Add list header drag and drop

Editor/Elements/TriListElement.cs

@@ -1,10 +1,12 @@
 using System;
 using System.Collections;
+using System.Linq;
 using TriInspectorUnityInternalBridge;
 using TriInspector.Utilities;
 using UnityEditor;
 using UnityEditorInternal;
 using UnityEngine;
+using Object = UnityEngine.Object;
 
 namespace TriInspector.Elements
 {
@@ -121,10 +123,15 @@ public override void OnGUI(Rect position)
         }
 
         private void AddElementCallback(ReorderableList reorderableList)
+        {
+            AddElementCallback(reorderableList, null);
+        }
+
+        private void AddElementCallback(ReorderableList reorderableList, Object addedReferenceValue)
         {
             if (_property.TryGetSerializedProperty(out _))
             {
-                ReorderableListProxy.defaultBehaviours.DoAddButton(reorderableList);
+                ReorderableListProxy.DoAddButton(reorderableList, addedReferenceValue);
                 _property.NotifyValueChanged();
                 return;
             }
@@ -139,6 +146,12 @@ private void AddElementCallback(ReorderableList reorderableList)
                 {
                     var array = Array.CreateInstance(_property.ArrayElementType, template.Length + 1);
                     Array.Copy(template, array, template.Length);
+
+                    if (addedReferenceValue != null)
+                    {
+                        array.SetValue(addedReferenceValue, array.Length - 1);
+                    }
+
                     value = array;
                 }
                 else
@@ -148,7 +161,10 @@ private void AddElementCallback(ReorderableList reorderableList)
                         value = (IList) Activator.CreateInstance(_property.FieldType);
                     }
 
-                    var newElement = CreateDefaultElementValue(_property);
+                    var newElement = addedReferenceValue != null
+                        ? addedReferenceValue
+                        : CreateDefaultElementValue(_property);
+
                     value.Add(newElement);
                 }
 
@@ -293,6 +309,29 @@ private void DrawHeaderCallback(Rect rect)
 
             var label = _reorderableListGui.count == 0 ? "Empty" : $"{_reorderableListGui.count} items";
             GUI.Label(arraySizeRect, label, Styles.ItemsCount);
+
+            if (Event.current.type == EventType.DragUpdated && rect.Contains(Event.current.mousePosition))
+            {
+                DragAndDrop.visualMode = DragAndDrop.objectReferences.All(obj => TryGetDragAndDropObject(obj, out _))
+                    ? DragAndDropVisualMode.Copy
+                    : DragAndDropVisualMode.Rejected;
+
+                Event.current.Use();
+            }
+            else if (Event.current.type == EventType.DragPerform && rect.Contains(Event.current.mousePosition))
+            {
+                DragAndDrop.AcceptDrag();
+
+                foreach (var obj in DragAndDrop.objectReferences)
+                {
+                    if (TryGetDragAndDropObject(obj, out var addedReferenceValue))
+                    {
+                        AddElementCallback(_reorderableListGui, addedReferenceValue);
+                    }
+                }
+
+                Event.current.Use();
+            }
         }
 
         private void DrawElementCallback(Rect rect, int index, bool isActive, bool isFocused)
@@ -336,6 +375,34 @@ private static Array CloneValue(TriProperty property)
             return template;
         }
 
+        private bool TryGetDragAndDropObject(Object obj, out Object result)
+        {
+            if (obj == null)
+            {
+                result = null;
+                return false;
+            }
+
+            var elementType = _property.ArrayElementType;
+            var objType = obj.GetType();
+
+            if (elementType == objType || elementType.IsAssignableFrom(objType))
+            {
+                result = obj;
+                return true;
+            }
+
+            if (obj is GameObject go && typeof(Component).IsAssignableFrom(elementType) &&
+                go.TryGetComponent(elementType, out var component))
+            {
+                result = component;
+                return true;
+            }
+
+            result = null;
+            return false;
+        }
+
         private static class Styles
         {
             public static readonly GUIStyle ItemsCount;

Unity.InternalAPIEditorBridge.012/ReorderableListProxy.cs

@@ -2,6 +2,7 @@
 using System.Reflection;
 using UnityEditorInternal;
 using UnityEngine;
+using Object = UnityEngine.Object;
 
 namespace TriInspectorUnityInternalBridge
 {
@@ -59,5 +60,10 @@ public static void ClearCacheRecursive(ReorderableList list)
             ClearCacheMethod?.Invoke(list, Array.Empty<object>());
 #endif
         }
+
+        public static void DoAddButton(ReorderableList list, Object value)
+        {
+            defaultBehaviours.DoAddButton(list, value);
+        }
     }
 }