Several lifetime feature updates for the ComboBox controls:

- Resolves startup issue that prevented the control being used (Unity changed the start order in some instances), this was causing null reference issues with comboboxes
- Added the ability to set a specific item on start and not just the first
- Added the ability to disable the dropdown to make a read-only dropdown

Resolves:
- https://github.com/Unity-UI-Extensions/com.unity.uiextensions/issues/426
- https://github.com/Unity-UI-Extensions/com.unity.uiextensions/issues/425

Runtime/Scripts/Controls/ComboBox/AutoCompleteComboBox.cs

@@ -1,6 +1,7 @@
 ///Credit perchik
 ///Sourced from - http://forum.unity3d.com/threads/receive-onclick-event-and-pass-it-on-to-lower-ui-elements.293642/
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -16,7 +17,6 @@ namespace UnityEngine.UI.Extensions
     [AddComponentMenu("UI/Extensions/ComboBox/AutoComplete ComboBox")]
     public class AutoCompleteComboBox : MonoBehaviour
     {
-        public Color disabledTextColor;
         public DropDownListItem SelectedItem { get; private set; } //outside world gets to get this, not set it
 
         /// <summary>
@@ -24,6 +24,7 @@ namespace UnityEngine.UI.Extensions
         /// <see cref="RemoveItem(string)"/> and <see cref="SetAvailableOptions(List{string})"/> methods as these also execute
         /// the required methods to update to the current collection.
         /// </summary>
+        [Header("AutoComplete Box Items")]
         public List<string> AvailableOptions;
 
         private bool _isPanelActive = false;
@@ -51,11 +52,13 @@ namespace UnityEngine.UI.Extensions
         private Dictionary<string, GameObject> panelObjects;
 
         private GameObject itemTemplate;
+        private bool _initialized;
 
         public string Text { get; private set; }
 
+        [Header("Properties")]
         [SerializeField]
-        private float dropdownOffset;
+        private bool isActive = true;
 
         [SerializeField]
         private float _scrollBarWidth = 20.0f;
@@ -81,8 +84,6 @@ namespace UnityEngine.UI.Extensions
             }
         }
 
-        public bool SelectFirstItemOnStart = false;
-
         [SerializeField]
         [Tooltip("Change input text color based on matching items")]
         private bool _ChangeInputTextColorBasedOnMatchingItems = false;
@@ -107,9 +108,19 @@ namespace UnityEngine.UI.Extensions
 
         public AutoCompleteSearchType autocompleteSearchType = AutoCompleteSearchType.Linq;
 
+        [SerializeField]
+        private float dropdownOffset;
+
         [SerializeField]
         private bool _displayPanelAbove = false;
 
+        public bool SelectFirstItemOnStart = false;
+
+        [SerializeField]
+        private int selectItemIndexOnStart = 0;
+
+        private bool shouldSelectItemOnStart => SelectFirstItemOnStart || selectItemIndexOnStart > 0;
+
         private bool _selectionIsValid = false;
 
         [System.Serializable]
@@ -121,12 +132,18 @@ namespace UnityEngine.UI.Extensions
         [System.Serializable]
         public class SelectionValidityChangedEvent : Events.UnityEvent<bool> { }
 
+        [System.Serializable]
+        public class ControlDisabledEvent : Events.UnityEvent<bool> { }
+
         // fires when input text is changed;
+        [Header("Events")]
         public SelectionTextChangedEvent OnSelectionTextChanged;
         // fires when an Item gets selected / deselected (including when items are added/removed once this is possible)
         public SelectionValidityChangedEvent OnSelectionValidityChanged;
         // fires in both cases
         public SelectionChangedEvent OnSelectionChanged;
+        // fires when item is changed;
+        public ControlDisabledEvent OnControlDisabled;
 
         public void Awake()
         {
@@ -135,16 +152,17 @@ namespace UnityEngine.UI.Extensions
 
         public void Start()
         {
-            if (SelectFirstItemOnStart && AvailableOptions.Count > 0)
+            if (shouldSelectItemOnStart && AvailableOptions.Count > 0)
             {
-                ToggleDropdownPanel(false);
+                SelectItemIndex(SelectFirstItemOnStart ? 0 : selectItemIndexOnStart);
-                OnItemClicked(AvailableOptions[0]);
             }
             RedrawPanel();
         }
 
         private bool Initialize()
         {
+            if (_initialized) return true;
+
             bool success = true;
             try
             {
@@ -184,6 +202,8 @@ namespace UnityEngine.UI.Extensions
             _prunedPanelItems = new List<string>();
             _panelItems = new List<string>();
 
+            _initialized = true;
+
             RebuildPanel();
             return success;
         }
@@ -218,6 +238,17 @@ namespace UnityEngine.UI.Extensions
             }
         }
 
+
+        /// <summary>
+        /// Update the drop down selection to a specific index
+        /// </summary>
+        /// <param name="index"></param>
+        public void SelectItemIndex(int index)
+        {
+            ToggleDropdownPanel(false);
+            OnItemClicked(AvailableOptions[index]);
+        }
+
         /// <summary>
         /// Sets the given items as new content for the comboBox. Previous entries will be cleared.
         /// </summary>
@@ -263,6 +294,11 @@ namespace UnityEngine.UI.Extensions
         /// </summary>
         private void RebuildPanel()
         {
+            if (!_initialized)
+            {
+                Start();
+            }
+
             if (_isPanelActive) ToggleDropdownPanel();
 
             //panel starts with all options
@@ -427,6 +463,8 @@ namespace UnityEngine.UI.Extensions
         /// <param name="directClick"> whether an item was directly clicked on</param>
         public void ToggleDropdownPanel(bool directClick = false)
         {
+            if (!isActive) return;
+
             _isPanelActive = !_isPanelActive;
 
             _overlayRT.gameObject.SetActive(_isPanelActive);
@@ -440,6 +478,20 @@ namespace UnityEngine.UI.Extensions
             }
         }
 
+
+        /// <summary>
+        /// Updates the control and sets its active status, determines whether the dropdown will open ot not
+        /// </summary>
+        /// <param name="status"></param>
+        public void SetActive(bool status)
+        {
+            if (status != isActive)
+            {
+                OnControlDisabled?.Invoke(status);
+            }
+            isActive = status;
+        }
+
         private void PruneItems(string currText)
         {
             if (autocompleteSearchType == AutoCompleteSearchType.Linq)

Runtime/Scripts/Controls/ComboBox/ComboBox.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using static UnityEditor.Progress;
 
 namespace UnityEngine.UI.Extensions
 {
@@ -10,11 +11,15 @@ namespace UnityEngine.UI.Extensions
     [AddComponentMenu("UI/Extensions/ComboBox/ComboBox")]
     public class ComboBox : MonoBehaviour
     {
-        public Color disabledTextColor;
         public DropDownListItem SelectedItem { get; private set; }
 
+        [Header("Combo Box Items")]
         public List<string> AvailableOptions;
 
+        [Header("Properties")]
+        [SerializeField]
+        private bool isActive = true;
+
         [SerializeField]
         private float _scrollBarWidth = 20.0f;
 
@@ -27,22 +32,32 @@ namespace UnityEngine.UI.Extensions
         [SerializeField]
         private bool _displayPanelAbove = false;
 
+        public bool SelectFirstItemOnStart = false;
+
+        [SerializeField]
+        private int selectItemIndexOnStart = 0;
+
+        private bool shouldSelectItemOnStart => SelectFirstItemOnStart || selectItemIndexOnStart > 0;
+
         [System.Serializable]
         public class SelectionChangedEvent : Events.UnityEvent<string> { }
 
+        [Header("Events")]
         // fires when item is changed;
         public SelectionChangedEvent OnSelectionChanged;
 
+        [System.Serializable]
+        public class ControlDisabledEvent : Events.UnityEvent<bool> { }
+
+        // fires when item is changed;
+        public ControlDisabledEvent OnControlDisabled;
+
         //private bool isInitialized = false;
         private bool _isPanelActive = false;
         private bool _hasDrawnOnce = false;
-
         private InputField _mainInput;
         private RectTransform _inputRT;
-
-
         private RectTransform _rectTransform;
-
         private RectTransform _overlayRT;
         private RectTransform _scrollPanelRT;
         private RectTransform _scrollBarRT;
@@ -51,14 +66,11 @@ namespace UnityEngine.UI.Extensions
         private RectTransform _itemsPanelRT;
         private Canvas _canvas;
         private RectTransform _canvasRT;
-
         private ScrollRect _scrollRect;
-
         private List<string> _panelItems; //items that will get shown in the drop-down
-
         private Dictionary<string, GameObject> panelObjects;
-        
         private GameObject itemTemplate;
+        private bool _initialized;
 
