_setCurrentIndex & _triggerAnimation

main.dart

_setCurrentIndex(int index) {
  // if we're actually changing the index
  if (index != _currentIndex) {
    setState(() {
      // change the index
      _currentIndex = index;
    });

    // trigger the button animation
    _triggerAnimation();
    // scroll the TabBar to the correct position (if we have a scrollable bar)
    _scrollTo(index);
  }
}

_triggerAnimation() {
  // reset the animations so they're ready to go
  _animationControllerOn.reset();
  _animationControllerOff.reset();

  // run the animations!
  _animationControllerOn.forward();
  _animationControllerOff.forward();
}