TwoLevelHeader class

this header help you implements twoLevel function easyily, the behaviour just like TaoBao,XieCheng(携程) App TwoLevel

just a example

TwoLevelHeader(
 textStyle: TextStyle(color: Colors.white),
 displayAlignment: TwoLevelDisplayAlignment.fromTop,
 decoration: BoxDecoration(
 image: DecorationImage(
 image: AssetImage("images/secondfloor.jpg"),
 fit: BoxFit.cover,
 // 很重要的属性,这会影响你打开二楼和关闭二楼的动画效果
 alignment: Alignment.topCenter),
),
twoLevelWidget: Container(
  decoration: BoxDecoration(
  image: DecorationImage(
  image: AssetImage("images/secondfloor.jpg"),
  alignment: Alignment.topCenter,
  fit: BoxFit.cover),
  ),
  Container(
    height: 60.0,
    child: GestureDetector(
    child: Icon(
      Icons.arrow_back_ios,
    color: Colors.white,
   ),
  onTap: () {
    SmartRefresher.of(context).controller.twoLevelComplete();
  },
  ),
  alignment: Alignment.bottomLeft,
),
),
);