An property drawer to allow selecting from a list of component types on the current selected instance based on type or type inheritance hierarchy

SubselectionDrawer.cs

using System;
using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif


namespace UnityEngineExtensions
{

#if UNITY_EDITOR
    // [CustomPropertyDrawer(typeof(CustomMonoBehaviour), true)]
    // public class CustomMonoBehaviourSubselectionDrawer : SubselectionDrawer<CustomMonoBehaviour>
    // { }
#endif


#if UNITY_EDITOR
    public abstract class SubselectionDrawer<T> : PropertyDrawer where T : Component
    {
        private const float LabelWidthScale = 1.0f;
        private const string Tok_Selected = "selected\t";
        private const string Tok_FoldoutOff = "sub";
        private const string Tok_FoldoutOn = " X ";
        private static readonly char[] Tok_TrimChars = new char[] { ' ', '(', ')' };


        private bool _foldout = false;
        private int _subComps = 0;

        // Draw the property inside the given rect
        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            float labelWidth = EditorGUIUtility.labelWidth;
            SerializedPropertyType pType = property.propertyType;
            if (pType == SerializedPropertyType.ExposedReference
                || pType == SerializedPropertyType.ObjectReference)
            {
                UnityEngine.Object objRef = property.exposedReferenceValue;
                if (objRef == null)
                    objRef = property.objectReferenceValue;

                if (objRef != null && objRef is Component)
                {
                    Component compRef = objRef as Component;

                    Component[] subComps = compRef.GetComponents(typeof(T));
                    _subComps = subComps.Length;
                    if (subComps.Length > 1)
                    {
                        Rect fldRect = position;
                        fldRect.xMin = fldRect.xMax - 26;
                        fldRect.height = EditorGUIUtility.singleLineHeight;

                        GUIStyle fldStyle = EditorStyles.miniButton;
                        fldStyle.alignment = TextAnchor.MiddleCenter;
                        fldStyle.margin = new RectOffset();
                        fldStyle.padding = new RectOffset();
                        fldStyle.fontSize = 10;


                        if (!_foldout && GUI.Button(fldRect, Tok_FoldoutOff, fldStyle)) _foldout = true;
                        if (_foldout)
                        {
                            if (GUI.Button(fldRect, Tok_FoldoutOn, fldStyle))
                                _foldout = false;

                            GUIStyle btnStyle = new GUIStyle(EditorStyles.miniButton);
                            Color clr = btnStyle.normal.textColor;
                            btnStyle.fontSize = 11;
                            btnStyle.padding = new RectOffset(5, 5, 2, 2);
                            btnStyle.alignment = TextAnchor.MiddleLeft;

                            for (int i = 0; i < subComps.Length; i++)
                            {
                                Rect btnPos = position;
                                btnPos.y += (EditorGUIUtility.singleLineHeight) * (i + 1);
                                btnPos.height = EditorGUIUtility.singleLineHeight;
                                Rect lblPos = btnPos;
                                lblPos.xMax = EditorGUIUtility.labelWidth * LabelWidthScale;
                                btnPos.xMin += EditorGUIUtility.labelWidth * LabelWidthScale;


                                GUIStyle lblStyle = new GUIStyle(EditorStyles.miniBoldLabel);
                                lblStyle.alignment = TextAnchor.MiddleRight;
                                if (compRef == subComps[i])
                                {
                                    GUI.color = Color.green;
                                    GUI.Label(lblPos, Tok_Selected, lblStyle);
                                    GUI.color = Color.white;
                                }
                                GUI.Label(lblPos, $"{i}#", lblStyle);


                                string btnLabel = $"{subComps[i]}";
                                btnLabel = btnLabel.Remove(0, btnLabel.IndexOf('(') + 1).Trim(Tok_TrimChars);

                                if (btnLabel.Contains('.'))
                                {
                                    btnLabel = btnLabel.Substring(btnLabel.LastIndexOf('.') + 1);
                                }

                                Texture icon = EditorGUIUtility.ObjectContent(subComps[i], subComps[i].GetType()).image;
                                if (GUI.Button(btnPos, new GUIContent(btnLabel, icon, btnLabel), btnStyle))
                                {
                                    property.objectReferenceValue = subComps[i];
                                    property.serializedObject.ApplyModifiedProperties();
                                    _foldout = false;
                                }
                            }
                        }
                    }
                }

                Rect rawObjRect = position;
                if (_subComps > 1)
                    rawObjRect.xMax -= 28;
                EditorGUI.PropertyField(rawObjRect, property, true);

            }
            else
            {
                EditorGUI.LabelField(position, $"Type: {property.propertyType} is not supported by {nameof(SubComponentAttribute)}.");
                _subComps = 0;
            }
        }
        public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
        {
            return base.GetPropertyHeight(property, label) + (_subComps * (EditorGUIUtility.singleLineHeight + EditorGUIUtility.standardVerticalSpacing) * (_foldout ? 1 : 0));
        }
    }

#endif

}