refactor: move factory constructor to the top

lib/src/services/connection_status.dart

@@ -14,6 +14,10 @@ class ConnectionStatus {
   /// Stream of current connection status. Only updates when there is a change in connection.
   Stream<bool> get connectionChange => _connectionChangeController.stream;
 
+  factory ConnectionStatus() {
+    return _singleton;
+  }
+
   /// Private internal constructor for the Singleton
   ConnectionStatus._internal() {
     // Only call the init function when there is a subscriber to the stream
@@ -60,8 +64,4 @@ class ConnectionStatus {
   void dispose() {
     _connectionChangeController.close();
   }
-
-  factory ConnectionStatus() {
-    return _singleton;
-  }
 }

lib/src/ui/contact_tab/contact_tab.dart

@@ -3,6 +3,10 @@ import "package:flutter/material.dart";
 import "./widgets/home_view.dart";
 
 class ContactTab extends StatefulWidget {
+  final GlobalKey<NavigatorState> navigatorKey;
+
+  ContactTab({@required this.navigatorKey});
+
   @override
   State<StatefulWidget> createState() {
     return _ContactTabState();
@@ -21,6 +25,7 @@ class _ContactTabState extends State<ContactTab> {
   @override
   Widget build(BuildContext context) {
     return Navigator(
+      key: widget.navigatorKey,
       initialRoute: "contact/home",
       onGenerateRoute: (RouteSettings settings) {
         WidgetBuilder builder;

lib/src/ui/conversation_tab/conversation_tab.dart

@@ -8,6 +8,10 @@ import "./widgets/new_group_info_view.dart";
 import "../../models/user_model.dart";
 
 class ConversationTab extends StatefulWidget {
+  final GlobalKey<NavigatorState> navigatorKey;
+
+  ConversationTab({@required this.navigatorKey});
+
   @override
   State<StatefulWidget> createState() {
     return _ConversationTabState();
@@ -18,6 +22,7 @@ class _ConversationTabState extends State<ConversationTab> {
   @override
   Widget build(BuildContext context) {
     return Navigator(
+      key: widget.navigatorKey,
       initialRoute: "conversation/home",
       onGenerateRoute: (RouteSettings settings) {
         WidgetBuilder builder;

lib/src/ui/home.dart

@@ -26,13 +26,18 @@ class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
   final loginManager = LoginManager();
 
   // Bottom Bar navigation
-  int _tabNumber = 1;
+  int _tabIndex = 1;
   List<IconData> itemsList = [
     Icons.contacts,
     Icons.chat,
     Icons.settings,
   ];
-  TabController controller;
+
+  List<GlobalKey<NavigatorState>> navigatorKeys = [
+    GlobalKey<NavigatorState>(),
+    GlobalKey<NavigatorState>(),
+    GlobalKey<NavigatorState>()
+  ];
 
   // Current conversaton
   String currentConversationId = "";
@@ -46,8 +51,6 @@ class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
   @override
   initState() {
     super.initState();
-    controller = TabController(vsync: this, length: 3);
-    controller.index = 1; // Set default page to conversation page
 
     // initialize conversation manager
     loginManager.getToken().then((authToken) async {
@@ -78,7 +81,6 @@ class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
 
