). You can call addObject3D() even before assiging EntityObject3D component to an entity because the function assigns it if the component is not assigned yet. Use built-in removeObject3D() utility function to remove an Three.js Object3D from an entity.

When an Object3D is assigned to an entity, its transform must be identity (identity matrix). Update the transform via EntityObject3DProxy.root after assigning.

import { IWorld } from "bitecs";
import { Mesh, MeshBasicMaterial, SphereGeometry } from "three";
import { addObject3D } from "@tiny-web-metaverse/client/src";

const addSphereMesh = (world: IWorld, eid: number): void => {
  const geometry = new SphereGeometry(1.0);
  const material = new MeshBasicMaterial();
  const mesh = new Mesh(geometry, material);
  addObject3D(world, mesh, eid);
  EntityObject3DProxy.get(eid).root.position.set(0.0, 0.0, 2.0);
};

Multiple Object3Ds can be assigned to an entity. addObject3D() and removeObject3D() form the following Three.js objects structure as optimization. When swapping the root object, they keep the transform (position/rotation/scale/matrix).

The number of assigned Object3Ds: 0

- EntityRootGroup (EntityObject3DProxy.root)

The number of assigned Object3Ds: 1

- Object3D (EntityObject3DProxy.root)

The number of assigned Object3Ds: 2-

- EntityRootGroup (EntityObject3DProxy.root)
  - Object3D_A
  - Object3D_B
  ...

root object can be swapped so it is a good practice to access EntityObject3DProxy.root right before using it.

// Bad
const root = EntityObject3DProxy.get(eid);
something(); // This function may add or remove Object3D from an entity
root.position.set(0.0, 0.0, 2.0);

// Good
something();
const root = EntityObject3DProxy.get(eid);
root.position.set(0.0, 0.0, 2.0);

TODO: Static type check can't detect misoperation like the bad one in the above example code. Can we introduce a mechanism to avoid the problem?

TODO: Remove this optimization? It can be simpler.

InScene

Add InScene built-in component to an entity to add its Three.js Object3Ds to Three.js scene. A built-in system adds them to the scene. If InScene component is removed from an entity, the system removes its Object3Ds from the scene.

import { addComponent, IWorld } from "bitecs";
import { Mesh, MeshBasicMaterial, SphereGeometry } from "three";
import { addObject3D, InScene } from "@tiny-web-metaverse/client/src";

const addSphereMesh = (world: IWorld, eid: number): void => {
  const geometry = new SphereGeometry(1.0);
  const material = new MeshBasicMaterial();
  const mesh = new Mesh(geometry, material);
  addObject3D(world, mesh, eid);
  EntityObject3DProxy.get(eid).root.position.set(0.0, 0.0, 2.0);
  addComponent(world, InScene, eid);
};

Scene hierarchy

In Tiny Web Metaverse Client, Three.js Scene is expected to have an identity matrix and Object3Ds assigned to an entity are expected to be not the children of other Object3Ds assigned to other entities for simplicity and optimization as

• The scene graph can be kept shallow and scene graph matrices update cost can be lower because a Three.js Object3D's transform update doesn't affect many Object3Ds in the scene.
• The local transform of a EntityObject3D's root match its world transform so even when world transform is needed world transform calculation is not needed.

If an entity's Object3D wants to be as if a child of other entity's Object3D you can manipulate the matrix like this.

import { entityExists, hasComponent, IWorld } from "bitecs";
import {
  EntityObject3D,
  EntityObject3DProxy
} from "@tiny-web-metaverse/client/src";

const asIfChild = (world: IWorld, eid: number, parentEid: number): void => {
  if (!entityExists(world, parentEid) ||
    !hasComponent(world, EntityObject3D, parentEid) {
    return;
  }

  const root = EntityObject3DProxy.get(eid).root;
  const parent = EntityObject3DProxy.get(parentEid).root;

  root.updateMatrix();
  parent.updateMatrix();
  root.matrix.premultiply(parent.matrix);
  root.matrix.decompose(root.position, root.quaternion, root.scale);
};

TODO: Introduce a mechanism that allows entity's Object3D to act as if a child of other entity's Object3D?

Matrices update

App updates the entire scene graph matrices at SystemOrder.MatricesUpdate in an animation loop. Systems that update transform(position/rotation/scale) should run before it. And systemt that need updated matrices and don't update transform should run after it for efficiency.

