Unity3D BitMaskAttribute

using UnityEngine;
using UnityEditor;
using System.Collections;

public static class EditorExtension
{
    public static int DrawBitMaskField (Rect aPosition, int aMask, System.Type aType, GUIContent aLabel)
    {
        var itemNames = System.Enum.GetNames(aType);
        var itemValues = System.Enum.GetValues(aType) as int[];

        int val = aMask;
        int maskVal = 0;
        for(int i = 0; i < itemValues.Length; i++)
        {
            if (itemValues[i] != 0)
            {
                if ((val & itemValues[i]) == itemValues[i])
                    maskVal |= 1 << i;
            }
            else if (val == 0)
                maskVal |= 1 << i;
        }
        int newMaskVal = EditorGUI.MaskField(aPosition, aLabel, maskVal, itemNames);
        int changes = maskVal ^ newMaskVal;

        for(int i = 0; i < itemValues.Length; i++)
        {
            if ((changes & (1 << i)) != 0)            // has this list item changed?
            {
                if ((newMaskVal & (1 << i)) != 0)     // has it been set?
                {
                    if (itemValues[i] == 0)           // special case: if "0" is set, just set the val to 0
                    {
                        val = 0;
                        break;
                    }
                    else
                        val |= itemValues[i];
                }
                else                                  // it has been reset
                {
                    val &= ~itemValues[i];
                }
            }
        }
        return val;
    }
}

[CustomPropertyDrawer(typeof(BitMaskAttribute))]
public class EnumBitMaskPropertyDrawer : PropertyDrawer
{
    public override void OnGUI (Rect position, SerializedProperty prop, GUIContent label)
    {
        var typeAttr = attribute as BitMaskAttribute;
        // Add the actual int value behind the field name
        label.text = label.text + "("+prop.intValue+")";
        prop.intValue = EditorExtension.DrawBitMaskField(position, prop.intValue, typeAttr.propType, label);
    }
}

public class BitMaskAttribute : PropertyAttribute
{
    public System.Type propType;
    public BitMaskAttribute(System.Type aType)
    {
        propType = aType;
    }
}

/* How to use it:
/ [BitMaskAttribute(typeof(WalkableTile.Direction))]
/  public WalkableTile.Direction direction;
*/