How do I add an animation while loading a request from Angular 4?

My service looks like this:

export class ServicoService {

    constructor(private http:Http) { }

    public listar() {
        return this.http.get('aqui a url')
        .toPromise()
        .then(response => response.json());
    }

}

The code is working correctly, but I would like to give Feedback to the user saying that the content is loading. How do I?