Default Editor : Enum as flags ?

I like this! Probably worth mentioning that the attribute class must exist in a non-editor script and folder, and the property drawer class must exist in an editor script and folder. I had them in the same file as seems to be implied above. That tripped me up and it took some staring at the PropertyDrawer docs to figure it out.

For some reason the built-in "Everything" option is uncheckable for me. I use a custom definition for "All" in the enum, which is nice to have control over. I also like to define "None" in my enum as well as some combos that I use for mask testing, but don't intend for the field to be set to. Guess I'll mess with the editor UI a bit to see if I can suppress certain options.

UPDATE: found an alternative complete solution

http://answers.unity3d.com/questions/393992/custom-inspector-multi-select-enum-dropdown.html

Note that this solution works if and only if your enum values are consecutive bits (1, 2, 4, 8, ...). If you have a 0 value ("None") or combined values, then EditorGUI.MaskField isn't sufficient.

For a complete solution, see the link mentioned in previous comment (http://answers.unity3d.com/questions/393992/custom-inspector-multi-select-enum-dropdown.html)

Warning: The code in this answer isn't friendly to multiple-selection. As soon as you select two objects, the enum flags get overwritten without warning and with no way to undo. Here's my version of OnGUI that checks for changes before assigning. This is way safer.

     public override void OnGUI(Rect _position, UnityEditor.SerializedProperty _property, GUIContent _label)
     {
         // Change check is needed to prevent values being overwritten during multiple-selection
         UnityEditor.EditorGUI.BeginChangeCheck ();
         int newValue = UnityEditor.EditorGUI.MaskField( _position, _label, _property.intValue, _property.enumNames );
         if (UnityEditor.EditorGUI.EndChangeCheck ()) {
             _property.intValue = newValue;
         }
     }
 

Very elegant but doesn't handle multi selection. Add this right after OnGUI{

 EditorGUI.showMixedValue = _property.hasMultipleDifferentValues;