Loading glTF

Tiny Web Metaverse supports glTF 3D file format. Use the built-in GltfLoader component to load a glTF file and adds a loaded object to an entity.

TODO: Rename GltfLoader component to avoid the conflict name with GLTFLoader in Three.js?

The built-in gltfLoad system downloads and parses a file specified with GltfLoader component, creates Three.js objects, adds them to an entity, and adds the built-in GltfRoot component that refers to the root Three.js object of the loaded glTF objects.

import {
  addComponent,
  addEntity,
  IWorld
} from "bitecs";
import { AnimationMixer } from "three";
import {
  EntityObject3D,
  EntityObject3DProxy,
  GltfLoader,
  GltfLoaderProxy,
  InScene,
  MixerAnimation,
  MixerAnimationProxy
} from "@tiny-web-metaverse/client/src";

const assetUrl = 'assets/models/foo.gltf';

export const FooPrefab = (world: IWorld): number => {
  const eid = addEntity(world);
  addComponent(world, InScene, eid);
  addComponent(world, MixerAnimation, eid);
  MixerAnimationProxy.get(eid).allocate(new AnimationMixer(null));

  addComponent(world, EntityObject3D, eid);
  EntityObject3DProxy.get(eid).allocate();

  addComponent(world, GltfLoader, eid);
  GltfLoaderProxy.get(eid).allocate(assetUrl);

  return eid;
};

If you want to access glTF objects, you can use GltfRoot component and bitECS query.

import {
  defineQuery,
  enterQuery,
  IWorld
} from "bitecs";
import { GltfRoot, GltfRootProxy } from "@tiny-web-metaverse/client/src";

const enterGltfQuery = enterQuery(defineQuery([GltfRoot]));

export const gltfSystem = (world: IWorld): void => {
  enterGltfQuery(world).forEach(eid => {
    const gltfRoot = GltfRootProxy.get(eid).root;
    ...
  });
};

GLTFLoader plugin

While not yet officially documented, Three.js GLTFLoader offers an extensibility mechanism through its plugin system. Registered plugins will be called at specified stages; before, after, or during parsing glTF content. The plugin system is usually used for handling unlatified, vendor-specific, or custom glTF extensions.

In Tiny Web Metaverse, you can add such plugins by creating entities and adding GltfLoaderPluginComponent component to them. The built-in gltfLoad system registers the plugins to a Three.js GLTFLoader when loading a glTF file.

import {
  addComponent,
  addEntity,
  IWorld
} from "bitecs";
import { GLTFParser } from "three";
import {
  GltfLoaderPluginComponent,
  GltfLoaderPluginProxy
} from "@tiny-web-metaverse/client/src";

class FooPlugin {
  private parser: GLTFParser;

  constructor(parser: GLTFParser) {
    this.parser = parser;
    this.name = 'EXT_foo';
  }

  ...
}

class BarPlugin {
  private parser: GLTFParser;

  constructor(parser: GLTFParser) {
    this.parser = parser;
    this.name = 'EXT_bar';
  }

  ...
}

const addPlugins = (world: IWorld): void => {
  const fooEid = addEntity(world);
  addComponent(world, GltfLoaderPluginComponent, fooEid);
  GltfLoaderPluginProxy.get(fooEid).allocate((parser: GLTFParser) => {
    return new FooPlugin(parser);
  });

  const barEid = addEntity(world);
  addComponent(world, GltfLoaderPluginComponent, barEid);
  GltfLoaderPluginProxy.get(barEid).allocate((parser: GLTFParser) => {
    return new BarPlugin(parser);
  });
};

Event handling

As written above, we avoid async/await in systems for simplicity and predictable execution order.

However, certain types of asynchronous event processing cannot be avoided, and it is difficult to process them synchronously as-is. For example, input events from input devices such as keyboard and mouse are generated by user operations, so the timing of their occurrence cannot be predicted. Therefore, it is generally common to listen for these asynchronous events and process them when they occur. These asynchronous events typically occur during idle time.

Tiny Web Metaverse Client uses a trick to make these asynchronous events processable synchronously within systems.

