How to spawn an entity in Zero Gravity with RealityKit?

To spawn an entity in zero gravity with RealityKit, we can utilize the Entity Component System (ECS) to create a custom gravity Component. Here's an example implementation:

class GravityComponent: Component {
    var gravity: Float
    var mass: Float 
    
    init(gravity:Float = 9.81, mass:Float) {
        self.gravity = gravity 
        self.mass = mass
    }
    
    func update(entity: Entity, with deltaTime: TimeInterval, at physicsOrigin: Entity) {
        // Check that gravity is not default
        guard gravity != 9.81 else { return }
        // Check that entity indeed has physicsBody
        guard let physicsBody = entity as? HasPhysicsBody else { return }       
        
        // Calculate gravity correction
        let newG: Float = 9.81 - gravity
        
        // Apply linear impulse to the physicsBody
        let impulse = mass * newG * Float(deltaTime)
        physicsBody.applyLinearImpulse([0.0, impulse, 0.0], relativeTo: physicsOrigin)
    }
}

This GravityComponent applies a linear impulse to the entity's physics body based on its mass and gravity properties, compensating for FPS changes by considering the time interval (deltaTime) between frames. To use it, add the component to the entity and call its update method in your update loop.

let gravity = GravityComponent(gravity: 0.0, mass: 1.0)
entity.components.set(gravity)
func updateScene(on event: SceneEvents.Update) {
    for entity in sceneEntities {
        if let gravity = entity.components[GravityComponent.self] as? GravityComponent {
            gravity.update(entity: entity, with: event.deltaTime, at: physicsOrigin)
        }
    }
}

Remember to register your component before use: GravityComponent.registerComponent(). This approach has some limitations, such as sensitivity to FPS changes, but it provides a starting point for more refined zero-gravity simulations.

For a quick demo, check out my Vimeo recording.