How to wrap an existing async API in a reactive stream

rxjavawrap.md

return Observable.create { emitter ->
      foo.fetchBar(object : FooCallback<Bar> {
          override fun onSuccess(bar: Bar) {
               emitter.onNext(bar)
          }

          override fun onError(error: Throwable) {
               emitter.onError(error)
          }
      })
 }