AnimatedCrossFade 状态没有改变

Question

我无法获得 AnimatedCrossFade 的基本实现。我能够成功更改状态，但是当状态更改时动画不会触发。这是我所拥有的：

class Home extends StatefulWidget {
  @override
  HomeState createState() => HomeState();
}

class HomeState extends State<Home> {
  bool showPost;

  @override
  void initState() {
    showPost = true;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    return CupertinoTabView(builder: (context) {
      return getPage();
    });
  }

  Widget getPage() {
    return Center(
        child: Row(children: [
      CupertinoButton(
        child: Text("press"),
        onPressed: () {
          setState(() {
            showPost = !showPost;
          });
          log(showPost.toString());
        },
      ),
      AnimatedCrossFade(
          duration: const Duration(seconds: 3),
          firstChild: Center(child: Text("First Option")),
          secondChild: Center(
              child: Text("Second Option")),
          crossFadeState:
              showPost ? CrossFadeState.showFirst : CrossFadeState.showSecond)
    ]));
  }
}