it('calls unsubscribe on the appropriate downstream observable', async () => {
    const retry = new RetryLink({
      delay: { initial: 1 },
      attempts: { max: 2 },
    });
    const data = { data: { hello: 'world' } };
    const unsubscribeStub = jest.fn();

    const firstTry = fromError(standardError);
    // Hold the test hostage until we're hit
    let secondTry;
    const untilSecondTry = new Promise(resolve => {
      secondTry = {
        subscribe(observer) {
          resolve(); // Release hold on test.

          Promise.resolve().then(() => {
            observer.next(data);
            observer.complete();
          });
          return { unsubscribe: unsubscribeStub };
        },
      };
    });

    const stub = jest.fn();
    stub.mockReturnValueOnce(firstTry);
    stub.mockReturnValueOnce(secondTry);
    const link = ApolloLink.from([retry, stub]);

    const subscription = execute(link, { query }).subscribe({});
    await untilSecondTry;
    subscription.unsubscribe();
    expect(unsubscribeStub).toHaveBeenCalledTimes(1);
  });

  return new ApolloLink(operation => {
    let chosenURI = selectURI(operation, uri);

    const context = operation.getContext();

    const contextConfig = {
      http: context.http,
      options: context.fetchOptions,
      credentials: context.credentials,
      headers: context.headers,
    };

    //uses fallback, link, and then context to build options
    const { options, body } = selectHttpOptionsAndBody(
      operation,
      fallbackHttpConfig,
      linkConfig,
      contextConfig,
    );

    let controller;
    if (!(options as any).signal) {
      const { controller: _controller, signal } = createSignalIfSupported();
      controller = _controller;
      if (controller) (options as any).signal = signal;
    }

    // If requested, set method to GET if there are no mutations.
    const definitionIsMutation = (d: DefinitionNode) => {
      return d.kind === 'OperationDefinition' && d.operation === 'mutation';
    };
    if (
      useGETForQueries &&
      !operation.query.definitions.some(definitionIsMutation)
    ) {
      options.method = 'GET';
    }

    if (options.method === 'GET') {
      const { newURI, parseError } = rewriteURIForGET(chosenURI, body);
      if (parseError) {
        return fromError(parseError);
      }
      chosenURI = newURI;
    } else {
      try {
        (options as any).body = serializeFetchParameter(body, 'Payload');
      } catch (parseError) {
        return fromError(parseError);
      }
    }

    return new Observable(observer => {
      fetcher(chosenURI, options)
        .then(response => {
          operation.setContext({ response });
          return response;
        })
        .then(parseAndCheckHttpResponse(operation))
        .then(result => {
          // we have data and can send it to back up the link chain
          observer.next(result);
          observer.complete();
          return result;
        })
        .catch(err => {
          // fetch was cancelled so its already been cleaned up in the unsubscribe
          if (err.name === 'AbortError') return;
          // if it is a network error, BUT there is graphql result info
          // fire the next observer before calling error
          // this gives apollo-client (and react-apollo) the `graphqlErrors` and `networErrors`
          // to pass to UI
          // this should only happen if we *also* have data as part of the response key per
          // the spec
          if (err.result && err.result.errors && err.result.data) {
            // if we dont' call next, the UI can only show networkError because AC didn't
            // get andy graphqlErrors
            // this is graphql execution result info (i.e errors and possibly data)
            // this is because there is no formal spec how errors should translate to
            // http status codes. So an auth error (401) could have both data
            // from a public field, errors from a private field, and a status of 401
            // {
            //  user { // this will have errors
            //    firstName
            //  }
            //  products { // this is public so will have data
            //    cost
            //  }
            // }
            //
            // the result of above *could* look like this:
            // {
            //   data: { products: [{ cost: "$10" }] },
            //   errors: [{
            //      message: 'your session has timed out',
            //      path: []
            //   }]
            // }
            // status code of above would be a 401
            // in the UI you want to show data where you can, errors as data where you can
//.........这里部分代码省略.........

 const stub = jest.fn(() => fromError(standardError));

    const batchHandler = (operations: Operation[]) => {
      const chosenURI = selectURI(operations[0], uri);

      const context = operations[0].getContext();

      const contextConfig = {
        http: context.http,
        options: context.fetchOptions,
        credentials: context.credentials,
        headers: context.headers,
      };

      //uses fallback, link, and then context to build options
      const optsAndBody = operations.map(operation =>
        selectHttpOptionsAndBody(
          operation,
          fallbackHttpConfig,
          linkConfig,
          contextConfig,
        ),
      );

      const loadedBody = optsAndBody.map(({ body }) => body);
      const options = optsAndBody[0].options;

      // There's no spec for using GET with batches.
      if (options.method === 'GET') {
        return fromError<FetchResult[]>(
          new Error('apollo-link-batch-http does not support GET requests'),
        );
      }

      try {
        (options as any).body = serializeFetchParameter(loadedBody, 'Payload');
      } catch (parseError) {
        return fromError<FetchResult[]>(parseError);
      }

      let controller;
      if (!(options as any).signal) {
        const { controller: _controller, signal } = createSignalIfSupported();
        controller = _controller;
        if (controller) (options as any).signal = signal;
      }

      return new Observable<FetchResult[]>(observer => {
        // the raw response is attached to the context in the BatchingLink
        fetcher(chosenURI, options)
          .then(parseAndCheckHttpResponse(operations))
          .then(result => {
            // we have data and can send it to back up the link chain
            observer.next(result);
            observer.complete();
            return result;
          })
          .catch(err => {
            // fetch was cancelled so its already been cleaned up in the unsubscribe
            if (err.name === 'AbortError') return;
            // if it is a network error, BUT there is graphql result info
            // fire the next observer before calling error
            // this gives apollo-client (and react-apollo) the `graphqlErrors` and `networErrors`
            // to pass to UI
            // this should only happen if we *also* have data as part of the response key per
            // the spec
            if (err.result && err.result.errors && err.result.data) {
              // if we dont' call next, the UI can only show networkError because AC didn't
              // get andy graphqlErrors
              // this is graphql execution result info (i.e errors and possibly data)
              // this is because there is no formal spec how errors should translate to
              // http status codes. So an auth error (401) could have both data
              // from a public field, errors from a private field, and a status of 401
              // {
              //  user { // this will have errors
              //    firstName
              //  }
              //  products { // this is public so will have data
              //    cost
              //  }
              // }
              //
              // the result of above *could* look like this:
              // {
              //   data: { products: [{ cost: "$10" }] },
              //   errors: [{
              //      message: 'your session has timed out',
              //      path: []
              //   }]
              // }
              // status code of above would be a 401
              // in the UI you want to show data where you can, errors as data where you can
              // and use correct http status codes
              observer.next(err.result);
            }

            observer.error(err);
          });

        return () => {
          // XXX support canceling this request
          // https://developers.google.com/web/updates/2017/09/abortable-fetch
//.........这里部分代码省略.........