您可以使用LayoutBuilder获得Sliver AppBar的最大高度。当maximum.height = 80.0时，实际上意味着条子appbar被折叠。
这是一个完整的示例：
import 'package:flutter/material.dart';void main() => runApp(MaterialApp( home: MyApp(), ));class MyApp extends StatefulWidget { @override _MyAppState createState() => _MyAppState();}class _MyAppState extends State<MyApp> { var top = 0.0; @override Widget build(BuildContext context) { return Scaffold( body: NestedScrollView( headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) { return <Widget>[ SliverAppBar( expandedHeight: 200.0, floating: false, pinned: true, flexibleSpace: LayoutBuilder( builder: (BuildContext context, BoxConstraints constraints) { // print('constraints=' + constraints.toString()); top = constraints.biggest.height; return FlexibleSpaceBar( centerTitle: true, title: AnimatedOpacity( duration: Duration(milliseconds: 300), //opacity: top == 80.0 ? 1.0 : 0.0, opacity: 1.0, child: Text( top.toString(), style: TextStyle(fontSize: 12.0), )), background: Image.network("https://images.unsplash.com/photo-1542601098-3adb3baeb1ec?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=5bb9a9747954cdd6eabe54e3688a407e&auto=format&fit=crop&w=500&q=60",fit: BoxFit.cover, )); })), ]; },body: ListView.builder( itemCount: 100, itemBuilder: (context,index){ return Text("List Item: " + index.toString()); }, ), )); }}
