Entity

Note: This documentation is for the old 0.2.0 version of A-Frame. Check out the documentation for the current 1.6.0 version

An entity is represented by the <a-entity> element. As defined in the entity-component-system pattern, entities are placeholder objects to which we plug in components to in order to provide them apperance, behavior, and functionality.

In A-Frame, entities have inherently have attached the position, rotation, and scale components.

Example

Consider the entity below. By itself, it has no appearance, behavior, or functionality. It does nothing:

<a-entity>

We can attach components to it to make it render something or do something. To give it shape and appearance, we can attach the geometry and material components:

<a-entity geometry="primitive: box" material="color: red">

Or to make it emit light, we can further attach the light component:

<a-entity geometry="primitive: box" material="color: red"
          light="type: point; intensity: 2.0">

Retrieving an Entity

We can simply retrieve an entity using DOM APIs.

<a-entity id="mario"></a-entity>

var el = document.querySelector('#mario');

Once we have an entity, we have access to all of its properties and methods, which are detailed below.

Properties

components

<a-entity>.components is an object of components attached to the entity. This gives us access to all of the entity’s components including their data, methods, and API.

For example, if we wanted to grab an entity’s three.js camera object or material object, we could reach into its components:

var camera = document.querySelector('a-entity[camera]').components.camera.camera;
var material = document.querySelector('a-entity[material]').components.material.material;

Or if a component exposes an API, we can call its methods:

document.querySelector('a-entity[sound]').components.sound.pause();

isPlaying

Whether or not the entity is active and playing. If the entity is paused, then isPlaying will be false.

object3D

<a-entity>.object3D is a reference to the entity’s three.js Object3D representation. More specifically, object3D will be a THREE.Group object that may contain different types of THREE.Object3Ds such as cameras, meshes, lights, or sounds:

// Gaining access to the internal three.js scene graph.
var groupObject3D = document.querySelector('a-entity').object3D;
console.log(groupObject3D.parent);
console.log(groupObject3D.children);

The different types Object3Ds can be accessed through object3DMap.

object3DMap

An entity’s object3DMap is an JavaScript object that gives access to the different types of THREE.Object3Ds (e.g., camera, meshes, lights, sounds) that may have been registered by components.

For an entity with a geometry and light components attached, object3DMap might look like:

{
  light: <THREE.Light Object>,
  mesh: <THREE.Mesh Object>
}

An entity’s THREE.Object3Ds can be managed using getOrCreateObject3D, setObject3D, and removeObject3D.

sceneEl

An entity has a reference to its scene element.

var sceneEl = document.querySelector('a-scene');
var entity = sceneEl.querySelector('a-entity');
console.log(entity.sceneEl === sceneEl);  // >> true.

Methods

addState (stateName)

addState will push a state onto the entity. This will emit the stateadded event, and the state can then be checked for existence using .is:

entity.addEventListener('stateadded', function (evt) {
  if (evt.state === 'selected') {
    console.log('Entity now selected!');
  }
});

entity.addState('selected');
entity.is('selected');  // >> true

emit (name, detail, bubbles)

emit emits a custom DOM event on the entity. For example, we can emit an event to trigger an animation:

// <a-entity>
//   <a-animation attribute="rotation" begin="rotate" to="0 360 0"></a-animation>
// </a-entity>
entity.emit('rotate');

We can also pass event detail or data as the second argument:

entity.emit('collide', { target: collidingEntity });

The event will bubble by default. we can tell it not to bubble by passing false for bubble:

entity.emit('sink', null, false);

getAttribute (attr)

getAttribute can be used to retrieve parsed component data. If attr is the name of a registered component. getAttribute will return only the component data defined in the HTML as a parsed object. getAttribute for components is the partial form of getComputedAttribute since the returned component data does not include applied mixins or default values:

// <a-entity geometry="primitive: box; width: 3">

entity.getAttribute('geometry');
// >> { primitive: "box", width: 3 }

entity.getAttribute('geometry').primitive;
// >> "box"

entity.getAttribute('geometry').height;
// >> undefined

If attr is not the name of a registered component, getAttribute will behave as it normally would:

// <a-entity data-position="0 1 1">

entity.getAttribute('data-position');
// >> "0 1 1"

getComputedAttribute (attr)

getComputedAttribute is similar to getAttribute, but it will return all of the component’s properties for multi-property components. It can be thought of as an analog to getComputedStyle, which in CSS returns all CSS properties after applying stylesheets and computations. getComputedAttribute will return all component properties after applying mixins and default values.

