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How A-Frame VR is Different from Other 3D Markup Languages

Rough diagram of the A-Frame entity-component system.

Upon first seeing A-Frame, branded as "building blocks for the web" displaying markup like <a-cube>, developers may conceive A-Frame as yet another 3DML (3D markup language) such as X3Dom or GLAM. What A-Frame brings to the game is that it is based off an entity-component system, a pattern used by universal game engines like Unity which favors composability over inheritance. As we'll see, this makes A-Frame extremely extendable.

And A-Frame VR is extremely mindful of how to start a developer ecosystem. There are tools, tutorials, guides, boilerplates, libraries being built and knowledge being readily shared on Slack.

Entity-Component System

The entity-component system is a pattern in which every entity, or object, in a scene are general placeholders. Then components are used to add appearance, behavior, and functionality. They're bags of logic and data that can be applied to any entity, and they can be defined to just about do anything, and anyone can easily develop and share their components. To imagine this visually, let's revisit this image:

A good diagram for imagining what an entity looks like.

An entity, by itself without components, doesn't render or do anything. A-Frame ships with over 15 basic components. We can add a geometry component to give it shape, a material component to give it appearance, or a light component and sound component to have it emit light or sound.

Each component has properties that further defines how it modifies the entity. And components can be mixed and matched at will, hence the "composable" word root of "component". In traditional terms, they can be thought of as plugins. And anyone can write them to do anything, even explode an entity. They are expected to become an integral part of the workflow of building advanced scenes.

Writing and Sharing Components

So at what point does the promise of the ecosystem come in? A component is simply a plain JavaScript object that defines several lifecycle handlers that manages the component's data. Here are some example third-party components that I and other people have written:

Small components can be as little as a few lines of code. Under the hood, they either do three.js object or JavaScript DOM manipulations. I will go into more detail how to write a component at a later date, but to get started building a sharable component, check out the component boilerplate.

Comparison with Other 3DMLs

Other 3DMLs, or any markup languages at all for that matter, are based on an inheritance pattern. This is sort of the default pattern to go towards given the heirarchichal nature of HTML and XML. Even A-Frame was initially built in this way. The problem is that this lacks composability. Customizing objects to do something more than basic becomes difficult, both to the user and to the library developer.

Difficulty of extending traditional objects in 3DML.

The functionality of the language then becomes dependent on how many features the maintainers and the library add. With A-Frame however, composability brings about limitless functionality:

Composability makes it easy to extend objects in A-Frame.

Putting it logically, the different kinds of functionality you can squeeze out of an object is the permutation of the number of components you have. With the basic 16 components that A-Frame comes with, that's 65536 different sets of components that could be used. Add in the fact that components can be further customized with properties, and that there is an ecosystem of components to tap into, the previous use of the word "limitless" was quite literal.

With other 3DML libraries, if they ship 50 different kinds of objects, then you get only get 50 different kinds of objects with fixed behavior.

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