ReorderableList Property Drawer

example.cs

public class MyComponent : MonoBehaviour {

  public LevelWaveList levelWaves;
  
  void Start() {
    DoLevelWave( levelWaves.list[0].length );
  }
}

[Serializable]
public struct LevelWave {
    public float length;
    public GameObject[] toSpawn;    // TODO patterns or something
}

[Serializable]
public class LevelWaveList : ReorderableList<LevelWave> { }

ReorderableList.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

/// In Unity pre 2020, each different type you want to make a list from needs its own subclass, like UnityEvents.
/// eg :
/// [Serializable] public class IntReorderableList : ReorderableList<int> {}

[Serializable]
public class ReorderableList<T> {
    public List<T> list;
}

ReorderableListDrawer.cs

/* ReorderableListDrawer.cs
 * © Eddie Cameron 2019
 * ----------------------------
 */
using System.Collections.Generic;
using UnityEditor;
using UnityEditorInternal;
using UnityEngine;

[CustomPropertyDrawer( typeof( ReorderableList<> ), useForChildren: true )]
public class ReorderableListDrawer : PropertyDrawer {

    Dictionary<string, ReorderableList> _listsPerProp = new Dictionary<string, ReorderableList>();

    ReorderableList GetReorderableList( SerializedProperty prop ) {
        SerializedProperty listProperty = prop.FindPropertyRelative( "list" );

        ReorderableList list;
        if ( _listsPerProp.TryGetValue( listProperty.propertyPath, out list ) ) {
            return list;
        }

        list = new ReorderableList( listProperty.serializedObject, listProperty, draggable: true, displayHeader: true, displayAddButton: true, displayRemoveButton: true );
        _listsPerProp[listProperty.propertyPath] = list;

        list.drawHeaderCallback += rect => {
            EditorGUI.LabelField( rect, prop.displayName );
        };

        list.drawElementCallback += ( rect, index, isActive, isFocused ) => {
            SerializedProperty elementProp = list.serializedProperty.GetArrayElementAtIndex( index );
            if ( elementProp.hasVisibleChildren ) {
                EditorGUI.PropertyField( rect, elementProp, includeChildren: true );
            }
            else {
                EditorGUI.PropertyField( rect, elementProp, includeChildren: true, label: GUIContent.none );   // dont draw label if its a single line
            }
        };

        list.elementHeightCallback += idx => {
            SerializedProperty elementProp = listProperty.GetArrayElementAtIndex( idx );
            return EditorGUI.GetPropertyHeight( elementProp );
        };

        return list;
    }

    public override void OnGUI( Rect rect, SerializedProperty serializedProperty, GUIContent label )
    {
        ReorderableList list = GetReorderableList( serializedProperty );

        list.DoList( rect );
    }

    public override float GetPropertyHeight( SerializedProperty serializedProperty, GUIContent label )
    {
        return GetReorderableList( serializedProperty ).GetHeight();
    }
}