1. Add asynchronous events that occurred during idle time to a queue.
2. At the beginning of the next animation frame, a system notifies events to entities that listen to that events by adding an event component to the entities.
3. A system processes the entities that received the event notification.
4. At the end of the animation frame, a system deletes the event component.

Let's take a look at the details with some specific code examples.

First create an event and its listener components. The component holds events as its component data.

// src/components/foo.ts

export const FooEvent = defineComponent();

export const enum FooEventType {
  Enter,
  Leave
};

export type FooEventValue = {
  type: FooEventType
};

export class FooEventProxy {
  private static instance: FooEventProxy = new FooEventProxy();
  private eid: number;
  private map: Map<number, FooEventValue[]>;

  private constructor() {
    this.eid = NULL_EID;
    this.map = new Map();
  }

  static get(eid: number): Foo {
    FooEventProxy.instance.eid = eid;
    return FooEventProxy.instance;
  }

  allocate(): void {
    this.map.set(this.eid, []);
  }

  free(): void {
    this.map.delete(this.eid);
  }

  get events(): FooEventValue[] {
    return this.map.get(this.eid)!;
  }
}

export const FooEventListener = defineComponent();

And create a system that stores async events happened during the idle time to a queue and notifies the events to entities that listen to that event by adding the event component in an animation loop.

Also create a system that removes the event component.

// src/systems/foo.ts

import {
  addComponent,
  defineQuery,
  enterQuery,
  hasComponent,
  IWorld
} from "bitecs";
import {
  FooEvent,
  FooEventListener,
  FooEventProxy,
  FooEventType
} from "../components/foo";
import { NullComponent } from "../components/null";

const eventQueue: { type: FooEventType }[] = [];

// enterQuery + NullComponent is a hack for executing only at the first call
const initializeQuery = enterQuery(defineQuery([NullComponent]));

const listenerQuery = defineQuery([FooEventListener]);
const eventQuery = defineQuery([FooEvent]);

export const fooEventNotificationSystem = (world: IWorld): void => {
  initializeQuery(world).forEach(() => {
    window.addEventListener('enterfoo', () => {
      eventQueue.push({ type: FooEventType.Enter });
    });

    window.addEventListener('leavefoo', () => {
      eventQueue.push({ type: FooEventType.Leave });
    });
  });

  for (const e of eventQueue) {
    listenerQuery(world).forEach(eid => {
      if (!hasComponent(world, FooEvent, eid)) {
        addComponent(world, FooEvent, eid);
        FooEventProxy.get(eid).allocate();
      }
      FooEventProxy.get(eid).push({ type: e.type });
    });
  }

  eventQueue.length = 0;
};

export const clearFooEventSystem = (world: IWorld): void => {
  eventQuery(world).forEach(eid => {
    const proxy = FooEventProxy.get(eid);
    proxy.events.length = 0;
    proxy.free();
    removeComponent(world, FooEvent, eid);
  });
};

And then write a system that fetches entities that have the event component and processes with the events.

export const fooEventSystem = (world: IWorld): void => {
  eventQuery(world).forEach(eid => {
    for (const e of FooEventProxy.get(eid).events) {
      ...
    }
  });
};

Finally, register the systems to App. We recommend SystemOrder.EventHandling for systems that notify events to entities and SystemOrder.TearDown for systems that remove event components. And systems that process with events need to run between them.

app.registerSystem(fooEventNotificationSyste, SystemOrder.EventHandling);
app.registerSystem(fooEventSystem, SystemOrder.BeforeMatricesUpdate);
app.registerSystem(clearFooEventSystem, SystemOrder.TearDown);

Event handling systems are ready. What you have to do last is to add a listener component to entities that want to know events occurrence.

const eid = addEntity(world);
addComponent(world, EventListener, eid);

Input source interaction

Different input devices, such as mice, touch panels, and VR controllers, have a variety of triggers, such as clicks, touches, and button presses. In some cases, we may want to perform the same processing for triggers from different devices. For example, we may want to process the left click of a mouse, the single-finger touch of a touch panel, and the button press of a VR controller events with the same processing. However, if we write the same processing for each of these input events, it will lead to duplicate code and poor maintainability.

