some_big_widget.dart

class SomeBigWidgetState extends State<SomeBigWidget> {
  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        _buildSomeWidget1(),
        _buildSomeWidget2(),
        _buildSomeWidget3(),
        _buildLikeButton(),
      ],
    );
  }
  
  Widget _buildLikeButton() {
    return Positioned(
      right: 10,
      bottom: 10,
      child: IconButton(
        icon: Icon(Icons.favorite),
        onPressed: () {
          // setStateが呼ばれると
          // SomeBigWidgetState全体がリビルドされる！
          setState(() {
            ...; 
          };
        },
      ),
    );
  }
}