feat(challenge 7): begin ngrx notification work

apps/ngrx-notification/src/app/backend/push.service.ts

@@ -0,0 +1,27 @@
+import { Injectable } from '@angular/core';
+import { BehaviorSubject, tap, timer } from 'rxjs';
+import { randStudent } from '../model/student.model';
+import { randTeacher } from '../model/teacher.model';
+
+@Injectable({ providedIn: 'root' })
+export class PushService {
+  private notificationSubject = new BehaviorSubject<any>(undefined);
+  notification$ = this.notificationSubject.asObservable();
+
+  init() {
+    this.startTeacherNotification();
+    this.startStudentNotification();
+  }
+
+  private startTeacherNotification() {
+    timer(0, 4000)
+      .pipe(tap(() => this.notificationSubject.next(randTeacher())))
+      .subscribe();
+  }
+
+  private startStudentNotification() {
+    timer(1000, 3000)
+      .pipe(tap(() => this.notificationSubject.next(randStudent())))
+      .subscribe();
+  }
+}