Added support for tab navigation (bottom widgets) for the custom app bar

lib/src/screens/event_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import '../utils.dart';
 import '../blocs/event_bloc.dart';
 import '../widgets/custom_app_bar.dart';
 
@@ -14,18 +15,31 @@ class EventScreen extends StatefulWidget {
   _EventScreenState createState() => _EventScreenState();
 }
 
-class _EventScreenState extends State<EventScreen> {
+class _EventScreenState extends State<EventScreen>
+    with SingleTickerProviderStateMixin {
   EventBloc bloc;
-
+  TabController _tabController;
   initState() {
     super.initState();
     bloc = EventBloc(eventName: widget.eventName);
+    _tabController = TabController(vsync: this, length: 2);
   }
 
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: CustomAppBar(
         title: widget.eventName,
+        bottom: TabBar(
+          controller: _tabController,
+          tabs: <Tab>[
+            Tab(
+              text: 'tab1',
+            ),
+            Tab(
+              text: 'tab2',
+            ),
+          ],
+        ),
       ),
     );
   }

lib/src/widgets/custom_app_bar.dart

@@ -1,8 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:font_awesome_flutter/font_awesome_flutter.dart';
 
-//TODO: add support for tabed navigation.
-
 /// A custom app bar to match the DO> design rules.
 ///
 /// This app bar is meant to be usen in screens that are not the home screen.
@@ -12,16 +10,17 @@ class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {
   final String title;
 
   /// Widget to be shown on the bottom of the app bar.
-  final PreferredSize bottom;
+  final PreferredSizeWidget bottom;
-
-  /// the preferred size for this widget.
-  final Size preferredSize;
 
+  /// The size of only the app bar part.
+  final double appBarHeight;
   CustomAppBar({
     this.title = '',
     this.bottom,
-  }) : preferredSize =
+  }) : appBarHeight = bottom == null ? 140.0 : 120.0;
-            Size.fromHeight(140.0 + (bottom?.preferredSize?.height ?? 0));
+
+  Size get preferredSize =>
+      Size.fromHeight(appBarHeight + (bottom?.preferredSize?.height ?? 0));
 
   Widget build(BuildContext context) {
     Widget result = Container(
@@ -56,8 +55,14 @@ class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {
 
     if (bottom != null) {
       result = Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
         children: <Widget>[
-          result,
+          Flexible(
+            child: ConstrainedBox(
+              constraints: const BoxConstraints(maxHeight: 140.0),
+              child: result,
+            ),
+          ),
           bottom,
         ],
       );