Bottom Navigation Bar Issues

We were able to use Bottom navigation bar, by adding parent view "HomeView" in which we are calling all the child views. Views are working just fine.

Now the issue is about the DataService is been called each time, on swap of views. Below is our code for locator.dart

```
locator.registerLazySingleton(() => DialogService());
locator.registerLazySingleton(() => NavigationService());
locator.registerLazySingleton(() => AuthenticationService());
locator.registerLazySingleton(() => DataService());

locator.registerSingleton(ViewModel1());
locator.registerSingleton(ViewModel2());
locator.registerSingleton(ViewModel3());
```
In the view we have added `initialiseSpecialViewModelsOnce: true` `viewModelBuilder: () => locator()` as well.

When we execute:
```
flutter: Error while creating DataService
flutter: Stack trace:
#0 defaultBinaryMessenger. (package:flutter/src/services/binary_messenger.dart:76:7)
#1 defaultBinaryMessenger (package:flutter/src/services/binary_messenger.dart:89:4)
#2 MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:140:62)
#3 MethodChannel.setMethodCallHandler (package:flutter/src/services/platform_channel.dart:375:5)
#4 new MethodChannelFirestore (package:cloud_firestore_platform_interface/src/method_channel/method_channel_firestore.dart:27:13)
#5 FirestorePlatform.instance (package:cloud_firestore_platform_interface/cloud_firestore_platform_interface.dart:63:19)
#6 new Firestore (package:cloud_firestore/src/firestore.dart:26:42)
#7 Firestore.instance (package:cloud_firestore/src/firestore.dart:29:36)
#8 new DataService (package:Vaccine/app/service/data.dart:14:67)
#9 setupLocator. (package:Vaccine/app/locator.dart:21:39)
#10 _ServiceFactory.getObject (package:get_it/get_it_impl.dart:128:40)
#11 _GetItImplementation.get (package:get_it/get_it_impl.dart:276:34)
#12 _GetItImplementation.call (package:get_it/get_it_impl.dart:288:12)
#13 new AuthenticationService (package:Vaccine/app/service/auth.dart:10:43)
#14 setupLocator. (package:Vaccine/app/locator.dart:19:39)
#15 _ServiceFactory.getObject (package:get_it/get_it_impl.dart:128:40)
#16 _GetItImplementation.get (package:get_it/get_it_impl.dart:276:34)
#17 _GetItImplementation.call (package:get_it/get_it_impl.dart:288:12)
#18 new ScheduleScreenModel (package:Vaccine/app/screenmodels/view_model_1.dart:14:63)
#19 setupLocator (package:Vaccine/app/locator.dart:24:50)
#20 main (package:Vaccine/main.dart:9:3)
#21 _runMainZoned.. (dart:ui/hooks.dart:241:25)
#22 _rootRun (dart:async/zone.dart:1184:13)
#23 _CustomZone.run (dart:async/zone.dart:1077:19)
#24 _runZoned (dart:async/zone.dart:1619:10)
#25 runZonedGuarded (dart:async/zone.dart:1608:12)
#26 _runMainZoned. (dart:ui/hooks.dart:233:5)
#27 _startIsolate. (dart:isolate-patch/isolate_patch.dart:301:19)
#28 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
flutter: Error while creating AuthenticationService
flutter: Stack trace:
#0 defaultBinaryMessenger. (package:flutter/src/services/binary_messenger.dart:76:7)
#1 defaultBinaryMessenger (package:flutter/src/services/binary_messenger.dart:89:4)
#2 MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:140:62)
#3 MethodChannel.setMethodCallHandler (package:flutter/src/services/platform_channel.dart:375:5)
#4 new MethodChannelFirestore (package:cloud_firestore_platform_interface/src/method_channel/method_channel_firestore.dart:27:13)
#5 FirestorePlatform.instance (package:cloud_firestore_platform_interface/cloud_firestore_platform_interface.dart:63:19)
#6 new Firestore (package:cloud_firestore/src/firestore.dart:26:42)
#7 Firestore.instance (package:cloud_firestore/src/firestore.dart:29:36)
#8 new DataService (package:Vaccine/app/service/data.dart:14:67)
#9 setupLocator. (package:Vaccine/app/locator.dart:21:39)
#10 _ServiceFactory.getObject (package:get_it/get_it_impl.dart:128:40)
#11 _GetItImplementation.get (package:get_it/get_it_impl.dart:276:34)
#12 _GetItImplementation.call (package:get_it/get_it_impl.dart:288:12)
#13 new AuthenticationService (package:Vaccine/app/service/auth.dart:10:43)
#14 setupLocator. (package:Vaccine/app/locator.dart:19:39)
#15 _ServiceFactory.getObject (package:get_it/get_it_impl.dart:128:40)
#16 _GetItImplementation.get (package:get_it/get_it_impl.dart:276:34)
#17 _GetItImplementation.call (package:get_it/get_it_impl.dart:288:12)
#18 new ScheduleScreenModel (package:Vaccine/app/screenmodels/view_model_1.dart:14:63)
#19 setupLocator (package:Vaccine/app/locator.dart:24:50)
#20 main (package:Vaccine/main.dart:9:3)
#21 _runMainZoned.. (dart:ui/hooks.dart:241:25)
#22 _rootRun (dart:async/zone.dart:1184:13)
#23 _CustomZone.run (dart:async/zone.dart:1077:19)
#24 _runZoned (dart:async/zone.dart:1619:10)
#25 runZonedGuarded (dart:async/zone.dart:1608:12)
#26 _runMainZoned. (dart:ui/hooks.dart:233:5)
#27 _startIsolate. (dart:isolate-patch/isolate_patch.dart:301:19)
#28 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
```
Code for view_mode_1.dart 14:63 is:
```
final AuthenticationService _authenticationService = locator();
final DataService _dataService = locator();

```

Kindly suggest how to rectify this?

_Originally posted by @akyjoe in https://github.com/FilledStacks/flutter-tutorials/issues/86#issuecomment-669177084_

Other issue is, how can we switch BottomNavigationBar tabs programmatically or by tapping any button in current view?

まず、main.dart と app/locator.dart から app/service/data.dart、app/service/auth.dart、app/screenmodels/view_model_1.dart を経由したサービス作成の流れを追跡します。HomeView の BottomNavigationBar フローを再現し、DataService がいつインスタンス化されるかを確認します。報告された作成エラーが解決され、issue で要求されたプログラムによるタブ切り替えの動作が文書化または実証されれば完了です。

索引モデルが issue の本文から書いたものです。