先看效果：

Untitled.gif

实现方法

目前网上的方法实现这个效果性能比较差,自己写了下.
总体思路:

• 借助Provider进行状态管理,避免其他widget重复build.
• ScrollController的监听滚动.

Code

1. 创建个viewModel用于Provider监听

class MyDetailVM extends ChangeNotifier {
  double opacityValue = 0;
  final _maxOffset = 100;
  MyDetailVM(ScrollController scroller) {
    print("MyDetailVM init"+scroller.toString());
    // 监听滚动
    scroller.addListener(() {
      double alpha = scroller.offset / _maxOffset;
      print("scrollerListener" + scroller.offset.toString());
      if (alpha < 0) {
        alpha = 0;
      } else if (alpha > 1) {
        alpha = 1;
      }
      if ((alpha < 0 && opacityValue == 0) ||
          (alpha > 1 && opacityValue == 1)) {
        return;
      } else {
        opacityValue = alpha;
        notifyListeners();
      }
    });
  }
}

2. 创建一个Widget

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    ScrollController _controller = ScrollController();
    return Scaffold(
//body在AppBar后面
        extendBodyBehindAppBar: true,
        body:  ListView(
padding: EdgeInsets.all(0),
controller: _controller, 
children: [
          Container(
            height: 200,
            color: Colors.red,
          ),
          Container(
            height: 400,
            color: Colors.yellow,
          ),
          Container(
            height: 400,
            color: Colors.red,
          ),
        ]),
//appBar必须是PreferredSize的类型
        appBar: PreferredSize(
          preferredSize: Size.fromHeight(kToolbarHeight),
          child: ChangeNotifierProvider(
            create: (ctx) => MyDetailVM(_controller),
            child: Consumer<MyDetailVM>(
              builder: (context, vm, child) {
                if (vm.opacityValue == 0) {
                  //为0的时候就不显示
                  return Container();
                }
                return Opacity(
                  opacity: vm.opacityValue,
                  child: child,
                );
              },
              child: AppBar(
                backgroundColor: Colors.white,
                title: Text(
                  "I'm AppBar",
                  style: TextStyle(color: Colors.black),
                ),
              ),
            ),
          ),
        ));
  }
}