add packages

lib/fcs/common/pages/staff/model/staff_model.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:fcs/fcs/common/domain/constants.dart';
 import 'package:fcs/fcs/common/domain/entities/role.dart';
@@ -9,6 +11,7 @@ import 'package:logging/logging.dart';
 
 class StaffModel extends BaseModel {
   final log = Logger('StaffModel');
+  StreamSubscription<QuerySnapshot> listener;
 
   List<User> employees = [];
   List<Privilege> privileges = [];
@@ -21,6 +24,7 @@ class StaffModel extends BaseModel {
 
   @override
   logout() async {
+    if (listener != null) listener.cancel();
     employees = [];
   }
 
@@ -28,7 +32,9 @@ class StaffModel extends BaseModel {
     if (user == null || !user.hasStaffs()) return;
 
     try {
-      Firestore.instance
+      if (listener != null) listener.cancel();
+
+      listener = Firestore.instance
           .collection("/$user_collection")
           .where("is_employee", isEqualTo: true)
           .where("is_sys_admin", isEqualTo: false)
@@ -41,8 +47,6 @@ class StaffModel extends BaseModel {
           return user;
         }).toList();
         notifyListeners();
-      }).onError((e) {
-        log.warning("Error! $e");
       });
     } catch (e) {
       log.warning("Error!! $e");