Tiny Web Metaverse provides an abstract concept called InputSource to unify multiple types of triggers. We can reduce duplicate code by writing processing for InputSource events as follows:

import {
  defineQuery,
  IWorld
} from "bitecs";
import {
  FirstSourceInteractionTriggerEvent
} from "@tiny-web-metaverse/client/src";

const triggerQuery = defineQuery([FirstSourceInteractionTriggerEvent]);

export const fooSystem = (world: IWorld): void => {
  triggerQuery(world).forEach(eid => {
    ...
  });
};

For example, a mouse often has two buttons, left and right. There are multiple InputSources, such as FirstSource, SecondSource, and so on, to distinguish between different types of triggers.

Which trigger should be assigned to which InputSource is application-dependent, so users must specify it in their own applications by writing systems like the following:

import {
  addComponent,
  defineQuery,
  IWorld
} from "bitecs";
import {
  FirstSourceInteractable,
  FirstSourceInteractionLeaveEvent,
  FirstSourceInteractionTriggerEvent,
  MouseButtonEvent,
  MouseButtonEventProxy,
  MouseButtonEventType,
  MouseButtonType,
  SecondSourceInteractable,
  SecondSourceInteractionLeaveEvent,
  SecondSourceInteractionTriggerEvent
} from "@tiny-web-metaverse/client/src";

const firstSourceEventQuery = defineQuery([FirstSourceInteractable, MouseButtonEvent]);
const secondSourceEventQuery = defineQuery([SecondSourceInteractable, MouseButtonEvent]);

export const mouseInteractionTriggerSystem = (world: IWorld) => {
  firstSourceEventQuery(world).forEach(eid => {
    for (const e of MouseButtonEventProxy.get(eid).events) {
      if (e.button !== MouseButtonType.Left) {
        continue;
      }
      if (e.type === MouseButtonEventType.Down) {
        addComponent(world, FirstSourceInteractionTriggerEvent, eid);
      } else if (e.type === MouseButtonEventType.Up) {
        addComponent(world, FirstSourceInteractionLeaveEvent, eid);
      }
    }
  });

  secondSourceEventQuery(world).forEach(eid => {
    for (const e of MouseButtonEventProxy.get(eid).events) {
      if (e.button !== MouseButtonType.Right) {
        continue;
      }
      if (e.type === MouseButtonEventType.Down) {
        addComponent(world, SecondSourceInteractionTriggerEvent, eid);
      } else if (e.type === MouseButtonEventType.Up) {
        addComponent(world, SecondSourceInteractionLeaveEvent, eid);
      }
    }
  });
};

Systems that perform the most typical assignments is provided in Addons. For more details, see packages/addons/src/systems/*_interaction.ts. The assignment provided by Addons is as follows:

| Trigger | InputSource | | ---- | ---- | | Mouse left click | FirstSource | | Mouse right click | SecondSource | | Touch | FirstSource | | VR First controller | FirstSource | | VR Second controller | SecondSource |

TODO: Should VR controllers be distinguished by Left/Right controller instead of First/Second controller?

2D UI

As mentioned above, Client core doesn't take care of 2D UI controls that overlaps 3D canvas. It is the responsibility of user-application (or addons);

If you want HTML DOM elements to interact entities or components, the implementation would be similar to EventHandling described above, like storing events in idle time and processing with them in animation loop. This is an example of system code.

// src/systems/ui_button.ts

import { IWorld } from "bitecs";
import { FooComponent } from "../components/foo";
import { NullComponent } from "../components/null";

const enum ButtonEventType {
  Clicked
};

const eventQueue: { type: ButtonEventType }[] = [];

const button = document.createElement('button');
button.innerText = 'Click';
button.style.buttom = '10px';
button.style.left = '50%';
button.style.position = 'absolute';
button.style.transform = 'translate(-50%)';
button.style.zIndex = '1000';

button.addEventListener(() => {
  eventQueue.push({ type: ButtonEventType.Clicked });
});

// enterQuery + NullComponent is a hack for executing only at the first call
const initializeQuery = enterQuery(defineQuery([NullComponent]));
const fooQuery = defineQuery([FooComponent]);

export const buttonUISystem = (world: IWorld): void => {
  initializeQuery(world).forEach(() => {
    document.body.appendChild(button);
  });

  for (const e of eventQueue) {
    fooQuery(world).forEach(eid => {
      ...
    });
  }

  eventQueue.length = 0;
};

Stream server connection

Tiny Web Metaverse Client connects to the Stream server for voice communication with remote clients.

