[FIX] #88 Added a forcedRedrawPreviews flag that will redraw the graph and update the nodes to reflect changes properly when an external source has been re-imported.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphEditWindow.cs

         [NonSerialized]
         bool m_HasError;
 
+        [NonSerialized]
+        public bool forceRedrawPreviews = false;
+
         GraphEditorView m_GraphEditorView;
 
         GraphEditorView graphEditorView
                     m_GraphEditorView.RemoveFromHierarchy();
                     m_GraphEditorView.Dispose();
                 }
+
                 m_GraphEditorView = value;
                 if (m_GraphEditorView != null)
                 {
                     graphEditorView = new GraphEditorView(this, materialGraph, asset.name) { persistenceKey = selectedGuid };
                 }
 
+                if (forceRedrawPreviews)
+                {
+                    // Redraw all previews
+                    foreach (INode node in m_GraphObject.graph.GetNodes<INode>())
+                        node.Dirty(ModificationScope.Node);
+                    forceRedrawPreviews = false;
+                }
+                    
                 graphEditorView.HandleGraphChanges();
                 graphObject.graph.ClearChanges();
             }
                 Undo.ClearUndo(graphObject);
                 DestroyImmediate(graphObject);
             }
+
             graphEditorView = null;
         }
 
                     var inputSlotRef = new SlotReference(remappedInputNodeGuid, inputSlot.slotId);
                     subGraph.Connect(outputSlotRef, inputSlotRef);
                 }
+
                 // one edge needs to go to outside world
                 else if (outputSlotExistsInSubgraph)
                 {

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Importers/ShaderGraphImporter.cs

 using System.Text;
 using UnityEditor;
 using UnityEditor.Experimental.AssetImporters;
+using UnityEditor.Experimental.UIElements;
+using UnityEditor.Graphing;
+using UnityEditor.ShaderGraph.Drawing;
 
 [ScriptedImporter(1, ShaderGraphImporter.ShaderGraphExtension)]
 public class ShaderGraphImporter : ScriptedImporter
 
     static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths)
     {
+        MaterialGraphEditWindow[] windows = Resources.FindObjectsOfTypeAll<MaterialGraphEditWindow>();
+        foreach (var matGraphEditWindow in windows)
+        {
+            matGraphEditWindow.forceRedrawPreviews = true;
+        }
-}
+}