Compare the output of the above example of getAttribute:

// <a-entity geometry="primitive: box; width: 3">

entity.getAttribute('geometry');
// >> { primitive: "box", depth: 2, height: 2, translate: "0 0 0", width: 3, ... }

entity.getAttribute('geometry').primitive;
// >> "box"

entity.getAttribute('geometry').height;
// >> 2

More often we will want to use getComputedAttribute to inspect the component’s data. Though sometimes we might want to use getAttribute to discern which properties were explicitly defined.

getObject3D (type)

getObject3D looks up a child THREE.Object3D of the entity that is registered under type for object3DMap:

AFRAME.registerComponent('example-mesh', {
  init: function () {
    var el = this.el;
    el.getOrCreateObject3D('mesh', THREE.Mesh);
    el.getObject3D('mesh');  // Returns THREE.Mesh that was just created.
  }
});

`getOrCreateObject3D (type, Constructor)

If the entity does not have a THREE.Object3D registered under type, getOrCreateObject3D will register an instantiated THREE.Object3D using the passed Constructor. If the entity does have an THREE.Object3D registered under type, getOrCreateObject3D will act as getObject3D:

AFRAME.registerComponent('example-geometry', {
  update: function () {
    var mesh = this.el.getOrCreateObject3D('mesh', THREE.Mesh);
    mesh.geometry = new THREE.Geometry();
  }
});

pause ()

pause will stop any dynamic behavior as defined by animations and components. When an entity is paused, it will stop all of its animations and call Component.pause on each of its components. It is up to the components to implement how they paused, but they generally remove event listeners and background behavior. An entity will call pause on all of its children when it is paused itself.

// <a-entity id="spinning-jumping-ball">
entity.pause();

For example, the look-controls component on pause will remove event handlers that listen for input.

play ()

play will start any dynamic behavior as defined by animations and components. This is automatically called when the entity is attached. When an entity calls play, it will call play on all of its children.

entity.pause();
entity.play();

For example, the sound component on play will begin playing the sound.

setAttribute (attr, value, componentAttrValue)

If attr is not the name of a registered component or the component is a single-property component, setAttribute behaves mostly as it normally would:

entity.setAttribute('visible', false);

Though if attr is the name of a registered component, it may handle special parsing for the value. For example, the position component is a single-property component, but its property type parser allows it to take an object:

entity.setAttribute('position', { x: 1, y: 2, z: 3 });

Putting Multi-Property Component Data

To set or replace component data for a multi-property component, we can pass the name of a registered component as the attr, and pass an object of properties as the value:

// All previous properties for the light component will be removed and overwritten.
entity.setAttribute('light', {
  type: 'spot',
  distance: 30,
  intensity: 2.0
});

Updating Multi-Property Component Data

To update individual properties for a multi-property component, we can pass the name of registered component as the attr, a property name as the second argument, and the property value to set as the third argument:

// All previous properties for the material component (besides the color)  will be unaffected.
entity.setAttribute('material', 'color', 'crimson');

setObject3D (type, obj)

setObject3D will register the passed obj, a THREE.Object3D, as type under the entity’s object3DMap. obj will be added as a child of the entity’s root object3D. Passing in the value null for obj has the effect of unregistered the THREE.Object3D previously registered under type.

AFRAME.registerComponent('example-orthogonal-camera', {
  update: function () {
    this.el.setObject3D('camera', new THREE.OrthogonalCamera());
  }
});

removeAttribute (attr)

If attr is the name of a registered component, along with removing the attribute from the DOM, removeAttribute will also detach the component from the entity, invoking the component’s remove lifecycle method.

entity.removeAttribute('sound');  // The entity will no longer play sound.

removeObject3D (type)

removeObject3D removes the object specified by type from the entity’s THREE.Group and thus from the scene. This will update the entity’s object3DMap, setting the value of the type key to null. This is generally called from a component, often within the remove handler:

AFRAME.registerComponent('example-light', {
  update: function () {
    this.el.setObject3D('light', new THREE.Light());
    // Light is now part of the scene.
    // object3DMap.light is now a THREE.Light() object.
  },

  remove: function () {
    this.el.removeObject3D('light');
    // Light is now removed from the scene.
    // object3DMap.light is now null.
  }
});

removeState (stateName)

removeState will pop a state from the entity. This will emit the stateremove event, and the state can then be checked for its removal using .is:

entity.addEventListener('stateremoved', function (evt) {
  if (evt.state === 'selected') {
    console.log('Entity no longer selected.');
  }
});

entity.addState('selected');
entity.is('selected');  // >> true

entity.removeState('selected');
entity.is('selected');  // >> false

Events

Listening for Component Changes

We can use the componentchanged event to listen for changes to the entity:

entity.addEventListener('componentchanged', function (evt) {
  if (evt.name === 'position') {
    console.log('Entity has moved from', evt.oldData, 'to', evt.newData, '!');
  }
});

Event Detail

Below is what the event detail will contain for componentchanged: