medium_asynchronous_schematics_p3.ts

function waitingRule(options: any): Rule {
    return (host: Tree) => {
        const observer = new Observable<Tree>((observer) => {
            fetch('someUrl.com/') // any async operation
                .then(res => res.json())
                .then(data => {
                    observer.next(host);
                    observer.complete();
                })
                .catch(function (err: any) {
                    observer.error(err);
                });
        });
        return observer;
    };
}