import { concat, of, Observable, EMPTY } from 'rxjs';

import { tap } from 'rxjs/operators';
import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpResponse, HttpHandler, HttpEvent } from '@angular/common/http';
import { NaHttpCache } from '../na-http-cache';

@Injectable({
  providedIn: 'root'
})
export class NaCachingInterceptor implements HttpInterceptor {
  constructor(private cache: NaHttpCache) { }

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    // Still skip non-GET requests.
    if (req.method !== 'GET') {
      return next.handle(req);
    }

    // This will be an Observable of the cached value if there is one,
    // or an empty Observable otherwise. It starts out empty.
    let maybeCachedResponse: Observable<HttpEvent<any>> = EMPTY;

    // Check the cache.
    const cachedResponse = this.cache.get(req);
    if (cachedResponse) {
      maybeCachedResponse = of(cachedResponse);
    }

    // Create an Observable (but don't subscribe) that represents making
    // the network request and caching the value.
    const networkResponse = next.handle(req).pipe(tap(event => {
      // Just like before, check for the HttpResponse event and cache it.
      if (event instanceof HttpResponse) {
        this.cache.put(req, event);
      }
    }));

    // Now, combine the two and send the cached response first (if there is
    // one), and the network response second.
    return concat(maybeCachedResponse, networkResponse);
  }
}