final ApiService service = retrofit.create(ApiService.class);
service.login()
    .subscribeOn(Schedulers.newThread())
    .observeOn(Schedulers.newThread())
    .flatMap(response -> {
        if (response.getErrCode != 0) {
            return null;
        }
        return service.getDataFromWeb();
    })
    .subscribe(response -> {
        // do something
    }, err -> {
        // handle err
    });