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

  intercept(req: HttpRequestany>, next: HttpHandler): ObservableHttpEventany>> {
// Cache all GET requests and Skip other requist like POST, PUT etc.
if (req.method !== 'GET') {
return next.handle(req);
    }

// Cached Response for all exists GET requests.
const cachedResponse = this.cache.get(req);

//If cached response is exists.
if (cachedResponse) {
return Observable.of(cachedResponse);
    }else{
//If cached response is not exists.
return next.handle(req).do(event => {
if (event instanceof HttpResponse) {
this.cache.put(req, event);// Update the cache.
          }
      });
    }
  }
}