ChannelEnumControl fixes

- Add modification listener
- Add slot ID to attribute
- Cleanup

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Controls/ChannelEnumControl.cs

 using System;
 using System.Reflection;
 using UnityEngine;
+using UnityEditor.Graphing;
 using UnityEngine.Experimental.UIElements;
 
 namespace UnityEditor.ShaderGraph.Drawing.Controls
     {
         string m_Label;
+        int m_SlotId;
-        public ChannelEnumControlAttribute(string label = null)
+        public ChannelEnumControlAttribute(string label = null, int slotId = 0)
+            m_SlotId = slotId;
-            return new ChannelEnumControlView(m_Label, node, propertyInfo);
+            return new ChannelEnumControlView(m_Label, m_SlotId, node, propertyInfo);
-    public class ChannelEnumControlView : VisualElement
+    public class ChannelEnumControlView : VisualElement, INodeModificationListener
+        int m_SlotId;
-        string[] popupEntries;
-
-        public ChannelEnumControlView(string label, AbstractMaterialNode node, PropertyInfo propertyInfo)
+        public ChannelEnumControlView(string label, int slotId, AbstractMaterialNode node, PropertyInfo propertyInfo)
+            m_SlotId = slotId;
             if (!propertyInfo.PropertyType.IsEnum)
                 throw new ArgumentException("Property must be an enum.", "propertyInfo");
             m_Label = new GUIContent(label ?? ObjectNames.NicifyVariableName(propertyInfo.Name));
         void OnGUIHandler()
         {
-            var entries = (Enum) m_PropertyInfo.GetValue(m_Node, null);
-            var value = (int)m_PropertyInfo.GetValue(m_Node, null);
+            UpdatePopup();
+        }
+
+        public void OnNodeModified(ModificationScope scope)
+        {
+            if(scope == ModificationScope.Graph)
+                UpdatePopup();
+        }
+        private void UpdatePopup()
+        {
+            var value = (int)m_PropertyInfo.GetValue(m_Node, null);
-                int count = (int)SlotValueHelper.GetChannelCount(m_Node.FindSlot<MaterialSlot>(0).concreteValueType);
-                popupEntries = new string[count];
-
+                int count = (int)SlotValueHelper.GetChannelCount(m_Node.FindSlot<MaterialSlot>(m_SlotId).concreteValueType);
+                if (value >= count)
+                {
+                    m_Node.owner.owner.RegisterCompleteObjectUndo("Change " + m_Node.name);
+                    m_PropertyInfo.SetValue(m_Node, 0, null);
+                    return;
+                }
+                var entries = (Enum)m_PropertyInfo.GetValue(m_Node, null);
+                string[] popupEntries = new string[count];
-
                 if (changeCheckScope.changed)
                 {
                     m_Node.owner.owner.RegisterCompleteObjectUndo("Change " + m_Node.name);