Collision detection

Hi @lijiaxu

You can detect collisions by subscribing to collision events on an entity. For example, if you have an entity with a collision component, myEntity, you can subscribe to its various collision events like so:

@State private var collisionBegan: EventSubscription?
@State private var collisionUpdated: EventSubscription?
@State private var collisionEnded: EventSubscription?

var body: some View {
    RealityView { content in
        // Create myEntity here.
        ...
        
        // Subscribe to collision events.
        collisionBegan = content.subscribe(to: CollisionEvents.Began.self, on: myEntity) { collisionEvent in
            print("Collision began.")
        }

        collisionUpdated = content.subscribe(to: CollisionEvents.Updated.self, on: myEntity) { collisionEvent in
            print("Collision updated.")
        }

        collisionEnded = content.subscribe(to: CollisionEvents.Ended.self, on: myEntity) { collisionEvent in
            print("Collision ended.")
        }
    }
}

In this case, I call a print statement when the collision occurs, but you can execute whatever instructions you like in the collision event callback.

Things get a little bit more complicated when an entity with an AnchoringComponent is involved, as you will need to set its physicsSimulation property to .none, as well as start a SpatialTrackingSession. Here's a snippet that demonstrates how to do that:

@State private var collisionBegan: EventSubscription?
@State private var collisionUpdated: EventSubscription?
@State private var collisionEnded: EventSubscription?

var body: some View {
    RealityView { content in
        // Start a spatial tracking session for the hands.
        let configuration = SpatialTrackingSession.Configuration(tracking: [.hand])
        let session = SpatialTrackingSession()
        await session.run(configuration)
        
        // Create a cube with collision.
        let collidableCube = ModelEntity(mesh: .generateBox(size: 0.2), materials: [SimpleMaterial(color: .green.withAlphaComponent(0.75), isMetallic: false)])
        collidableCube.position = [0, 1, -0.5]
        collidableCube.generateCollisionShapes(recursive: false)
        content.add(collidableCube)
        
        // Create an anchor entity with collision tracking the right index finger tip.
        let rightIndexTipAnchor = AnchorEntity(.hand(.right, location: .indexFingerTip))
        rightIndexTipAnchor.anchoring.physicsSimulation = .none
        rightIndexTipAnchor.components.set(ModelComponent(mesh: .generateSphere(radius: 0.01), materials: [SimpleMaterial()]))
        rightIndexTipAnchor.components.set(CollisionComponent(shapes: [.generateSphere(radius: 0.01)]))
        content.add(rightIndexTipAnchor)
        
        // Subscribe to collision events.
        collisionBegan = content.subscribe(to: CollisionEvents.Began.self, on: rightIndexTipAnchor) { collisionEvent in
            print("Collision began.")
        }
        collisionUpdated = content.subscribe(to: CollisionEvents.Updated.self, on: rightIndexTipAnchor) { collisionEvent in
            print("Collision updated.")
        }
        collisionEnded = content.subscribe(to: CollisionEvents.Ended.self, on: rightIndexTipAnchor) { collisionEvent in
            print("Collision ended.")
        }
    }
}

For this approach, be sure to add an entry for NSHandsTrackingUsageDescription to your app’s information property list to provide a usage description that explains how your app uses the hand tracking information.