optimize song list load

src/app/modules/songs/services/song-list.resolver.ts

@@ -0,0 +1,17 @@
+import {Injectable} from '@angular/core';
+import {Resolve} from '@angular/router';
+import {Observable} from 'rxjs';
+import {SongService} from './song.service';
+import {Song} from './song';
+import {filter} from 'rxjs/operators';
+
+@Injectable({
+  providedIn: 'root',
+})
+export class SongListResolver implements Resolve<Song[]> {
+  public constructor(private songService: SongService) {}
+
+  public resolve(): Observable<Song[]> {
+    return this.songService.list$().pipe(filter(_ => _.length > 0));
+  }
+}