         public string Text { get; private set; }
 
@@ -89,11 +101,17 @@ namespace UnityEngine.UI.Extensions
 
         public void Start()
         {
+            if (shouldSelectItemOnStart && AvailableOptions.Count > 0)
+            {
+                SelectItemIndex(SelectFirstItemOnStart ? 0 : selectItemIndexOnStart);
+            }
             RedrawPanel();
         }
 
         private bool Initialize()
         {
+            if (_initialized) return true;
+
             bool success = true;
             try
             {
@@ -133,11 +151,22 @@ namespace UnityEngine.UI.Extensions
 
             _panelItems = AvailableOptions.ToList();
 
+            _initialized = true;
+
             RebuildPanel();
-            //RedrawPanel();  - causes an initialisation failure in U5
             return success;
         }
 
+        /// <summary>
+        /// Update the drop down selection to a specific index
+        /// </summary>
+        /// <param name="index"></param>
+        public void SelectItemIndex(int index)
+        {
+            ToggleDropdownPanel(false);
+            OnItemClicked(AvailableOptions[index]);
+        }
+
         public void AddItem(string item)
         {
             AvailableOptions.Add(item);
@@ -187,6 +216,11 @@ namespace UnityEngine.UI.Extensions
         /// </summary>
         private void RebuildPanel()
         {
+            if (!_initialized)
+            {
+                Start();
+            }
+
             //panel starts with all options
             _panelItems.Clear();
             foreach (string option in AvailableOptions)
@@ -310,6 +344,8 @@ namespace UnityEngine.UI.Extensions
         /// <param name="directClick"> whether an item was directly clicked on</param>
         public void ToggleDropdownPanel(bool directClick)
         {
+            if (!isActive) return;
+
             _isPanelActive = !_isPanelActive;
 
             _overlayRT.gameObject.SetActive(_isPanelActive);
@@ -322,5 +358,18 @@ namespace UnityEngine.UI.Extensions
                 // scrollOffset = Mathf.RoundToInt(itemsPanelRT.anchoredPosition.y / _rectTransform.sizeDelta.y); 
             }
         }
+
+        /// <summary>
+        /// Updates the control and sets its active status, determines whether the dropdown will open ot not
+        /// </summary>
+        /// <param name="status"></param>
+        public void SetActive(bool status)
+        {
+            if (status != isActive)
+            {
+                OnControlDisabled?.Invoke(status);
+            }
+            isActive = status;
+        }
     }
 }

Runtime/Scripts/Controls/ComboBox/DropDownList.cs

@@ -15,7 +15,14 @@ namespace UnityEngine.UI.Extensions
 		public Color disabledTextColor;
 		public DropDownListItem SelectedItem { get; private set; } //outside world gets to get this, not set it
 
-		public List<DropDownListItem> Items;
+        [Header("Dropdown List Items")]
+        public List<DropDownListItem> Items;
+
+        [Header("Properties")]
+
+        [SerializeField]
+        private bool isActive = true;
+
         public bool OverrideHighlighted = true;
 
 		//private bool isInitialized = false;
@@ -37,12 +44,10 @@ namespace UnityEngine.UI.Extensions
 
 		private ScrollRect _scrollRect;
 
-		private List<DropDownListButton> _panelItems;
+		private List<DropDownListButton> _panelItems = new List<DropDownListButton>();
 
-		private GameObject _itemTemplate;
+        private GameObject _itemTemplate;
-
+		private bool _initialized;
-		[SerializeField]
-		private float dropdownOffset;
 
         [SerializeField]
 		private float _scrollBarWidth = 20.0f;
@@ -71,28 +76,45 @@ namespace UnityEngine.UI.Extensions
 			}
 		}
 
-		public bool SelectFirstItemOnStart = false;
+        [SerializeField]
+        private float dropdownOffset;
 
-		[SerializeField]
+        [SerializeField]
-		private bool _displayPanelAbove = false;
+        private bool _displayPanelAbove = false;
 
