Flutter中Dio Cache Interceptor无法缓存请求的问题求助

我正尝试为Flutter应用添加dio_cache_interceptor，使用的版本如下：

dio: ^5.6.0
dio_cache_interceptor: ^3.5.0

但发起请求时，请求始终发送至Web服务器，缓存似乎从未生效。

已尝试或排查的内容：

• 响应状态码为200
• 调整过CacheOptions.keyBuilder，生成的请求缓存键始终一致
• 响应头信息如下：
  • etag:
  • content-type: application/json;charset=utf-8
  • cache-control: private, max-age=86400, stale-if-error=21600
  • date: Fri, 16 Aug 2024, 09:42:46 GMT
  • content-length: 2332
  • x-aspnet-version: 4.0.30319
    （该API由我维护，可添加自定义响应头）
• 调整过拦截器顺序（缓存拦截器已放在首位）
• 已安装http: ^1.2.1包，但未在当前文件中使用
• 查阅过Dio Cache Interceptor官方文档，但未找到解决方法

已花费一整天排查仍无头绪，希望能得到方向指引。

精简后的代码：

import 'dart:async';
import 'package:dio/dio.dart';
import 'package:dio_cache_interceptor/dio_cache_interceptor.dart';

class WebAPIProvider {
  late final Dio dio;
  late final CacheOptions cacheOptions;

  /// WebAPIProvider Constructor
  WebAPIProvider() {
    // Set cache options
    cacheOptions = CacheOptions(
      store: MemCacheStore(),
      policy: CachePolicy.request,
      hitCacheOnErrorExcept: [403],
      maxStale: const Duration(days: 7),
    );

    // Set base options for every request and add interceptors
    dio = Dio(
      BaseOptions(
        baseUrl: 'https://my.domain.com/webapi/v1',
        connectTimeout: const Duration(seconds: 7),
        receiveTimeout: const Duration(seconds: 4),
        responseType: ResponseType.json,
      ),
    )..interceptors.addAll([
        DioCacheInterceptor(
          options: cacheOptions,
        ),
        InterceptorsWrapper(
          onRequest: onDioRequest,
          onResponse: onDioResponse,
          onError: onDioError,
        ),
      ]);
  }

  /// Fired before every Dio request
  /// 
  /// @Stackoverflow: This adds the accessToken (and other stuff) to the header
  FutureOr<void> onDioRequest(
    RequestOptions options,
    RequestInterceptorHandler handler,
  ) async {
    // Set contentType
    options.headers["contentType"] = "application/json";

    // Set accessToken
    options.headers["accessToken"] = "access-token-here";

    // Continue
    return handler.next(options);
  }

  /// Fired at every response Dio receives
  /// 
  /// @Stackoverflow: Using this for logging the reponse
  FutureOr<void> onDioResponse(
    Response response,
    ResponseInterceptorHandler handler,
  ) async {
    // Some logging here...
        
    // Continue
    return handler.next(response);
  }

  /// Fired at any error during the request
  /// 
  /// @Stackoverflow: Using this for logging the error and resolve with an "empty" dataset
  FutureOr<void> onDioError(
    DioException exception,
    handler,
  ) async {
    // Continue with empty response
    return handler.resolve(Response(
      extra: {"exceptionType": exception.type},
      requestOptions: exception.requestOptions,
    ));
  }

  /// 🌐 Invokes the API [endpoint] with optional [requestHeaders] and [queryParameters].
  /// 
  /// @Stackoverflow: This is the method I'm invoking, i.e. data = api.get('/my/endpoint')
  Future<Response> get(
    String endpoint, [
    Map<String, String>? requestHeaders,
    Map<String, dynamic>? queryParameters,
  ]) async {
    // Send request
    Response<dynamic> response = await dio.get(
      endpoint,
      queryParameters: queryParameters,
      options: Options(
        headers: requestHeaders,
      ),
    );

    // Here I would convert it into a specific type, matching my API

    return response;
  }
}

补充说明：尝试将onDioRequest()改为同步方法（原本因从安全存储读取accessToken设为异步），但响应始终包含extra: {@cache_key@: always-same-key, @fromNetwork@: true}的内容。

1. 修复请求选项覆盖缓存配置的问题
   当前全局设置的CacheOptions会被get方法中传入的空Options对象覆盖，导致缓存规则未生效。修改get方法的请求代码，把全局缓存配置合并到请求选项中：

   Response<dynamic> response = await dio.get(
     endpoint,
     queryParameters: queryParameters,
     options: cacheOptions.copyWith(headers: requestHeaders).toOptions(),
   );
   

2. 强制忽略响应头缓存指令
   响应头中的private指令可能干扰缓存行为，可在全局CacheOptions中添加参数，强制使用自定义的缓存规则：

   cacheOptions = CacheOptions(
     store: MemCacheStore(),
     policy: CachePolicy.request,
     hitCacheOnErrorExcept: [403],
     maxStale: const Duration(days: 7),
     overrideCacheHeaders: true, // 强制忽略响应头的缓存指令
   );
   

3. 排查自定义拦截器的影响
   暂时移除自定义的InterceptorsWrapper，测试缓存是否生效。如果生效，再逐步恢复onRequest/onResponse/onError方法，排查是否有代码修改了请求/响应参数导致缓存无法正常存储或读取。

4. 验证缓存存储状态
   在请求后手动调用缓存存储的获取方法，确认缓存是否被写入：

   final cacheKey = cacheOptions.keyBuilder(RequestOptions(path: endpoint, baseUrl: dio.options.baseUrl));
   final cachedResponse = await cacheOptions.store.get(cacheKey);
   print(cachedResponse); // 查看是否有缓存数据
   

5. 测试持久化缓存存储
   MemCacheStore是内存缓存，应用重启后会丢失，可临时换成HiveCacheStore（需添加依赖dio_cache_interceptor_hive_store）测试，确认是否是存储类型的问题。

内容的提问来源于stack exchange，提问作者Boris