save and restore scroll position of song list

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);
+  }
+}