indexIsChanging property Null safety
True while we're animating from previousIndex to index as a consequence of calling animateTo.
This value is true during the animateTo animation that's triggered when the user taps a TabBar tab. It is false when offset is changing as a consequence of the user dragging (and "flinging") the TabBarView.
Implementation
bool get indexIsChanging => _indexIsChangingCount != 0;