module-inspector/Editor/Drawers/ReadableScriptableObjectAttributeDrawer.cs

using Module.Inspector.Editor.Utilities;
using UnityEditor;
using UnityEngine;

namespace Module.Inspector.Editor
{
    [CustomPropertyDrawer(typeof(ReadableScriptableObjectAttribute))]
    internal sealed class ReadableScriptableObjectAttributeDrawer : DrawerPropertyDrawer
    {
        public override bool Draw(Rect position, DrawerPropertyAttribute attribute, SerializedProperty property, GUIContent label, EditorPropertyUtility.Result result)
        {
            if (!IsSupported(property))
                return false;

            var att = (ReadableScriptableObjectAttribute)attribute;

            EditorGUI.BeginChangeCheck();
            EditorGUI.BeginProperty(position, label, property);

            {
                if (property.objectReferenceValue != null)
                {
                    property.isExpanded = DrawFoldout(position, property, label);

                    if (property.isExpanded)
                        DrawContent(position, property.objectReferenceValue, att.editable);
                }
                else
                {
                    // Note: Why not use EditorGUI.PropertyField?
                    //       The reason is that the foldout uses Toggle and ObjectField and if we don't use 
                    //       Label and ObjectField here, then ObjectField will be given a new control id when changing 
                    //       objectReferenceValue from an existing object to null and back again. The new object will
                    //       not be set, due to null giving a new control id.
                    var rectObj = new Rect 
                    { 
                        x = position.x + EditorGUIUtility.labelWidth + 2.0f,
                        y = position.y,
                        width = position.width - EditorGUIUtility.labelWidth - 2.0f,
                        height = position.height
                    };

                    GUI.Label(position, label);
                    property.objectReferenceValue = EditorGUI.ObjectField(rectObj, property.objectReferenceValue, property.GetValueType(), false);
                }
            }

            EditorGUI.EndProperty();
            bool hasChanged = EditorGUI.EndChangeCheck();

            if (hasChanged)
                property.serializedObject.ApplyModifiedProperties();

            return true;
        }

        private static bool DrawFoldout(Rect position, SerializedProperty property, GUIContent label)
        {
            // Note: Why not use EditorGUI.BeginFoldoutHeaderGroup?
            //       FoldoutHeaderGroup doesn't support nesting, so any ScriptableObject with a field that uses
            //       a FoldoutHeaderGroup will not be drawn, e.g. arrays.

            const float FOLDOUT_OFFSET = 14.0f;
            const float FIELD_OFFSET = 16.0f;

            position.x -= FOLDOUT_OFFSET;
            position.width += FOLDOUT_OFFSET;
            position.height = EditorGUIUtility.singleLineHeight;

            var rectObj = new Rect 
            { 
                x = position.x + EditorGUIUtility.labelWidth + FIELD_OFFSET,
                y = position.y,
                width = position.width - EditorGUIUtility.labelWidth - FIELD_OFFSET,
                height = position.height
            };

            GUIStyle style = property.objectReferenceValue != null ? EditorStyles.foldoutHeader : EditorStyles.label;
            bool foldout = property.isExpanded;

            if (Event.current.type == EventType.Repaint)
            {
                GUI.Toggle(position, foldout, label, style);
                EditorGUI.ObjectField(rectObj, property.objectReferenceValue, property.GetValueType(), false);
            }
            else
            {
                property.objectReferenceValue = EditorGUI.ObjectField(rectObj, property.objectReferenceValue, property.GetValueType(), false);
                foldout = GUI.Toggle(position, foldout, label, style);
            }

            return foldout;
        }

        private static void DrawContent(Rect rect, Object obj, bool editable)
        {
            var serializedObject = new SerializedObject(obj);
            SerializedProperty it = serializedObject.GetIterator();

            it.NextVisible(true);
            rect.y += EditorGUIUtility.singleLineHeight;

            EditorGUI.indentLevel++;
            EditorGUI.BeginChangeCheck();
            {
                bool prevEnabled = GUI.enabled;
                GUI.enabled = prevEnabled && editable;

                while (it.NextVisible(false))
                {
                    rect.height = EditorGUI.GetPropertyHeight(it);
                    EditorGUI.PropertyField(rect, it, new GUIContent(it.displayName), true);
                    rect.y += rect.height;
                }

                GUI.enabled = prevEnabled;
            }
            bool hasChanged = EditorGUI.EndChangeCheck();
            EditorGUI.indentLevel--;

            if (hasChanged)
                serializedObject.ApplyModifiedProperties();
        }

        public override string GetErrorMessage(SerializedProperty property)
        {
            return IsSupported(property)
                       ? "Unknown error"
                       : "Only supports types which inherits from ScriptableObject type";
        }

        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            if (property.objectReferenceValue == null || !property.isExpanded)
                return base.GetPropertyHeight(property, label);

            var so = new SerializedObject(property.objectReferenceValue);
            SerializedProperty it = so.GetIterator();
            var height = 0.0f;
            it.NextVisible(true);
            height += EditorGUI.GetPropertyHeight(it);

            while (it.NextVisible(false))
            {
                height += EditorGUI.GetPropertyHeight(it);
            }

            return height;
        }

        private static bool IsSupported(SerializedProperty property)
        {
            return property.propertyType == SerializedPropertyType.ObjectReference &&
                   typeof(ScriptableObject).IsAssignableFrom(property.GetValueType());
        }
    }
}