Body of the app

main.dart

@override
Widget build(BuildContext context) {
  return Scaffold(
      appBar: AppBar(
        title: Text('Button Tabs (IOS Style)'),
      ),
      backgroundColor: Colors.white,
      body: Column(children: <Widget>[
        // this is the TabBar
        Container(
            height: 49.0,
            // this generates our tabs buttons
            child: ListView.builder(
                // this gives the TabBar a bounce effect when scrolling farther than it's size
                physics: BouncingScrollPhysics(),
                controller: _scrollController,
                // make the list horizontal
                scrollDirection: Axis.horizontal,
                // number of tabs
                itemCount: _icons.length,
                itemBuilder: (BuildContext context, int index) {
                  return Padding(
                      // each button's key
                      key: _keys[index],
                      // padding for the buttons
                      padding: EdgeInsets.all(6.0),
                      child: ButtonTheme(
                          child: AnimatedBuilder(
                        animation: _colorTweenBackgroundOn,
                        builder: (context, child) => FlatButton(
                            // get the color of the button's background (dependent of its state)
                            color: _getBackgroundColor(index),
                            // make the button a rectangle with round corners
                            shape: RoundedRectangleBorder(
                                borderRadius: new BorderRadius.circular(7.0)),
                            onPressed: () {
                              setState(() {
                                _buttonTap = true;
                                // trigger the controller to change between Tab Views
                                _controller.animateTo(index);
                                // set the current index
                                _setCurrentIndex(index);
                                // scroll to the tapped button (needed if we tap the active button and it's not on its position)
                                _scrollTo(index);
                              });
                            },
                            child: Icon(
                              // get the icon
                              _icons[index],
                              // get the color of the icon (dependent of its state)
                              color: _getForegroundColor(index),
                            )),
                      )));
                })),
        Flexible(
            // this will host our Tab Views
            child: TabBarView(
          // and it is controlled by the controller
          controller: _controller,
          children: <Widget>[
            // our Tab Views
            Icon(_icons[0]),
            Icon(_icons[1]),
            Icon(_icons[2]),
            Icon(_icons[3]),
            Icon(_icons[4]),
            Icon(_icons[5])
          ],
        )),
      ]));
}

...

_getBackgroundColor(int index) {
  if (index == _currentIndex) {
    // if it's active button
    return _colorTweenBackgroundOn.value;
  } else if (index == _prevControllerIndex) {
    // if it's the previous active button
    return _colorTweenBackgroundOff.value;
  } else {
    // if the button is inactive
    return _backgroundOff;
  }
}

_getForegroundColor(int index) {
  // the same as the above
  if (index == _currentIndex) {
    return _colorTweenForegroundOn.value;
  } else if (index == _prevControllerIndex) {
    return _colorTweenForegroundOff.value;
  } else {
    return _foregroundOff;
  }
}