HTML & Primitives
This section will go over the concepts of A-Frame’s primitive elements and their relation to the entity-component framework. If you’re looking for a guide on using HTML and primitives, check out the Building a Basic Scene guide.
HTML
A-Frame is based on top of HTML and the DOM using a polyfill for Custom Elements. HTML is the building block of the Web, providing one of the most accessible computing languages around. There are no installations or build steps required, creating with HTML involves just text in an HTML file and opening the HTML file in a browser. Since most of the Web was built on top of HTML, most existing tools and libraries work with A-Frame including React, Vue.js, Angular, d3.js, and jQuery.
If you don’t have too much experience with HTML, no problem! It’s fairly easy to pick up and perhaps even easier to grasp than 2D HTML. Once you pick up the general structure or syntax of HTML (opening tag, attributes, closing tag), then you’re good to go! Read an introduction to HTML on MDN.
Primitives
While the HTML layer looks basic, HTML and the DOM are only the outermost abstraction layer of A-Frame. Underneath, A-Frame is an entity-component framework for three.js that is exposed declaratively.
A-Frame provides a handful of elements such as <a-box>
or <a-sky>
called
primitives that wrap the entity-component pattern to make it appealing for
beginners. At the bottom of the documentation navigation sidebar, we can see
every primitive that A-Frame provides out of the box. Developers can create
their own primitives as well.
Example
Below is the Hello, WebVR example that uses a few basic primitives. A-Frame provides primitives to create meshes, render 360° content, customize the environment, place the camera, etc.
<html> |
Under the Hood
Primitives act as a convenience layer (i.e., syntactic sugar) primarily for
newcomers. Keep in mind for now that primitives are <a-entity>
s under the
hood that:
- Have a semantic name (e.g.,
<a-box>
) - Have a preset bundle of components with default values
- Map or proxy HTML attributes to component data
Primitives are similar to prefabs in Unity. Some literature on the entity-component-system pattern refer to them as assemblages. They abstract the core entity-component API to:
- Pre-compose useful components together with prescribed defaults
- Act as a shorthand for complex-but-common types of entities (e.g.,
<a-sky>
) - Provide a familiar interface for beginners since A-Frame takes HTML in a new direction
Under the hood, this <a-box>
primitive:
<a-box color="red" width="3"></a-box> |
represents this entity-component form:
<a-entity geometry="primitive: box; width: 3" material="color: red"></a-entity> |
<a-box>
defaults the geometry.primitive
property to box
. And the
primitive maps the HTML width
attribute to the underlying geometry.width
property as well as the HTML color
attribute to the underlying
material.color
property.
Attaching Components to Primitives
Primitives are just <a-entity>
s under the hood. This means primitives have
the same API as entities such as positioning, rotating, scaling, and attaching
components.
Example
Let’s attach community physics components to primitives. We include the source
for Don McCurdy’s
aframe-physics-system
and attach
the physics components via HTML attributes:
If you are using A-Frame 1.6.0 or later:
aframe-physics-system
and you’re having issues, make sure you’re using THREE.BufferGeometry, not the now-deprecated THREE.Geometry. Recent versions of three.js rename generators such as PlaneBufferGeometry to just PlaneGeometry, but support the old name as an alias. More info on this GitHub issue.
<html> |
Registering a Primitive
We can register our own primitives (i.e., register an element) using
AFRAME.registerPrimitive(name, definition)
. name
is a string and must contain a dash (i.e. 'a-foo'
). definition
is a JavaScript
object defining these properties:
Property | Description | Example |
---|---|---|
defaultComponents | Object specifying default components of the primitive. The keys are the components’ names and the values are the components’ default data. | {geometry: {primitive: 'box'}} |
mappings | Object specifying mapping between HTML attribute name and component property names. Whenever the HTML attribute name is updated, the primitive will update the corresponding component property. The component property is defined using a dot syntax ${componentName}.${propertyName} . |
{depth: 'geometry.depth', height: 'geometry.height', width: 'geometry.width'} |
Example
For example, below is A-Frame’s registration for the <a-box>
primitive:
var extendDeep = AFRAME.utils.extendDeep; |
For example, Don McCurdy’s aframe-extras
package includes an
<a-ocean>
primitive that wraps his ocean
component. Here is the definition
for <a-ocean>
.
AFRAME.registerPrimitive('a-ocean', { |
With the <a-ocean>
primitive registered, we’d be able to create oceans using
a line of traditional HTML:
<a-ocean color="aqua" depth="100" width="100"></a-ocean> |