For starting the voice communication, there are two steps required. First connect to the Stream server and then join a room.

To connect to the Stream server, create an entity and add the built-in StreamConnectRequestor component to it. A built-in system will send a connection request to the Stream server.

To detect the connection, use the built-in ConnectedStreamEventListener component and observe the built-in StreamMessageType.Connected event.

To join a room, create an entity and add the built-in StreamJoinRequestor component to it. A built-in system will send a join request to the Stream server.

To detect the join, use the built-in JoinedStreamEventListener component and observe the built-in StreamMessageType.Joined event.

This is an example to connect and join the stream server.

// src/systems/stream_event.ts

import {
  addEntity,
  defineQuery,
  enterQuery,
  IWorld
} from "bitecs";
import {
  ConnectedStreamEventListener,
  JoinedStreamEventListener,
  NullComponent,
  StreamConnectRequestor,
  StreamEvent,
  StreamEventProxy,
  StreamMessageType,
  StreamJoinRequestor
} from "@tiny-web-metaverse/client/src";

const initialQuery = enterQuery(defineQuery([NullComponent]));
const eventQuery = defineQuery([StreamEvent]);

export const streamEventSystem = (world: IWorld): void => {
  initialQuery(world).forEach(() => {
    // Send a connection request in the first call.
    addComponent(world, StreamConnectRequestor, addEntity(world));
  });

  eventQuery(world).forEach(eid => {
    for (const event of StreamEventProxy.get(eid).events) {
      switch (event.type) {
        case StreamMessageType.Connected:
          // Connected. Send a join request.
          addComponent(world, StreamJoinRequestor, addEntity(world));
          break;
        case StreamMessageType.Joined:
          // Joined. Ready to start the voice conversation.
          ...
          break;
      }
    }
  });
};

// src/app.ts

import {
  addComponent,
  addEntity,
  IWorld
} from "bitecs";
import {
  ConnectedStreamEventListener,
  JoinedStreamEventListener
} from "@tiny-web-metaverse/client/src";

// App initialization

...

// Create an entity for listening stream events to mange stream server
// connection
const eid = addEntity(world);
addComponent(world, ConnectedStreamEventListener, eid);
addComponent(world, JoinedStreamEventListener, eid);

...

State server connection

Tiny Web Metaverse Client connects to the State server and synchronizes the state of entities state with remote clients through the state server.

App automatically connects to the State server in the constructor.

TODO: Instead of automatically connecting, connect to the state server when requested similar to the stream server?

Network sync

In Tiny Web Metaverse, the state of Entities can be selectively synchronized with remote clients in the same room. Here, the state of an Entities refers to the entity existence and the specified component data.

A synchronized Entity is called a Networked Entity, and its synchronized components are called Networked Components.

Some built-in network systems perform network synchronization processing.

The network send system periodically checks for changes to the data in the networked component. Only when changes are detected the system sends the updated data to the remote client via the server. This helps to prevent network data flooding.

The network receive system observes the network data sent from the remote client. When data is received, the system reflects the data to the networked components. Because data is only sent periodically, the system may interpolate the data before reflecting it.

Network entities are bound to a client that created them. If a client leaves the room, the network entities created by the client will be removed.

You need to do the following steps to create networked components and entities.

• Define networked components
• Write and register serializers/deserializers
• Write and register prefabs
• Create networked entity

Let's take a loot at one by one.

Networked type

There are three networked types.

• Local: Local networked components/entities should be directly controlled only by local client. Changes made to Local components/entities are sent to other clients by built-in network send system. On remote clients, Local components/entities appear as Remote.
• Remote: Remote networked components/entities should be directly controlled only by a remote client that created them. Updates to remote entities are received and applied to corresponding components/entities by built-in network receive system. Remote components/entities appear as Local on a remote client that created them and as Remote on other clients. on their creator client and as Remote on other clients.
• Shared: Shared networked components/entities are controlled by any client. Built-in network send and receive systems make them synced. Shared components/entities appear as Shared on all clients.

Networked Components

First you need to create a component that indicates another component is networked. For instance, if you want to make FooComponent component network-enabled, create NetworkedFooComponent. When both FooComponent and NetworkedFooComponent are added to an entity, FooComponent becomes networked. If only FooComponent is added, it remains non-networked.

