Flutter Called setState and rebuilt, but the UI has not changed?

main.dart

//This is probably my own problem, but I don't know what caused it , how to solve it.

//example:
//Clicking on the blue circle button, clicking on it will add a tabitem, but now clicking it will not respond.

import 'dart:math';
import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
    home: Scaffold(
      body:NestedScrollExample()
      )
  ));
}

class NestedScrollExample extends StatefulWidget {
  @override
  _NestedScrollExampleState createState() => _NestedScrollExampleState();

  const NestedScrollExample();
}

class _NestedScrollExampleState extends State<NestedScrollExample>
    with TickerProviderStateMixin {
  String string = 'tabItem';
  List<Widget> viewList = [
    Page1('page1'),
  ];

  @override
  Widget build(BuildContext context) {

    XCTabBar tabBar = createTabBar(
        viewList: viewList,
        provider: this,
        tabStrings: string,
    );

    return NestedScrollView(
      body: TabBarView(
      children: viewList,
      controller: tabBar.tabController,
    ),
      headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
        return <Widget>[
          SliverToBoxAdapter(
            child: Container(
              color: Colors.green,
              padding: const EdgeInsets.all(20.0),
              child: FloatingActionButton(onPressed: addTab),
            ),
          ),
          SliverPersistentHeader(
            floating: true,
            pinned: true,
            delegate: _SliverAppBarDelegate(
                maxHeight: 44.0,
                minHeight: 44.0,
                child: Container(
                  color: Colors.white,
                  child: tabBar,
                )
            ),
          ),
        ];
      },
    );
  }

  void addTab() {
    setState(() {
      string += ',tabItem_add';
      viewList.add(Page1(string));
    });
  }

}

class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverAppBarDelegate({
    @required this.minHeight,
    @required this.maxHeight,
    @required this.child,
  });

  final double minHeight;
  final double maxHeight;
  final Widget child;

  @override
  double get minExtent => minHeight;

  @override
  double get maxExtent => max((minHeight ?? kToolbarHeight), minExtent);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return child;
  }

  @override
  bool shouldRebuild(_SliverAppBarDelegate oldDelegate) {
//     return maxHeight != oldDelegate.maxHeight ||
//         minHeight != oldDelegate.minHeight ||
//         child != oldDelegate.child;
      print('shouldRebuild');
      return true;
  }
}

class Page1 extends StatelessWidget {
  final String content;
  Page1(this.content);
  
  @override
  Widget build(BuildContext context) {
    print('build Page1');

    return Center(
      child: Text(this.content),
    );
  }
}


XCTabBar createTabBar({@required List<Widget> viewList, @required TickerProvider provider, List<Widget> tabList, String tabStrings}) {
  if(tabList == null && tabStrings != null) {
    tabList = tabStrings.split(',').map((item) {
      return Text(
        '$item',
        style: TextStyle(fontSize: 15),
      );
    }).toList();
  }
  return XCTabBar(viewList, tabList, TabController(vsync: provider, length: tabList.length));
}

class XCTabBar extends StatefulWidget {
  final TabController tabController;
  final List<Widget> viewList;
  final List<Widget> tabList;

  XCTabBar(this.viewList, this.tabList, this.tabController);

  @override
  State<StatefulWidget> createState() {
    return XCTabBarState();
  }

  TabBarView getTabView() {
    return TabBarView(
      children: viewList,
      controller: tabController,
    );
  }
}

class XCTabBarState extends State<XCTabBar> with SingleTickerProviderStateMixin {
  Color selectColor, unselectedColor;
  TextStyle selectStyle, unselectedStyle;

  @override
  void initState() {
    super.initState();

    selectColor = Colors.black;
    unselectedColor = Color.fromARGB(255, 117, 117, 117);
    selectStyle = TextStyle(fontSize: 18, color: selectColor, fontWeight: FontWeight.bold);
    unselectedStyle = TextStyle(fontSize: 18, color: selectColor);
  }

  @override
  void dispose() {
    widget.tabController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.only(top: 0.0, bottom: 0.0),
      child: TabBar(
        tabs: widget.tabList,
        isScrollable: true,
        controller: widget.tabController,
        indicatorColor: selectColor,
        labelColor: selectColor,
        labelStyle: selectStyle,
        unselectedLabelColor: unselectedColor,
        unselectedLabelStyle: unselectedStyle,
        indicatorSize: TabBarIndicatorSize.label,
      ),
    );
  }
}