[shader graph]Tidy up generation of shaders / preview.

UnityProject/Assets/UnityShaderEditor/Editor/Source/BaseMaterialGraph.cs

             foreach (var node in nodes)
             {
                 var pNode = node as BaseMaterialNode;
-                if (pNode != null)
+                if (pNode != null && pNode.hasPreview)
                 {
                     MaterialWindow.DebugMaterialGraph("Updating preview Properties on Node: " + pNode);
                     pNode.UpdatePreviewProperties();

UnityProject/Assets/UnityShaderEditor/Editor/Source/Nodes/BaseMaterialNode.cs

         private const int kPreviewHeight = 64;
 
         [NonSerialized]
-        private UnityEngine.Material m_Material;
+        private Material m_Material;
 
         [SerializeField]
         private List<SlotDefaultValue> m_SlotDefaultValues;
             }
         }
 
-        public UnityEngine.Material previewMaterial
+        public Material previewMaterial
-                    m_Material = new UnityEngine.Material(defaultPreviewShader) {hideFlags = HideFlags.DontSave};
+                    m_Material = new Material(defaultPreviewShader) {hideFlags = HideFlags.DontSave};
-                    UpdatePreviewProperties();
                 }
 
                 return m_Material;
         #region Previews
         public virtual bool UpdatePreviewMaterial()
         {
-            MaterialWindow.DebugMaterialGraph("RecreateShaderAndMaterial : " + name + "_" + GetInstanceID());
-
+            InternalUpdatePreviewShader(resultShader);
+            return true;
+        }
+        protected void InternalUpdatePreviewShader(string resultShader)
+        {
+            MaterialWindow.DebugMaterialGraph("RecreateShaderAndMaterial : " + name + "_" + GetInstanceID());
-
+            UpdatePreviewProperties();
-            return true;
         }
 
         // this function looks at all the nodes that have a
             return previewUtil.EndPreview();
         }
 
-        private void SetPreviewMaterialProperty(PreviewProperty previewProperty)
+        private static void SetPreviewMaterialProperty(PreviewProperty previewProperty, Material mat)
-                    previewMaterial.SetTexture(previewProperty.m_Name, previewProperty.m_Texture);
+                    mat.SetTexture(previewProperty.m_Name, previewProperty.m_Texture);
-                    previewMaterial.SetColor(previewProperty.m_Name, previewProperty.m_Color);
+                    mat.SetColor(previewProperty.m_Name, previewProperty.m_Color);
-                    previewMaterial.SetVector(previewProperty.m_Name, previewProperty.m_Vector4);
+                    mat.SetVector(previewProperty.m_Name, previewProperty.m_Vector4);
                     break;
             }
         }
                 node.CollectPreviewMaterialProperties(pList);
 
             foreach (var prop in pList)
-                SetPreviewMaterialProperty (prop);
+                SetPreviewMaterialProperty (prop, previewMaterial);
         }
 
         #endregion

UnityProject/Assets/UnityShaderEditor/Editor/Source/Nodes/PixelShaderNode.cs

 
         public override bool UpdatePreviewMaterial()
         {
-            Dictionary<string, Texture> defaultTextures;
-            
+            Dictionary<string, Texture> defaultTextures;
-            if (previewMaterial.shader != defaultPreviewShader)
-                DestroyImmediate(previewMaterial.shader, true);
-            previewMaterial.shader = ShaderUtil.CreateShaderAsset(resultShader);
-            EditorMaterialUtility.SetShaderDefaults(previewMaterial.shader, defaultTextures.Keys.ToArray(), defaultTextures.Values.ToArray());
-            previewMaterial.shader.hideFlags = HideFlags.DontSave;
+            InternalUpdatePreviewShader(resultShader);
             return true;
         }
     }