// src/components/foo.ts

import { defineComponent, Types } from "bitecs";

export const FooComponent = defineComponent({
  data: Types.f32
});
export const NetworkedFooComponent = defineComponent();

// Refer to the following "Serializer" section about this component
export const FooComponentInterpolation = defineComponent({
  target: Types.f32
});

Serializer

Next, you have to write diff checkers, serializers, and deserializers for networked components.

Diff checker is a function periodically called to check for changes of networked component data.

Serializer is a function that serializes networked component data to send. Deserializer is a function that deserializes serialized network component data and reflects to component data.

You can define two deserializer types, one with interpolation and another one for without interpolation. The non-interpolation one is used for component data initialization and the other one is used for others.

In general, interpolation would be like

Normally, interpolation is a process of gradually approaching the target value. Such processing should be done in a system, and you need to write a system. In the deserializer, you will need to add a component to drive the system.

// src/serializers/position.ts

import {
  addComponent,
  hasComponent
} from "bitecs";
import {
  FooComponent,
  FooComponentInterpolation,
  NetworkedFooComponent
} from "../components/foo";

const EPSILON = 0.0001;
export type SerializedFoo = { data: number };

const checkFooDiff = (world: IWorld, eid: number, cache: SerializedFoo): boolean => {
  if (!hasComponent(world, FooComponent, eid)) {
    throw new Error('checkFooDiff requires FooComponent component.');
  }
  return Math.abs(cache.data - FooComponent.data[eid]) > EPSILON;
};

const serializeFoo = (world: IWorld, eid: number): SerializedFoo => {
  if (!hasComponent(world, FooComponent, eid)) {
    throw new Error('serializeFoo requires FooComponent component.');
  }
  return { data: FooComponent.data[eid] };
};

const deserializeFoo = (world: IWorld, eid: number, data: SerializedFoo): void => {
  if (!hasComponent(world, FooComponent, eid)) {
    throw new Error('deserializeFoo requires FooComponent component.');
  }
  FooComponent.data[eid] = data.data;
};

const networkDeserializeFoo = (world: IWorld, eid: number, data: SerializedFoo): void => {
  if (!hasComponent(world, FooComponent, eid)) {
    throw new Error('networkDeserializeFoo requires FooComponent component.');
  }
  // Add a component to drive a interpolation system.
  // Interpolation system implementation is omitted in this example.
  addComponent(world, FooComponentInterpolation, eid);
  FooComponentInterpolation.target[eid] = data.data;
};

export const fooSerializers = {
  deserializer: deserializeFoo,
  diffChecker: checkFooDiff,
  networkDeserializer: deserializeFoo,
  serializer: serializeFoo
};

And then you have to register the functions with the built-in registerSerializers() function to establish a mapping between the functions and the networked component.

The second argument is a unique key string within the application that identifies this mapping. This key is used in built-in network systems.

The third and fourth arguments specify a networked component and functions for mapping. These functions must be contained within an object that provides the diffChecker, serializer, deserializer, and networkDeserializer. Deserializer function without interpolation should be passed as deserializer, while deserializer function with interpolation should be passed as networkDeserializer.

import { registerSerializers } from "@tiny-web-metaverse/client/src";
import { NetworkedFoo } from "../components/networked_position";
import { fooSerializers } from "../serializers/foo";

registerSerializers(world, 'foo', NetworkedFoo, fooSerializers);

Prefab

Next, you have to write a prefab. Prefab is a function that takes bitECS world and an optional parameter, and creates an entity with preset components. Prefab may be said an entity template function.

A networked entity is created from a prefab. Networked components for a networked entity must be set up in a prefab.

// src/prefabs/foo.ts

import {
  addComponent,
  addEntity,
  IWorld
} from "bitecs";
import { FooComponent, NetworkedFoo } from "../components/foo";

export const FooPrefab = (world: IWorld, params: { data: number }): number => {
  const eid = addEntity(world);

  addComponent(world, FooComponent, eid);
  FooComponent.data[eid] = params.data;
  addComponent(world, NetworkedFoo, eid);

  return eid;
};

Similar to serializers, prefab have to be registered with the built-in registerPrefab() function.

