1. override
void didUpdateConfig(BottomNavigationBar oldConfig)

Called whenever the configuration changes.

If the parent widget rebuilds and request that this location in the tree update to display a new widget with the same runtimeType and key, the framework will update the config property of this State object to refer to the new widget and then call the this method with the previous widget as an argument.

Override this method to respond to changes in the config widget (e.g., to start implicit animations).

The framework always calls build after calling didUpdateConfig, which means any calls to setState in didUpdateConfig are redundant.

If a State's build method depends on an object that can itself change state, for example a ChangeNotifier or Stream, or some other object to which one can subscribe to receive notifications, then the State should subscribe to that object during initState, unsubscribe from the old object and subscribe to the new object when it changes in didUpdateConfig, and then unsubscribe from the object in dispose.

If you override this, make sure your method starts with a call to super.didUpdateConfig(oldConfig).

Source 

@override
void didUpdateConfig(BottomNavigationBar oldConfig) {
  if (config.currentIndex != oldConfig.currentIndex) {
    if (config.type == BottomNavigationBarType.shifting)
      _pushCircle(config.currentIndex);
    _controllers[oldConfig.currentIndex].reverse();
    _controllers[config.currentIndex].forward();
  }
}