optimize read calls

src/app/modules/shows/services/show-data.service.ts

@@ -1,19 +1,24 @@
 import {Injectable} from '@angular/core';
-import {BehaviorSubject, Observable} from 'rxjs';
+import {Observable} from 'rxjs';
 import {DbService} from '../../../services/db.service';
 import {Show} from './show';
-import {map} from 'rxjs/operators';
+import {map, shareReplay} from 'rxjs/operators';
 
 @Injectable({
   providedIn: 'root',
 })
 export class ShowDataService {
-  public list$ = new BehaviorSubject<Show[]>([]);
   private collection = 'shows';
+  public list$: Observable<Show[]> = this.dbService.col$<Show>(this.collection).pipe(
+    // server-side ordering cuts client work and keeps stable order across subscribers
+    map(shows => [...shows].sort((a, b) => a.date.toMillis() - b.date.toMillis())),
+    shareReplay({
+      bufferSize: 1,
+      refCount: true,
+    })
+  );
 
-  public constructor(private dbService: DbService) {
-    this.dbService.col$<Show>(this.collection).subscribe(_ => this.list$.next(_));
-  }
+  public constructor(private dbService: DbService) {}
 
   public listRaw$ = () => this.dbService.col$<Show>(this.collection);