The second argument is a unique key string within the application that identifies this prefab. This key is used when creating a networked entity.

The third argument specifies a prefab to register.

import { registerPrefab } from "@tiny-web-metaverse/client/src";
import { NetworkedFoo } from "../components/networked_position";
import { fooSerializers } from "../serializers/foo";

registerPrefab(world, 'foo', FooPrefab);

createNetworkedEntity()

The set up for networked entity creation has been done. The last thing you have to do is create networked entities.

You can create a networked entity with the built-in createNetworkedEntity().

The second argument specifies the network type with the built-in NetworkedType enum, Local or Shared. Remote entities creation is fired from a remote client so they are created in built-in network systems. createNetworkedEntity() is only for Local or Shared networked entities.

The third argument is a registered prefab key used to create a networked entity from.

import {
  createNetworkedEntity,
  NetworkedType
} from "@tiny-web-metaverse/client/src";

const eid = createNetworkedEntity(world, NetworkedType.Local, 'foo');

Networked entities have the built-in Networked component. And also they have either Local, Remote, or Shared built-in component corresponding to network type.

Audio processing

T.B.D.

Positional audio

T.B.D.

Custom audio effect

T.B.D.

Other utilities

NULL_EID and NullComponent

App first creates an entity with the built-in NullComponent component in the constructor. This first entity itself should not be used for any processing.

However, its Entity ID (which should be zero) can be used to indicate that the entity does not exist. For instance, you could write a function that searches for entities with a specific component and returns their Entity ID. If no such entity is found, it returns zero. There is a built-in variable, NULL_EID, which represents a nonexistent Entity ID.

import {
  Component,
  defineQuery
  IWorld,
  removeQuery
} from "bitecs";
import { NULL_EID } from "@tiny-web-metaverse/client/src";
import { FooComponent } from "../components/foo";

const searchAnyEntity = (world: IWorld, c: Component): number => {
  const query = defineQuery([c]);
  const eids = query(world);
  removeQuery(world, query);
  return eids.length > 0 ? eids[0] : NULL_EID;
};

const func = (world: IWorld): void => {
  const eid = searchAnyEntity(world, FooComponent);
  if (eid !== NULL_EID) {
    // If found
  } else {
    // If not found
  }
};

And built-in NullComponent component should be added only to that world-first entity. This NullComponent can be used for letting a system process something only in the first call by using bitECS defineQuery() and enterQuery().

import {
  defineQuery,
  enterQuery,
  IWorld
} from "bitecs";
import { NullComponent } from "@tiny-web-metaverse/client/src";

const initializeQuery = enterQuery(defineQuery([NullComponent]));

export const barSystem = (world: IWorld): void => {
  initializeQuery(world).forEach(() => {
    // This code is only executed once, the first time the system is called.
  });
};

User app examples

So far, you have learned the Client core concepts necessary to create your own Client (User app). You can create your own Client on top of the Client Core by adding Entities, Components, Systems, Prefabs, Serializers, UI, and so on. You can also import the Readme of addons package or create and publish addons for others to reuse.

A good starting point is to create a Client that displays avatars.

If you need more practical examples, see the examples package.

Avatar example

This is a very basic example to handle networked avatars.

• An avatar appears as a sphere object in the scene
• Local avatar is moved with the arrow keys
• Remote avatars shown moved by remote clients

// src/prefabs/avatar.ts

import {
  addComponent,
  addEntity,
  IWorld
} from "bitecs";
import { Mesh, MeshBasicMaterial, SphereGeometry } from "three";
import {
  addObject3D,
  Avatar,
  InScene,
  NetworkedPosition,
  NetworkedQuaternion,
  NetworkedScale  
} from "@tiny-web-metaverse/client/src";

export const AvatarPrefab = (world: IWorld): number => {
  const eid = addEntity(world);

  addComponent(world, Avatar, eid);
  addComponent(world, NetworkedPosition, eid);
  addComponent(world, NetworkedQuaternion, eid);
  addComponent(world, NetworkedScale, eid);
  addComponent(world, InScene, eid);

  const avatarObject = new Mesh(
    new SphereGeometry(0.25),
    new MeshBasicMaterial({ color: 0xaaaacc })    
  );
  addObject3D(world, avatarObject, eid);

  return eid;
};

