private void OnSpaceDown(KeyDownEvent evt)
 {
     if (evt.keyCode == KeyCode.Space)
     {
         var graphViewPos = m_graph.viewTransform.position;
         var pos          = evt.imguiEvent.mousePosition - new Vector2(graphViewPos.x, graphViewPos.y);
         m_graph.AddNode(new MyNode(m_graph, pos));
     }
 }
        public void OnEnable()
        {
            var root = this.GetRootVisualContainer();

            m_graph = new MyGraphView(this);
            m_graph.RegisterCallback <KeyDownEvent>(OnSpaceDown);

            m_graph.AddManipulator(new ContentDragger());
            m_graph.AddManipulator(new RectangleSelector());
            //graph.AddManipulator(new SelectionDragger());

            m_graph.AddNode(new MyNode(m_graph, Vector2.zero));

            root.Add(m_graph);
        }