Component

In the entity-component-system pattern, a component is a reusable and modular chunk of data that we plug into an entity to add appearance, behavior, and/or functionality.

In A-Frame, components modify entities which are 3D objects in the scene. We mix and compose components together to build complex objects. They let us encapsulate three.js and JavaScript code into modules that we can use declaratively from HTML.

As an abstract analogy, if we define a smartphone as an entity, we might use components to give it appearance (color, shape), to define its behavior (vibrate when called, shut down on low battery), or to add functionality (camera, screen).

Components are roughly analogous to CSS. Like how CSS rules modify the appearance of elements, component properties modify the appearance, behavior, and functionality of entities.

Component HTML Form

A component holds a bucket of data in the form of one or more component properties. Components use this data to modify entities. Consider an engine component, we might define properties such as horsepower or cylinders.

HTML attributes represent component names and the value of those attributes represent component data.

Single-Property Component

If a component is a single-property component, meaning its data consists of a single value, then in HTML, the component value looks like a normal HTML attribute:

<!-- `position` is the name of the position component. -->
<!-- `1 2 3` is the data of the position component. -->
<a-entity position="1 2 3"></a-entity>

Multi-Property Component

If a component is a multi-property component, meaning the data consists of multiple properties and values, then in HTML, the component value resembles inline CSS styles:

<!-- `light` is the name of the light component. -->
<!-- The `type` property of the light is set to `point`. -->
<!-- The `color` property of the light is set to `crimson`. -->
<a-entity light="type: point; color: crimson"></a-entity>

Register a Component

`AFRAME.registerComponent (name, definition)`

Register an A-Frame component. We must register components before we use them anywhere in <a-scene>. Meaning from an HTML file, components should come in order before <a-scene>.

{string} name - Component name. The component’s public API as represented through an HTML attribute name.
{Object} definition - Component definition. Contains schema and lifecycle handler methods.

// Registering component in foo-component.js
AFRAME.registerComponent('foo', {
  schema: {},
  init: function () {},
  update: function () {},
  tick: function () {},
  remove: function () {},
  pause: function () {},
  play: function () {}
});

<!-- Usage of `foo` component. -->
<html>
  <head>
    <script src="aframe.min.js"></script>
    <script src="foo-component.js"></script>
  </head>
  <body>
    <a-scene>
      <a-entity foo></a-entity>
    </a-scene>
  </body>
</html>

Schema

The schema is an object that defines and describes the property or properties of the component. The schema’s keys are the names of the property, and the schema’s values define the types and values of the property (in case of a multi-property component):

AFRAME.registerComponent('bar', {
  schema: {
    color: {default: '#FFF'},
    size: {type: 'int', default: 5}
  }
}

<a-scene>
  <a-entity bar="color: red; size: 20"></a-entity>
</a-scene>

propsimage Image by Ruben Mueller from vrjump.de

Property Types

Property types primarily define how the schema parses incoming data from the DOM for each property. The parsed data will then be available via the data property on the component’s prototype. Below are A-Frame’s built-in property types:

Property Type	Description	Default Value
array	Parses comma-separated values to array (i.e., `"1, 2, 3" to ['1', '2', '3'])`.	[]
asset	For URLs pointing to general assets. Can parse URL out of a string in the form of `url(<url>)`. If the value is an element ID selector (e.g., `#texture`), this property type will call `getElementById` and `getAttribute('src')` to return a URL. The `asset` property type may or may not change to handle XHRs or return MediaElements directly (e.g., `<img>` elements).	‘’
audio	Same parsing as the `asset` property type. Will possibly be used by the A-Frame Inspector to present audio assets.	‘’
boolean	Parses string to boolean (i.e., `"false"` to false, everything else truthy).	false
color	Currently doesn’t do any parsing. Primarily used by the A-Frame Inspector to present a color picker. Also, it is required to use color type for color animations to work.	#FFF
int	Calls `parseInt` (e.g., `"124.5"` to `124`).	0
map	Same parsing as the `asset` property type. Will possibly be used by the A-Frame Inspector to present texture assets.	‘’
model	Same parsing as the `asset` property type. Will possibly be used by the A-Frame Inspector to present model assets.	‘’
number	Calls `parseFloat` (e.g., `"124.5"` to `124.5`).	0
selector	Calls `querySelector` (e.g., `"#box"` to `<a-entity id="box">`).	null
selectorAll	Calls `querySelectorAll` and converts `NodeList` to `Array` (e.g., `".boxes"` to [<a-entity class=”boxes”, …]),	null
string	Doesn’t do any parsing.	‘’
vec2	Parses two numbers into an `{x, y}` object (e.g., `1 -2` to `{x: 1, y: -2}`.	{x: 0, y: 0}
vec3	Parses three numbers into an `{x, y, z}` object (e.g., `1 -2 3` to `{x: 1, y: -2, z: 3}`.	{x: 0, y: 0, z: 0}
vec4	Parses four numbers into an `{x, y, z, w}` object (e.g., `1 -2 3 -4.5` to `{x: 1, y: -2, z: 3, w: -4.5}`.	{x: 0, y: 0, z: 0, w: 0}

