RXjava 和 Retrofit 处理空数据响应

问题描述

我使用 Rxjava 和 Retrofit 来处理来自 HTTP 服务器的响应.

I use Rxjava and Retrofit to handle the response from an HTTP server.

大多数时候，数据响应采用这种格式:

Most of the time, data response is in this format:

{
  code: 'OK',
  data: {.....}
}

有时，data 属性不存在:

{
  code: 'OK'
}

我创建了一个 BaseResponse 类.

public class BaseResponse<T> implements Serializable {
    private String code;
    private String msg;
    private T data;

    public String getCode() {
        return code;
    }

    public String getMsg() {
        return msg;
    }

    public T getData() {
        return data;
    }
}

并制作一个Map函数来解析它:

And make a Map function to parse it:

public static <T> Function<BaseResponse<T>, T> applyUnwrapResponse () {
        return new Function<BaseResponse<T>, T>() {
            @Override
            public T apply(@NonNull BaseResponse<T> baseResponse) throws Exception {
                if (baseResponse.getCode().equals(Constants.SUCCESS_CODE)) {
                    // http returned from server
                    T data = baseResponse.getData();
                    if (data == null) {
                        return null;  // throw exception: The mapper function returned a null value
                        return return new T(); // syntax error
                        // how ?
                    }
                    return data;
                } else if (baseResponse.getCode().equals(Constants.USER_NOT_LOGGED_IN)) {
                    // auth token error
                    throw new UserNotLoginException(baseResponse.getMsg());
                }
                // unknown error
                throw new UnknownServerException(baseResponse.getMsg());
            }
        };
    }

当 data 属性不存在时，我尝试返回 null，但这在 Rxjava2.
在自定义Observer中，我在onNext()中处理了所有success场景，所以我需要在data == null时发出错误.
但是我不能返回null，也不能通过return new T()返回一个T，那我该怎么办?谁能帮帮我?

When data property doesn't exist, I try to return null, but this is not allowed in Rxjava2.
In the customize Observer, I handle all success scenarios in onNext(), so I need to emit an error when data == null.
but I can not return null, and also cannot return a T by return new T(), so how should I do? Anyone can help me out?

推荐答案

几个建议的选项:

• 由于 Gson 解析器会在没有数据的字段中放置 null，因此您可以返回整个 BaseResponse 对象并在订阅者/观察者处检查数据属性对象的无效性.

• 你可以简单地用一些 Optional 类似的接口包装你的 T 值，它可以有一个值，也可以什么都没有(Java8 Optional 可能不适用于android，但你可以自己想出简单的结构，比如 Optional)，然后在你的 onNext() 上你可以检查你是否有一个值.

• As Gson parser will put null in fields with no data, so you can return the entire BaseResponse object and check the nullity of data property object at the Subscriber/Observer.

• You can simply wrap your T value with some Optional alike interface, which can have a value, or can have nothing (Java8 Optional probably not available with android, but you can come up with simple structure like Optional yourself), then on your onNext() you can check whether you have a value or not.

RxJava2 具有新的 Observable 类型，称为 也许，要么发出单个项目 - onSuccess 通知，要么什么都不发出 - onComplete() 通知，(还有错误):

RxJava2 has new Observable type called Maybe, which either emit a single item - onSuccess notification, or nothing - onComplete() notification, (and also errors):

onSubscribe (onSuccess | onCompleted | onError)?

您可以将 flatMapMaybe 与 Observable 或 Single(来自 Retrofit 服务接口)一起使用，将一些响应值映射到 Maybe，在 flatMapMaybe 中，您可以使用 Maybe.fromCallable 返回 null，就像您在此处所做的那样，Maybe.fromCallable 将处理由于未发出任何项目而返回 null，您将获得 onComplete()，因此您可以区分结果情况 (onNext) 和无项目情况 (onComplete).

You can use flatMapMaybe with both Observable or Single (from the Retrofit service interface), to map some response value to a Maybe, in the flatMapMaybe you can use Maybe.fromCallable to return null like you did here, Maybe.fromCallable will treat returned null as no items emitted, and you'll get onComplete(), so you can distinguish between result case (onNext) to the no items case (onComplete).

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