Flutter Dart List.map() 获取下标

class HomePageState extends State{
  final topTitles = ['审批单', '机票列表', '客服'];
  final topIcons = ['assets/home/approval.png', 'assets/home/air_ticket.png', 'assets/home/service.png'];
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return new Scaffold(
      appBar: AppBar(
        title: Text('首页')
      ),
      body: Column(
        children: <Widget>[
          //顶部
          Row(
            children: topTitles.asMap().keys.map((f)=>
　　　　　　　　　　//f拿到的就是下标：
                Expanded(
                  flex: ,
                  child: Column(
                    children: <Widget>[
                      Image(
                        image: new AssetImage(topIcons[f]),
                        height: , width: ),
                      Text(topTitles[f])
                    ],
                  ),
                )).toList(),
          ),
          //列表

        ],
      ),
    );
  }

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    print("initStateHome");
  }

}