Sketch has incredibly robust exporting built in. You can specify relative sizes (@2x, @3x), or absolute sizes (320w, 90h) as well as a suffix ( _xxxhdpi), and multiple formats (jpg, png, svg, etc).
I've even been using Sketch to batch resize jpg images with 20 export sizes per image (product photos and thumbnails for web, iOS and Android). It is the only thing I've found that can do what I need without having to resort to scripting.
And I haven't used Photoshop for UI work in probably 4 years, but Slicy was a pretty great tool back when I did: http://macrabbit.com/slicy/
Specctr Just released version 2.0 today. Is there something that's missing with it? I haven't heard of any alternatives for it.
A lot of Mac uses are trying out Sketch which has plugins that do similar. http://bohemiancoding.com/sketch/
Also, another useful tool along these lines: http://macrabbit.com/slicy/
Yeah there isn't really a way to do this, I mean you could write your own tool (either by scripting photoshop or by writing a PSD importer) but it's definitely outside the scope of what Futile does. Have a look at something like Slicy too: http://macrabbit.com/slicy/