How to get current tab index in Flutter : <p>In flutter implementing a tab layout is easy and straightforward. This is a simple example from the official <a href="https://flutter.io/cookbook/design/tabs/" rel="noreferrer">documentation</a>:</p>

<pre><code>import 'package:flutter/material.dart';

void main() {
  runApp(new TabBarDemo());
}

class TabBarDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new DefaultTabController(
        length: 3,
        child: new Scaffold(
          appBar: new AppBar(
            bottom: new TabBar(
              tabs: [
                new Tab(icon: new Icon(Icons.directions_car)),
                new Tab(icon: new Icon(Icons.directions_transit)),
                new Tab(icon: new Icon(Icons.directions_bike)),
              ],
            ),
            title: new Text('Tabs Demo'),
          ),
          body: new TabBarView(
            children: [
              new Icon(Icons.directions_car),
              new Icon(Icons.directions_transit),
              new Icon(Icons.directions_bike),
            ],
          ),
        ),
      ),
    );
  }
}
</code></pre>

<p>But here is the thing, I want to get the active tab index so I can apply some logic on certain tabs. I search the documentation but I wasn't able to figure it out. Can you guys help and thanks?</p>