-		[System.Serializable]
+        public bool SelectFirstItemOnStart = false;
+
+        [SerializeField]
+        private int selectItemIndexOnStart = 0;
+        private bool shouldSelectItemOnStart => SelectFirstItemOnStart || selectItemIndexOnStart > 0;
+
+        [System.Serializable]
 		public class SelectionChangedEvent : Events.UnityEvent<int> { }
-		// fires when item is changed;
-		public SelectionChangedEvent OnSelectionChanged;
 
-		public void Start()
+        // fires when item is changed;
-		{
+        [Header("Events")]
-			Initialize();
+        public SelectionChangedEvent OnSelectionChanged;
-			if (SelectFirstItemOnStart && Items.Count > 0) {
-				ToggleDropdownPanel (false);
-				OnItemClicked (0);
-			}
-			RedrawPanel();
-		}
 
-		private bool Initialize()
+        [System.Serializable]
+        public class ControlDisabledEvent : Events.UnityEvent<bool> { }
+
+        // fires when item is changed;
+        public ControlDisabledEvent OnControlDisabled;
+
+        public void Start()
+        {
+            Initialize();
+            if (shouldSelectItemOnStart && Items.Count > 0)
+            {
+                SelectItemIndex(SelectFirstItemOnStart ? 0 : selectItemIndexOnStart);
+            }
+            RedrawPanel();
+        }
+
+        private bool Initialize()
 		{
+			if (_initialized) return true;
+
 			bool success = true;
 			try
 			{
@@ -125,23 +147,32 @@ namespace UnityEngine.UI.Extensions
 				Debug.LogError("Something is setup incorrectly with the dropdownlist component causing a Null Reference Exception");
 				success = false;
 			}
+            _initialized = true;
 
-			_panelItems = new List<DropDownListButton>();
+            RebuildPanel();
-
-			RebuildPanel();
 			RedrawPanel();
 			return success;
 		}
 
-		// currently just using items in the list instead of being able to add to it.
+        /// <summary>
-		/// <summary>
+		/// Update the drop down selection to a specific index
-		/// Rebuilds the list from a new collection.
 		/// </summary>
-		/// <remarks>
+		/// <param name="index"></param>
-		/// NOTE, this will clear all existing items
+		public void SelectItemIndex(int index)
-		/// </remarks>
+        {
-		/// <param name="list"></param>
+            ToggleDropdownPanel(false);
-		public void RefreshItems(params object[] list)
+            OnItemClicked(index);
+        }
+
+        // currently just using items in the list instead of being able to add to it.
+        /// <summary>
+        /// Rebuilds the list from a new collection.
+        /// </summary>
+        /// <remarks>
+        /// NOTE, this will clear all existing items
+        /// </remarks>
+        /// <param name="list"></param>
+        public void RefreshItems(params object[] list)
 		{
 			Items.Clear();
 			List<DropDownListItem> ddItems = new List<DropDownListItem>();
@@ -249,6 +280,11 @@ namespace UnityEngine.UI.Extensions
 		{
 			if (Items.Count == 0) return;
 
+			if (!_initialized)
+			{
+				Start();
+			}
+
 			int indx = _panelItems.Count;
 			while (_panelItems.Count < Items.Count)
 			{
@@ -374,6 +410,8 @@ namespace UnityEngine.UI.Extensions
 		/// <param name="directClick"> whether an item was directly clicked on</param>
 		public void ToggleDropdownPanel(bool directClick)
 		{
+            if (!isActive) return;
+            
 			_overlayRT.transform.localScale = new Vector3(1, 1, 1);
 			_scrollBarRT.transform.localScale = new Vector3(1, 1, 1);
 			_isPanelActive = !_isPanelActive;
@@ -387,5 +425,18 @@ namespace UnityEngine.UI.Extensions
 				// scrollOffset = Mathf.RoundToInt(itemsPanelRT.anchoredPosition.y / _rectTransform.sizeDelta.y); 
 			}
 		}
-	}
+
+        /// <summary>
+        /// Updates the control and sets its active status, determines whether the dropdown will open ot not
+        /// </summary>
+        /// <param name="status"></param>
+        public void SetActive(bool status)
+        {
+            if (status != isActive)
+            {
+                OnControlDisabled?.Invoke(status);
+            }
+            isActive = status;
+        }
+    }
 }