Custom transform on a Laravel model

I’m trying to do a little bit of tidy up on my Laravel models.

Right now, my toSearchableArray() method is quite large and I’d like to extract the transformation into its own class.

As I understand it, for that I need a custom transformer as documented here Customize searchable data | Laravel | Algolia

What the docs don’t make clear however is what’s contained with the App\Contracts\TransformService.

Can anyone shed any light?