Property Type Inference

The schema will try to infer a property type given only a default value:

schema: {default: 10}  // type: "number"
schema: {default: "foo"}  // type: "string"
schema: {default: [1, 2, 3]}  // type: "array"

The schema will set a default value if not provided, given the property type:

schema: {type: 'number'}  // default: 0
schema: {type: 'string'}  // default: ''
schema: {type: 'vec3'}  // default: {x: 0, y: 0, z: 0}

Custom Property Type

We can also define our own property type by providing a parse and stringify function in place of a type:

schema: {
  // Parse slash-delimited string to an array (e.g., `foo="myProperty: a/b"` to `['a', 'b']`),
  // stringify array to string (e.g., `['a', 'b']` to `foo="myProperty: a/b"`)
  myProperty: {
    default: [],
    parse: function (value) {
      return value.split('/');
    },
    stringify: function (value) {
      return value.join('/');
    }
  }
}

parse is called when component properties are updated by setAttribute method. stringify is called when DOM is updated by flushToDom method.

Single-Property Schema

A component can either be a single-property component (consisting of one anonymous value) or a multi-property component (consisting of multiple named values). A-Frame will infer whether a component is single-property vs. multi-property based on the structure of the schema.

A single-property component’s schema contains type and/or default keys, and the schema’s values are plain values rather than objects:

AFRAME.registerComponent('foo', {
  schema: {type: 'int', default: 5}
});

<a-scene>
  <a-entity foo="20"></a-entity>
</a-scene>

Definition Lifecycle Handler Methods

With the schema being the anatomy, the lifecycle methods are the physiology; the schema defines the shape of the data, the lifecycle handler methods use the data to modify the entity. The handlers will usually interact with the Entity API.

methodsimage Lifecycle method handlers. Image by Ruben Mueller from vrjump.de

Overview of Methods

Method	Description
init	Called once when the component is initialized. Used to set up initial state and instantiate variables.
update	Called both when the component is initialized and whenever any of the component’s properties is updated (e.g, via setAttribute). Used to modify the entity.
remove	Called when the component is removed from the entity (e.g., via removeAttribute) or when the entity is detached from the scene. Used to undo all previous modifications to the entity.
tick	Called on each render loop or tick of the scene. Used for continuous changes or checks.
tock	Called on each render loop or tick of the scene after the scene has rendererd. Used for post processing effects or other logic that needs to happen after the scene has been drawn.
play	Called whenever the scene or entity plays to add any background or dynamic behavior. Also called once when the component is initialized. Used to start or resume behavior.
pause	Called whenever the scene or entity pauses to remove any background or dynamic behavior. Also called when the component is removed from the entity or when the entity is detached from the scene. Used to pause behavior.
updateSchema	Called whenever any of the component’s properties is updated. Can be used to dynamically modify the schema.

Component Prototype Properties

Within the methods, we have access to the component prototype via this:

Property	Description
this.data	Parsed component properties computed from the schema default values, mixins, and the entity’s attributes. Important: Do not modify the data attribute directly. It is updated internally by A-Frame. To modify a component, use setAttribute.
this.el	Reference to the entity as an HTML element.
this.el.sceneEl	Reference to the scene as an HTML element.
this.id	If the component can have multiple instances, the ID of the individual instance of the component (e.g., `foo` from `sound__foo`).

`.init ()`

.init () is called once at the beginning of the component’s lifecycle. An entity can call the component’s init handler:

When the component is statically set on the entity in the HTML file and the page is loaded.
When the component is set on an attached entity via setAttribute.
When the component is set on an unattached entity, and the entity is then attached to the scene via appendChild.

