Reality Kit, Get distance between floor and ceiling

I haven't tested this, but I think something like this should work:

struct MyView: View {
    
    @State private var floor: AnchorEntity?
    @State private var ceiling: AnchorEntity?

    var body: some View {
        RealityView { content, attachments in
            
            // Run a spatial tracking session to get end user approval for world sensing.
            let session = SpatialTrackingSession()
            let config = SpatialTrackingSession.Configuration(tracking: [.plane])
            await session.run(config)
            
            // Add two anchor entities for floor and ceiling to the RealityView.
            let floor = AnchorEntity(.plane(.horizontal, classification: .floor, minimumBounds: [0.1, 0.1]))
            let ceiling = AnchorEntity(.plane(.horizontal, classification: .ceiling, minimumBounds: [0.1, 0.1]))
            content.add(floor)
            content.add(ceiling)
            self.floor = floor
            self.ceiling = ceiling
        }
        .task {
            while true {
                try? await Task.sleep(nanoseconds: 1_000_000_000)
                
                guard let floor, let ceiling else { continue }

                // Compute room height if floor and ceiling have been detected.
                if floor.isAnchored && ceiling.isAnchored {
                    // Y axis is gravity aligned, so absolute distance between the anchors on the Y axis is the room height.
                    let roomHeight = let roomHeight = abs(floor.position.y - ceiling.position.y)
                    print("roomHeight: \(roomHeight)")
                    break
                } else {
                   maybeShowSomeUIToUserAskingThemToLookAtTheFloorOrCeiling()
                }
            }
        }
    }
}