PreferredSize自定义AppBar高度

Widget build(BuildContext context) {
    return Container(
      child: DefaultTabController(
        length: _tabTitles.length,
        child: Scaffold(
          appBar: AppBar(
            elevation: 0,
            title: Text(
              '新闻中心',
              style: TextStyle(color: Color(0xffffffff)),
            ),
            centerTitle: true,
            bottom: PreferredSize(
                child: Material(
                  color: Colors.blue,
                  child: TabBar(
                    indicatorColor: Colors.red,
                    labelColor: Colors.red,
                    unselectedLabelColor: Colors.yellow,
                    tabs: _tabTitles
                        .map((title) => Tab(
                              text: title,
                            ))
                        .toList(),
                  ),
                ),
                preferredSize: Size.fromHeight(48)),
          ),
          body: TabBarView(children: [
            Center(
              child: Text('选址技巧'),
            ),
            Center(
              child: Text('经营攻略'),
            ),
            Center(
              child: Text('公司资讯'),
            ),
            Center(
              child: Text('经典案例'),
            ),
          ]),
        ),
      ),
    );
  }