The init handler is often used to:

Set up initial state and variables
Bind methods
Attach event listeners

For example, a cursor component’s init would set state variables, bind methods, and add event listeners:

AFRAME.registerComponent('cursor', {
  // ...
  init: function () {
    // Set up initial state and variables.
    this.intersection = null;
    // Bind methods.
    this.onIntersection = this.onIntersection.bind(this);
    // Attach event listener.
    this.el.addEventListener('raycaster-intersection', this.onIntersection);
  }
  // ...

`.update (oldData)`

.update (oldData) is called whenever the component’s properties change, including at the beginning of the component’s lifecycle. An entity can call a component’s update handler:

After init () is called, at the beginning of component’s lifecycle.
When the component’s properties are updated with .setAttribute.

The update handler is often used to:

Do most of the work in making modifications to the entity, using this.data.
Modify the entity whenever one or more component properties change.

Granular modifications to the entity can be done by diffing the current dataset (this.data) with the previous dataset before the update (oldData).

A-Frame calls .update() both at the beginning of a component’s lifecycle and every time a component’s data changes (e.g., as a result of setAttribute). The update handler often uses this.data to modify the entity. The update handler has access to the previous state of a component’s data via its first argument. We can use the previous data of a component to tell exactly which properties changed to do granular updates.

For example, the visible component’s update sets the visibility of the entity.

AFRAME.registerComponent('visible', {
  /**
   * this.el is the entity element.
   * this.el.object3D is the three.js object of the entity.
   * this.data is the component's property or properties.
   */
  update: function (oldData) {
    this.el.object3D.visible = this.data;
  }
  // ...
});

`.remove ()`

.remove () is called whenever the component is detached from the entity. An entity can call a component’s remove handler:

When the component is removed from the entity via removeAttribute.
When the entity is detached from the scene (e.g., removeChild).

The remove handler is often used to:

Remove, undo, or clean up all of the component’s modifications to the entity.
Detach event listeners.

For example, when the light component is removed, the light component will remove the light object that it had previously set on the entity, thus removing it from the scene.

AFRAME.registerComponent('light', {
  // ...
  remove: function () {
    this.el.removeObject3D('light');
  }
  // ...
});

`.tick (time, timeDelta)`

.tick () is called on each tick or frame of the scene’s render loop. The scene will call a component’s tick handler:

On each frame of the render loop.
On the order of 60 to 120 times per second.
If the entity or scene is not paused (e.g., the Inspector is open).
If the entity is still attached to the scene.

The tick handler is often used to:

Continuously modify the entity on each frame or on an interval.
Poll for conditions.

The tick handler is provided the global uptime of the scene in milliseconds (time) and the time difference in milliseconds since the last frame (timeDelta). These can be used for interpolation or to only run parts of the tick handler on a set interval.

For example, the tracked controls component will progress the controller’s animations, update the controller’s position and rotation, and check for button presses.

AFRAME.registerComponent('tracked-controls', {
  // ...
  tick: function (time, timeDelta) {
    this.updateMeshAnimation();
    this.updatePose();
    this.updateButtons();
  }
  // ...
});

`.tock (time, timeDelta, camera)`

Identical to the tick method but invoked after the scene has rendered.

The tock handler is used to run logic that needs access to the drawn scene before it’s pushed into the headset like postprocessing effects.

`.pause ()`

.pause () is called when the entity or scene pauses. The entity can call a component’s pause handler:

Before the component is removed, before the remove handler is called.
When the entity is paused with Entity.pause ().
When the scene is paused with Scene.pause () (e.g., the Inspector is opened).

The pause handler is often used to:

Remove event listeners.
Remove any chances of dynamic behavior.

For example, the sound component will pause the sound and remove an event listener that would have played a sound on an event:

AFRAME.registerComponent('sound', {
  // ...
  pause: function () {
    this.pauseSound();
    this.removeEventListener();
  }
  // ...
});

`.play ()`

.play () is called when the entity or scene resumes. The entity can call a component’s play handler:

When the component is first attached, after the update handler is called.
When the entity was paused but then resumed with Entity.play ().
When the scene was paused but then resumed with Scene.play ().

The play handler is often use to:

Add event listeners.

