ClickRayCastObservable.cs

    [RequireComponent(typeof(Camera))]
    public class ClickRayCastObservable : MonoBehaviour
    {
        public LayerMask targetLayer;
        public float distance;
        public Mouse moueButton;

        private Subject<RaycastHit> raycastStream = new Subject<RaycastHit>();

        public IObservable<RaycastHit> ClickRaycastObservable
        {
            get { return raycastStream.AsObservable(); }
        }

        private Subject<Unit> raycastFailedStream = new Subject<Unit>();

        public IObservable<Unit> RaycastFailedAsObservable
        {
            get { return raycastFailedStream.AsObservable();  }
        } 

        void Start()
        {
            var selfCamera = GetComponent<Camera>();

            this.UpdateAsObservable()
                .Where(_ => Input.GetMouseButtonDown((int)moueButton))
                .Select(_ => selfCamera.ScreenPointToRay(Input.mousePosition))
                .Subscribe(ray => {
                    RaycastHit hit;
                    if(Physics.Raycast(ray, out hit, distance, targetLayer.value))
                    {
                        raycastStream.OnNext(hit);
                    }
                    else
                    {
                        raycastFailedStream.OnNext(Unit.Default);
                    }
                });
                

        }
    }

    public enum Mouse
    {
        Left = 0,
        Right = 1
    }