allow adding nodes again.

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/MaterialGraphDataSource.cs

 namespace UnityEditor.Graphing.Drawing
 {
     [Serializable]
-    public class MaterialGraphDataSource : IGraphElementDataSource
+    public class MaterialGraphDataSource : ScriptableObject, IGraphElementDataSource
+        [SerializeField]
         private List<GraphElementData> m_Elements = new List<GraphElementData>();
 
         public IGraphAsset graphAsset { get; private set; }
             }
         }
 
-        public MaterialGraphDataSource(IGraphAsset graphAsset)
+        private void UpdateData()
-            this.graphAsset = graphAsset;
-
-            if (graphAsset == null)
-                return;
+            m_Elements.Clear();
 
             var drawableNodes = new List<MaterialNodeData>();
             foreach (var node in graphAsset.graph.GetNodes<INode>())
 
                         var targetAnchors = targetNode.elements.OfType<MaterialNodeAnchorData>();
                         var targetAnchor = targetAnchors.FirstOrDefault(x => x.slot == toSlot);
-	                    var edgeData = ScriptableObject.CreateInstance<EdgeData>();
-	                    edgeData.left = sourceAnchor;
-	                    edgeData.right = targetAnchor;
+                        var edgeData = ScriptableObject.CreateInstance<EdgeData>();
+                        edgeData.left = sourceAnchor;
+                        edgeData.right = targetAnchor;
                         drawableEdges.Add(edgeData);
                     }
                 }
             m_Elements.AddRange(drawableEdges.OfType<GraphElementData>());
+        }
+
+        public MaterialGraphDataSource(IGraphAsset graphAsset)
+        {
+            this.graphAsset = graphAsset;
+
+            if (graphAsset == null)
+                return;
+
+            UpdateData();
+        }
+
+
+        public void AddNode(INode node)
+        {
+            graphAsset.graph.AddNode(node);
+            UpdateData();
         }
 
         public IEnumerable<GraphElementData> elements

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/MaterialGraphView.cs

             var drawstate = node.drawState;
             drawstate.position = new Rect(posObj.m_Pos.x, posObj.m_Pos.y, 0 , 0);
             node.drawState = drawstate;
-            /*m_DataSource.Addnode(node);
-            Rebuild();
-            Repaint();*/
+
+            var graphDataSource = dataSource as MaterialGraphDataSource;
+            graphDataSource.AddNode(node);
         }
     }
 }