Unity3D Editor: Scene popup

ScenePopup.cs

using UnityEngine;
using System.Collections;
	
public class ScenePopup : PropertyAttribute {
	
	public bool required = false;

	public ScenePopup() {}
	
	public ScenePopup(bool required) {
		this.required = required;
	}

}

ScenePopupDrawer.cs

using UnityEngine;
using UnityEditor;
using System.IO;
using System.Collections;
using System.Collections.Generic;

[CustomPropertyDrawer(typeof(ScenePopup))]
public class ScenePopupDrawer : PropertyDrawer {

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) {
		if (property.propertyType != SerializedPropertyType.Integer) {
			EditorGUI.PropertyField(position, property);
			return;
		}

		List<GUIContent> sceneNames = new List<GUIContent>();
		List<int> sceneValues = new List<int>();

		ScenePopup scenePopup = attribute as ScenePopup;
		if (!scenePopup.required) {
			sceneNames.Add(new GUIContent("- None -"));
			sceneValues.Add(-1);
		}

		for (int i = 0; i < EditorBuildSettings.scenes.Length; i++) {
			EditorBuildSettingsScene scene = EditorBuildSettings.scenes[i];
			if (scene.enabled) {
				string name = Path.GetFileName(scene.path);
				sceneNames.Add(new GUIContent(name));
				sceneValues.Add(i);
			}
		}

		EditorGUI.IntPopup(position, property, sceneNames.ToArray(), sceneValues.ToArray());
	}

}