fix errors

lib/pages/discount/model/discount_model.dart

@@ -2,7 +2,9 @@ import 'dart:async';
 
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:fcs/data/services/services.dart';
+import 'package:fcs/domain/constants.dart';
 import 'package:fcs/domain/entities/discount.dart';
+import 'package:fcs/helpers/paginator.dart';
 import 'package:fcs/pages/main/model/base_model.dart';
 import 'package:logging/logging.dart';
 
@@ -11,7 +13,28 @@ class DiscountModel extends BaseModel {
 
   StreamSubscription<QuerySnapshot> listener;
 
-  List<Discount> discounts = [];
+  List<Discount> _discounts = [];
+  List<Discount> get discounts =>
+      _selectedIndex == 1 ? _discounts : List<Discount>.from(_used.values);
+
+  Paginator _used;
+  bool isLoading = false;
+  int _selectedIndex = 1;
+  set selectedIndex(int index) {
+    _selectedIndex = index;
+    notifyListeners();
+  }
+
+  get selectedIndex => _selectedIndex;
+
+  initData() {
+    _selectedIndex = 1;
+    _load();
+
+    if (_used != null) _used.close();
+    _used = _getUsed();
+    _used.load();
+  }
 
   void initUser(user) {
     super.initUser(user);
@@ -22,7 +45,7 @@ class DiscountModel extends BaseModel {
     if (listener != null) listener.cancel();
     try {
       listener = Firestore.instance
-          .collection("/discounts")
+          .collection("/$discounts_collection")
           .orderBy("code", descending: false)
           .snapshots()
           .listen((snaps) {
@@ -37,9 +60,42 @@ class DiscountModel extends BaseModel {
     }
   }
 
+  Paginator _getUsed() {
+    if (user == null || !user.hasFcsShipments()) return null;
+
+    var pageQuery = Firestore.instance
+        .collection("/$discounts_collection")
+        .where("status", isEqualTo: fcs_shipment_shipped_status)
+        .orderBy("code", descending: false);
+    var paginator = new Paginator(pageQuery, rowPerLoad: 20, toObj: (data, id) {
+      return Discount.fromMap(data, id);
+    });
+    return paginator;
+  }
+
+  Future<void> loadMore() async {
+    if (_used.ended || _selectedIndex == 1) return;
+    isLoading = true;
+    notifyListeners();
+    await _used.load(onFinished: () {
+      isLoading = false;
+      notifyListeners();
+    });
+  }
+
+  Future<void> refresh() async {
+    if (_selectedIndex == 1) return;
+    await _used.refresh(onFinished: () {
+      notifyListeners();
+    });
+  }
+
   @override
   logout() async {
-    discounts = [];
+    if (listener != null) await listener.cancel();
+    if (_used != null) _used.close();
+
+    _discounts = [];
   }
 
   Future<void> addDiscount(Discount discount) async {