There are transformer and operator counterparts for Single, Maybe, and Completable. When you want to create an Observable or Flowable operator that yields Single, you might find it easier to convert it back into an Observable/Flowable by calling its toObservable() or toFlowable() operators. This also applies to Maybe.
If, on some rare occasion, you need to create a transformer or operator specifically to take a Single and transform it into another Single, you want to use SingleTransformer or SingleOperator. The Maybe and Completable have counterparts with MaybeTransformer/MaybeOperator and CompletableTransformer/CompletableOperator, respectively.
The implementation of apply() for all of these should largely be the same experience, and you will use SingleObserver, MaybeObserver, and CompletableObserver to proxy the upstream...