import { tap } from 'rxjs/operators';
import { Observable, of } from 'rxjs';
import { Injectable } from '@angular/core';
import {
  HttpRequest,
  HttpHandler,
  HttpEvent,
  HttpInterceptor,
  HttpResponse
} from '@angular/common/http';
import { TransferStateService } from '../services/transfer-state.service';

@Injectable()
export class TransferStateInterceptor implements HttpInterceptor {
  constructor(private transferStateService: TransferStateService) {
  }

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    /**
     * Skip this interceptor if the request method isn't GET.
     */
    if (req.method !== 'GET') {
      return next.handle(req);
    }

    const cachedResponse = this.transferStateService.getCache(req.url);
    if (cachedResponse) {
      // A cached response exists which means server set it before. Serve it instead of forwarding
      // the request to the next handler.
      return of(new HttpResponse<any>({ body: cachedResponse }));
    }

    /**
     * No cached response exists. Go to the network, and cache
     * the response when it arrives.
     */
    return next.handle(req).pipe(
      tap(event => {
        if (event instanceof HttpResponse) {
          this.transferStateService.setCache(req.url, event.body);
        }
      })
    );
  }
}