save and restore scroll position of song list

src/app/services/scroll.service.spec.ts

@@ -0,0 +1,16 @@
+import {TestBed} from '@angular/core/testing';
+
+import {ScrollService} from './scroll.service';
+
+describe('ScrollService', () => {
+  let service: ScrollService;
+
+  beforeEach(() => {
+    TestBed.configureTestingModule({});
+    service = TestBed.inject(ScrollService);
+  });
+
+  it('should be created', () => {
+    expect(service).toBeTruthy();
+  });
+});

src/app/services/scroll.service.ts

@@ -0,0 +1,25 @@
+import {Injectable} from '@angular/core';
+import {BehaviorSubject} from 'rxjs';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class ScrollService {
+  private scrollPosition: number;
+  private scrollSlots = {};
+  private _restoreScrollPosition$ = new BehaviorSubject<number>(0);
+  public restoreScrollPosition$ = this._restoreScrollPosition$.asObservable();
+
+  public saveScrollPosition(pos: number) {
+    this.scrollPosition = pos;
+  }
+
+  public storeScrollPositionFor(slot: string) {
+    this.scrollSlots[slot] = this.scrollPosition;
+  }
+
+  public restoreScrollPositionFor(slot: string) {
+    const pos = this.scrollSlots[slot];
+    this._restoreScrollPosition$.next(pos);
+  }
+}