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

import { Observable } from 'rxjs/Observable';
import { tap } from 'rxjs/operators';
import 'rxjs/add/observable/of';

import { RequestCache } from '../services/request-cache.service';

@Injectable()
export class CachingInterceptor implements HttpInterceptor {
  constructor(private cache: RequestCache) { }

  intercept(req: HttpRequest<any>, next: HttpHandler) {
    const cachedResponse = this.cache.get(req);
    return cachedResponse
      ? Observable.of(cachedResponse)
      : this.sendRequest(req, next);
  }

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