多值可观察流仅给出单值

Question

以official Angular docs中的HttpInterceptor缓存示例为例，它演示了使用startWith将多值可观察流用于缓存的响应。

示例代码：

import {Injectable} from '@angular/core';
import {HttpEvent, HttpHandler, HttpInterceptor, HttpRequest, HttpResponse} from '@angular/common/http';

import {Observable} from 'rxjs';
import {tap, startWith} from 'rxjs/operators';

@Injectable()
export class CacheInterceptor implements HttpInterceptor {
    intercept(req: HttpRequest<any>, next: HttpHandler) {
        if (req.method !== 'GET') {
            return next.handle(req);
        }

        const cachedResponse = cache.get(req);
        const results$ = sendRequest(req, next);
        return cachedResponse ? results$.pipe(startWith(cachedResponse)) : results$;
    }
}

function sendRequest(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    return next.handle(req).pipe(
        tap(event => {
            if (event instanceof HttpResponse) {
                cache.put(req, event);
            }
        })
    );
}

export class HttpCache {
    cache = new Map();

    get(req: HttpRequest<any>): HttpResponse<any> | undefined {
        const url = req.urlWithParams;
        const cached = this.cache.get(url);

        if (!cached) {
            return undefined;
        }

        return cached.response;
    }

    put(req: HttpRequest<any>, response: HttpResponse<any>): void {
        const url = req.urlWithParams;
        const entry = {url, response, lastRead: Date.now()};

        this.cache.set(url, entry);
    }
}

const cache: HttpCache = new HttpCache();

该代码无效。它总是等待第二个可观察值，并且永远不会发出startWith中包含的第一个值。有什么作用？