// src/systems/avatar_key_controls.ts

import {
  addComponent,
  defineQuery,
  IWorld,
  removeComponent
} from "bitecs";
import {
  Avatar,
  KeyEvent,
  KeyEventProxy,
  KeyEventType,
  LinearMoveBackward,
  LinearMoveForward,
  LinearMoveLeft,
  LinearMoveRight,
  Local
} from "@tiny-web-metaverse/client/src";

const eventQuery = defineQuery([Avatar, KeyEvent, Local]);

export const avatarKeyControlsSystem = (world: IWorld): void => {
  eventQuery(world).forEach(eid => {
    const speed = 1.0;
    for (const e of KeyEventProxy.get(eid).events) {
      if (e.code === 37) { // Left
        if (e.type === KeyEventType.Down) {
          addComponent(world, LinearMoveLeft, eid);
          LinearMoveLeft.speed[eid] = speed;
        } else if (e.type === KeyEventType.Up) {
          removeComponent(world, LinearMoveLeft, eid);
        }
      } else if (e.code === 38) { // Up
        if (e.type === KeyEventType.Down) {
          addComponent(world, LinearMoveForward, eid);
          LinearMoveForward.speed[eid] = speed;
        } else if (e.type === KeyEventType.Up) {
          removeComponent(world, LinearMoveForward, eid);
        }
      } else if (e.code === 39) { // Right
        if (e.type === KeyEventType.Down) {
          addComponent(world, LinearMoveRight, eid);
          LinearMoveRight.speed[eid] = speed;
        } else if (e.type === KeyEventType.Up) {
          removeComponent(world, LinearMoveRight, eid);
        }
      } else if (e.code === 40) { // Down
        if (e.type === KeyEventType.Down) {
          addComponent(world, LinearMoveBackward, eid);
          LinearMoveBackward.speed[eid] = speed;
        } else if (e.type === KeyEventType.Up) {
          removeComponent(world, LinearMoveBackward, eid);
        }
      }
    }
  });
};

// src/app.ts

import { addComponent, addEntity } from "bitecs";
import {
  App,
  AudioDestination,
  createNetworkedEntity,
  KeyEventListener,
  registerPrefab,
  SystemOrder
} from "@tiny-web-metaverse/client/src";
import { AvatarPrefab } from "./prefabs/avatar";
import { avatarKeyControlsSystem } from "./systems/avatar_key_controls";

const roomId = '1234';
const canvas = document.createElement('canvas');

const app = new App({ canvas, roomId });
document.body.appendChild(canvas);

app.registerSystem(avatarKeyControls, SystemOrder.BeforeMatricesUpdate);

const world = app.getWorld();

registerPrefab(world, 'avatar', AvatarPrefab);

const avatarEid = createNetworkedEntity(world, NetworkedType.Local, 'avatar');
EntityObject3DProxy.get(avatarEid).root.position.set(0.0, 0.75, 2.0);
addComponent(world, KeyEventListener, avatarEid);
addComponent(world, AudioDestination, avatarEid);

app.start();

Avatar example with addons

This example is based on the above example. Instead of implementing a custom avatar key controls system, it imports controls addons and set them up.

// src/app.ts

import { addComponent, addEntity } from "bitecs";
import { avatarKeyControlsSystem } from "@tiny-web-metaverse/addons/src";
import {
  App,
  AudioDestination,
  createNetworkedEntity,
  KeyEventListener,
  registerPrefab,
  SystemOrder
} from "@tiny-web-metaverse/client/src";
import { AvatarPrefab } from "./prefabs/avatar";

const roomId = '1234';
const canvas = document.createElement('canvas');

const app = new App({ canvas, roomId });
document.body.appendChild(canvas);

app.registerSystem(avatarKeyControls, SystemOrder.BeforeMatricesUpdate);

const world = app.getWorld();

registerPrefab(world, 'avatar', AvatarPrefab);

const avatarEid = createNetworkedEntity(world, NetworkedType.Local, 'avatar');
EntityObject3DProxy.get(avatarEid).root.position.set(0.0, 0.75, 2.0);
addComponent(world, KeyEventListener, avatarEid);
addComponent(world, AudioDestination, avatarEid);

app.start();

Built-in components

T.B.D.

Network sync internal

T.B.D.