下拉刷新时，老师的能整体向下拖动，为什么我的只会出现刷新那个图标呢？是正常现象吗？

body: _currentIndex == 0
            ? RefreshIndicator(
                child: ListView(//刷新组件,需要配合列表使用
                  children: <Widget>[
                    Container(
                      decoration: BoxDecoration(
                        gradient: const LinearGradient(colors: [
                          Colors.purple,
                          Colors.lightGreen,
                          Colors.amber
                        ]),
                      ),
                      alignment: Alignment.center,
                      child: Column(
                        children: <Widget>[
//              Text('I am text1',style: TextStyle(color: Colors.amber,fontSize: 50.0),),
//              Text('I am text2',style: TextStyle(color: Colors.red,fontSize: 40.0),),
                          Text(
                            'I am text3',
                            style: TextStyle(color: Colors.greenAccent),
                          ),
                          Icon(
                            Icons.access_time,
                            size: 70,
                            color: Colors.greenAccent,
                          ),
                          CloseButton(),
                          BackButton(),
                          Chip(
                            //小部件
                            avatar: Icon(Icons.person),
                            label: Text('thie is a pperson'),
                          ),
                          Divider(
                            //分割线
                            height: 1, //容器的高度，不是线的高度
                            indent: 10, // 左侧间距
                            color: Colors.white,
                          ),
                          Card(
                            // 带有圆角、阴影、边框等效果的卡片
                            color: Colors.grey,
                            elevation: 5, //卡片阴影
                            margin: const EdgeInsets.all(10.0),
                            child: Container(
                              padding: const EdgeInsets.all(10),
                              child: Text(
                                '这是卡片Card',
                                style: TextStyle(fontSize: 5),
                              ),
                            ),
                          ),
                          AlertDialog(
                            //弹框
                            backgroundColor: Colors.greenAccent,
                            title: Text('盘他'),
                            content: Text('你个糟老头坏得很'),
                          )
                        ],
                      ),
                    ),
                  ],
                ),
                onRefresh: _handleRefresh)
            : Text('列表'),

_handleRefresh：

Future<Null> _handleRefresh() async {
    await Future.delayed(Duration(microseconds: 200));
    return null;
  }