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Primitives

Remarks:

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][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.

Registering a Primitive

We can register our own primitives (i.e., register an element) using AFRAME.registerPrimitive(name, definition). definition is a JavaScript object defining these properties:

Property	Description
defaultComponents	Object specifying default components of the primitive. The keys are the components' names and the values are the components' default data.
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}`.

For example, below is A-Frame's registration for the <a-box> primitive:

var extendDeep = AFRAME.utils.extendDeep;

// The mesh mixin provides common material properties for creating mesh-based primitives.
// This makes the material component a default component and maps all the base material properties.
var meshMixin = AFRAME.primitives.getMeshMixin();

AFRAME.registerPrimitive('a-box', extendDeep({}, meshMixin, {
  // Preset default components. These components and component properties will be attached to the entity out-of-the-box.
  defaultComponents: {
    geometry: {primitive: 'box'}
  },

  // Defined mappings from HTML attributes to component properties (using dots as delimiters).
  // If we set `depth="5"` in HTML, then the primitive will automatically set `geometry="depth: 5"`.
  mappings: {
    depth: 'geometry.depth',
    height: 'geometry.height',
    width: 'geometry.width'
  }
}));

Which we can use then

<a-box depth="1.5" height="1.5" width="1.5"></a-box>

represents this entity-component form:

<a-entity geometry="primitive: box; depth: 1.5; height: 1.5; width:1.5;"></a-entity>

Contributors

Topic Id: 10074

Example Ids: 30918

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