Tri-Inspector/Editor.Extras/Drawers/InlineEditorDrawer.cs

using TriInspector;
using TriInspector.Drawers;
using TriInspector.Elements;
using TriInspector.Utilities;
using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;

[assembly: RegisterTriAttributeDrawer(typeof(InlineEditorDrawer), TriDrawerOrder.Drawer - 100,
    ApplyOnArrayElement = true)]

namespace TriInspector.Drawers
{
    public class InlineEditorDrawer : TriAttributeDrawer<InlineEditorAttribute>
    {
        public override string CanDraw(TriProperty property)
        {
            if (!typeof(Object).IsAssignableFrom(property.FieldType))
            {
                return "[InlineEditor] valid only on Object fields";
            }

            return null;
        }

        public override TriElement CreateElement(TriProperty property, TriElement next)
        {
            var element = new TriBoxGroupElement(new DeclareBoxGroupAttribute(""));
            element.AddChild(new ObjectReferenceFoldoutDrawerElement(property));
            element.AddChild(new InlineEditorElement(property));
            return element;
        }

        private class ObjectReferenceFoldoutDrawerElement : TriElement
        {
            private readonly TriProperty _property;

            public ObjectReferenceFoldoutDrawerElement(TriProperty property)
            {
                _property = property;
            }

            public override float GetHeight(float width)
            {
                return EditorGUIUtility.singleLineHeight;
            }

            public override void OnGUI(Rect position)
            {
                var prefixRect = new Rect(position)
                {
                    height = EditorGUIUtility.singleLineHeight,
                    xMax = position.xMin + EditorGUIUtility.labelWidth,
                };
                var pickerRect = new Rect(position)
                {
                    height = EditorGUIUtility.singleLineHeight,
                    xMin = prefixRect.xMax,
                };

                using (TriGuiHelper.PushIndentLevel())
                {
                    TriEditorGUI.Foldout(prefixRect, _property);
                }

                EditorGUI.BeginChangeCheck();

                var allowSceneObjects = _property.PropertyTree.TargetIsPersistent;

                var value = (Object) _property.Value;
                value = EditorGUI.ObjectField(pickerRect, GUIContent.none, value,
                    _property.FieldType, allowSceneObjects);

                if (EditorGUI.EndChangeCheck())
                {
                    _property.SetValue(value);
                }
            }
        }
    }
}