rxretrofit.java

.addCallAdapterFactory(new CallAdapter.Factory() {
    //relevant SO:  https://github.com/square/retrofit/pull/1117
    RxJavaCallAdapterFactory delegate = RxJavaCallAdapterFactory.create();

    @Override
    public CallAdapter<?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
        //noinspection unchecked
        CallAdapter<Observable<?>> delegate2 = (CallAdapter<Observable<?>>) delegate.get(returnType, annotations, retrofit);
        return new CallAdapter<Object>() {
            @Override
            public Type responseType() {
                return delegate2.responseType();
            }

            @Override
            public <R> Object adapt(Call<R> call) {
                return delegate2.adapt(call).subscribeOn(Schedulers.io());
            }
        };
    }
})