private let eyeTracker = EyeTracker()

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    if EyeTracker.isSupported {
        eyeTracker.delegate = self
        eyeTracker.start()
    }
}

// MARK: - EyeTrackerDelegate

func eyeTracker(_ eyeTracker: EyeTracker, didUpdateTrackingState state: EyeTracker.TrackingState) {
    switch state {
    case let .screenIn(screenPos):
        // ...
    case let .screenOut(edge, screenPos):
        // ...
    case .notTracked:
        // ...
    case .pausing:
        // ...
    }
}