For example, the sound component will play the sound and update the event listener that would play a sound on an event:

AFRAME.registerComponent('sound', {
  // ...
  play: function () {
    if (this.data.autoplay) { this.playSound(); }
    this.updateEventListener();
  }
  // ...
});

`.updateSchema (data)`

.updateSchema (), if defined, is called on every update in order to check if the schema needs to be dynamically modified.

The updateSchema handler is often used to:

Dynamically update or extend the schema, usually depending on the value of a property.

For example, the geometry component checks if the primitive property changed to determine whether to update the schema for a different type of geometry:

AFRAME.registerComponent('geometry', {
  // ...
  updateSchema: (newData) {
    if (newData.primitive !== this.data.primitive) {
      this.extendSchema(GEOMETRIES[newData.primitive].schema);
    }
  }
  // ...
});

Definition Properties

`dependencies`

dependencies allows for control on ordering of component initialization if a component depends on one or more other components. Component names specified in the dependencies array will be initialized left-to-right before initializing the current component. If the dependency have other dependency components, those other dependency components will be ordered in the same manner:

// Initializes last.
AFRAME.registerComponent('a', {
  dependencies: ['b']
});

// Initializes second.
AFRAME.registerComponent('b', {
  dependencies: ['c']
});

// Initializes first.
AFRAME.registerComponent('c', {});

`multiple`

The multiple flag allows for a component to have multiple instances of itself on an entity. Since multiple is set to false by default, an entity could only have a single instance of a component. For example, an entity could only have one geometry component.

But if a component has multiple set to true, then the component can have multiple instances:

AFRAME.registerComponent('foo', {
  multiple: true,
  // ...
});

In the DOM, we can differentiate between instances of the component by giving a suffix of a double underscore and ID (__<ID>). For example, to attach multiple instances of the sound component:

<a-scene>
  <a-entity
    sound="src: url(sound.mp3)"
    sound__beep="src: url(beep.mp3)"
    sound__boop="src: url(boop.mp3)"
  ></a-entity>
</a-scene>

From the component lifecycle handler methods, we can differentiate between instances of the component with this.id. If a component instance is set with foo__bar, then this.id would be "bar":

AFRAME.registerComponent('foo', {
  multiple: true,

  update: function () {
    console.log('This component instance has the ID', this.id);
  }
});

If we’re doing a setObject3D(), we’ll usually want to use this.attrName. If a component instance is set with foo__bar, then this.attrName would be foo__bar. This gives us a namespace and an ID to set an object3D on the entity’s object3DMap:

AFRAME.registerComponent('foo', {
  multiple: true,

  update: function () {
    // An object3D will be set using `foo__bar` as the key.
    this.el.setObject3D(this.attrName, new THREE.Mesh());
  }
});

`sceneOnly`

The sceneOnly flag indicates if a component can only be applied to the scene entity. Since sceneOnly is set to false by default, the component can be added to any entity. For example, any entity could have a geometry component.

But if a component has sceneOnly set to true, then the component can only be applied to <a-scene>:

AFRAME.registerComponent('foo', {
  sceneOnly: true,
  // ...
});

`events`

The events object allows for conveniently defining event handlers that get binded and automatically attached and detached at appropriate times during the component’s lifecycle:

Attached on .play()
Detached on .pause() and .remove()

Using events ensures that event handlers properly clean themselves up when the entity or scene is paused, or the component is detached. If a component’s event handlers are registered manually and not detached properly, the event handler can still fire even after the component no longer exists.

AFRAME.registerComponent('foo', {
  events: {
    click: function (evt) {
      console.log('This entity was clicked!');
      this.el.setAttribute('material', 'color', 'red');
    }
  }
});

Component Prototype Methods

`.flushToDOM ()`

To save on CPU time on stringification, A-Frame will only update in debug mode the component’s serialized representation in the actual DOM. Calling flushToDOM () will manually serialize the component’s data and update the DOM:

document.querySelector('[geometry]').components.geometry.flushToDOM();

Read more about component-to-DOM serialization.

Accessing a Component’s Members and Methods

A component’s members and methods can be accessed through the entity from the .components object. Look up the component from the entity’s map of components, and we’ll have access to the component’s internals. Consider this example component:

AFRAME.registerComponent('foo', {
  init: function () {
    this.bar = 'baz';
  },

  qux: function () {
    // ...
  }
});

Let’s access the bar member and qux method:

var fooComponent = document.querySelector('[foo]').components.foo;
console.log(fooComponent.bar);
fooComponent.qux();