angular 6 snipped: attach spinner in observable

angularspinner.ts

import { NgxSpinnerService } from 'ngx-spinner';
import { map, finalize } from 'rxjs/operators';
import { HttpClient, HttpHeaders } from '@angular/common/http'; 

...

private attachSpinner(ob : Observable<any>): Observable<any> {
    this.spinner.show();
    return ob.pipe(finalize(() => {
        this.spinner.hide()
    }));
}

...

let ob = this.http.get<ServerData[]>(this.url)
        .pipe(map((data: any) => {
            let serverList: ServerData[] = [];
            data.serverlist.forEach(s => {
                serverList.push({server: s);
            })
            return serverList;
        }));                
return this.attachSpinner(ob);