AVUser user = new AVUser();
    user.setUsername(fieldsJson.getString("username"));
    user.setPassword(fieldsJson.getString("password"));
    user.signUpInBackground().subscribe(new Observer<AVUser>() {

        @Override
        public void onSubscribe(Disposable disposable) {
        }

        @Override
        public void onNext(AVUser avUser) {
            result.success(avUser.toJSONObject().toJSONString());
        }

        @Override
        public void onError(Throwable throwable) {

            Log.i("----->message:", throwable.toString());//这里throwable.toString()和getMessage(),都只能获得错误消息,比如"Username has already been taken."而无法获取对应到的错误码202

            result.error("leancloud-error", throwable.getMessage(), null);
        }