   @override
   dispose() {
-    controller.dispose();
     super.dispose();
   }
 
@@ -108,59 +110,65 @@ class _HomeState extends State<Home> with SingleTickerProviderStateMixin {
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      key: _scaffoldKey,
-      body: TabBarView(
-          physics: NeverScrollableScrollPhysics(),
-          controller: controller,
-          children: <Widget>[
-            ContactTab(),
-            ConversationTab(),
-            SettingsTab(toWelcomePage: () {
-              Navigator.pushNamed(context, '/welcome');
-            }),
-          ]),
-      bottomNavigationBar:
-          Column(mainAxisSize: MainAxisSize.min, children: <Widget>[
-        BottomBar(conversationId: currentConversationId),
-        BottomNavigationBar(
-            onTap: (int index) {
-              setState(() {
-                _tabNumber = index;
-                controller.index = _tabNumber;
-              });
-            },
-            items: itemsList.map((data) {
-              return BottomNavigationBarItem(
-                icon: itemsList[_tabNumber] == data
-                    ? ShaderMask(
-                        blendMode: BlendMode.srcIn,
-                        shaderCallback: (Rect bounds) {
-                          return ui.Gradient.linear(
-                            Offset(4.0, 24.0),
-                            Offset(24.0, 4.0),
-                            [
-                              Theme.of(context).primaryColor,
-                              Theme.of(context).primaryColorDark,
-                            ],
-                          );
-                        },
-                        child: Icon(data, size: 25.0),
-                      )
-                    : Icon(data, color: Colors.grey, size: 20),
-                title: Container(),
-              );
-            }).toList()),
-        Visibility(
-          visible: _connectionStatusVisible,
-          child: Container(
-              padding: EdgeInsets.only(top: 5.0, bottom: 5.0),
-              width: MediaQuery.of(context).size.width,
-              alignment: Alignment(0.0, 0.0),
-              color: _isOnline ? Colors.green : Colors.red,
-              child: Text(_isOnline ? 'Online' : 'Offline')),
+    List<Widget> tabsList = [
+      ContactTab(navigatorKey: navigatorKeys[0]),
+      ConversationTab(navigatorKey: navigatorKeys[1]),
+      SettingsTab(
+          navigatorKey: navigatorKeys[2],
+          toWelcomePage: () {
+            Navigator.of(context).pushNamedAndRemoveUntil('/welcome', (Route<dynamic> route) => false);
+          }),
+    ];
+
+    return WillPopScope(
+      onWillPop: () async => !await navigatorKeys[_tabIndex].currentState.maybePop(),
+      child: Scaffold(
+        key: _scaffoldKey,
+        body: IndexedStack(
+          index: _tabIndex,
+          children: tabsList,
         ),
-      ]),
+        bottomNavigationBar:
+            Column(mainAxisSize: MainAxisSize.min, children: <Widget>[
+          BottomBar(conversationId: currentConversationId),
+          BottomNavigationBar(
+              onTap: (int index) {
+                setState(() {
+                  _tabIndex = index;
+                });
+              },
+              items: itemsList.map((data) {
+                return BottomNavigationBarItem(
+                  icon: itemsList[_tabIndex] == data
+                      ? ShaderMask(
+                          blendMode: BlendMode.srcIn,
+                          shaderCallback: (Rect bounds) {
+                            return ui.Gradient.linear(
+                              Offset(4.0, 24.0),
+                              Offset(24.0, 4.0),
+                              [
+                                Theme.of(context).primaryColor,
+                                Theme.of(context).primaryColorDark,
+                              ],
+                            );
+                          },
+                          child: Icon(data, size: 25.0),
+                        )
+                      : Icon(data, color: Colors.grey, size: 20),
+                  title: Container(),
+                );
+              }).toList()),
+          Visibility(
+            visible: _connectionStatusVisible,
+            child: Container(
+                padding: EdgeInsets.only(top: 5.0, bottom: 5.0),
+                width: MediaQuery.of(context).size.width,
+                alignment: Alignment(0.0, 0.0),
+                color: _isOnline ? Colors.green : Colors.red,
+                child: Text(_isOnline ? 'Online' : 'Offline')),
+          ),
+        ]),
+      ),
     );
   }
 }

lib/src/ui/settings_tab/settings_tab.dart

@@ -8,8 +8,9 @@ import "./widgets/home_view.dart";
 
 class SettingsTab extends StatefulWidget {
   final VoidCallback toWelcomePage;
+  final GlobalKey<NavigatorState> navigatorKey;
 
-  SettingsTab({@required this.toWelcomePage});
+  SettingsTab({@required this.toWelcomePage, @required this.navigatorKey});
 
   @override
   State<StatefulWidget> createState() {
@@ -21,6 +22,7 @@ class _SettingsTabState extends State<SettingsTab> {
   @override
   Widget build(BuildContext context) {
     return Navigator(
+      key: widget.navigatorKey,
       initialRoute: "settings/home",
       onGenerateRoute: (RouteSettings settings) {
         WidgetBuilder builder;