Flutter How to check if Sliver AppBar is expanded or collapsed?

问题

I am using a SliverAppBar in Flutter, with a background widget.

The thing is When it's expanded, the title and icons (leading and actions) should be white in order to be seen correctly, and when it's collapsed, they should be changed to black.

Any ideas on how I can get a bool out of it? Or other ways of resolving this problem.

Thank you.

class SliverExample extends StatefulWidget {
  final Widget backgroundWidget;
  final Widget bodyWidgets;

  SliverExample({
    this.backgroundWidget,
    this.body,
  });
  @override
  _SliverExampleState createState() => _SliverExampleState();
}

class _SliverExampleState extends State<SliverExample> {

  // I need something like this
  // To determine if SliverAppBar is expanded or not.
  bool isAppBarExpanded = false;

  @override
  Widget build(BuildContext context) {

    // To change the item's color accordingly
    // To be used in multiple places in code
    Color itemColor = isAppBarExpanded ? Colors.white : Colors.black;

    // In my case PrimaryColor is white,
    // and the background widget is dark

    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            pinned: true,
            leading: BackButton(
              color: itemColor, // Here
            ),
            actions: <Widget>[
              IconButton(
                icon: Icon(
                  Icons.shopping_cart,
                  color: itemColor, // Here
                ),
                onPressed: () {},
              ),
            ],
            expandedHeight: 200.0,
            flexibleSpace: FlexibleSpaceBar(
              centerTitle: true,
              title: Text(
                'title',
                style: TextStyle(
                  fontSize: 18.0,
                  color: itemColor, // Here
                ),
              ),
              // Not affecting the question.              
              background: widget.backgroundWidget,
            ),
          ),
          // Not affecting the question.
          SliverToBoxAdapter(child: widget.body),
        ],
      ),
    );
  }
}

回答1:

You can use LayoutBuilder to get sliver AppBar biggest height. When biggest.height = 80.0, it actually means sliver appbar is collapsed.

Here is a full example:

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());
        },
      ),
    ));
  }
}

Final result:

回答2:

You need to use ScrollController to achieve the desired effect

try this code

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: SliverExample(
        bodyWidgets: Text(
            'Hello Body gggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg'),
        backgroundWidget: Text('Hello Background'),
      ),
    );
  }
}

class SliverExample extends StatefulWidget {
  final Widget backgroundWidget;
  final Widget bodyWidgets;

  SliverExample({
    this.backgroundWidget,
    this.bodyWidgets,
  });

  @override
  _SliverExampleState createState() => _SliverExampleState();
}

class _SliverExampleState extends State<SliverExample> {

  ScrollController _scrollController;
  Color _theme ;

  @override
  void initState() {
    super.initState();
    _theme = Colors.black;

    _scrollController = ScrollController()
      ..addListener(
        () => _isAppBarExpanded ?
            _theme != Colors.white ?
        setState(
          () {
            _theme = Colors.white;
            print(
                'setState is called');
          },
        ):{}
            : _theme != Colors.black ?
        setState((){
          print(
              'setState is called');
          _theme = Colors.black;
        }):{},

      );
  }

  bool get _isAppBarExpanded {
    return _scrollController.hasClients &&
        _scrollController.offset > (200 - kToolbarHeight);
  }

  @override
  Widget build(BuildContext context) {
    // To change the item's color accordingly
    // To be used in multiple places in code
    //Color itemColor = isAppBarExpanded ? Colors.white : Colors.black;

    // In my case PrimaryColor is white,
    // and the background widget is dark

    return Scaffold(
      body: CustomScrollView(
        controller: _scrollController,
        slivers: <Widget>[
          SliverAppBar(
            pinned: true,
            leading: BackButton(
              color: _theme, // Here
            ),
            actions: <Widget>[
              IconButton(
                icon: Icon(
                  Icons.shopping_cart,
                  color: _theme, // Here
                ),
                onPressed: () {},
              ),
            ],
            expandedHeight: 200.0,
            flexibleSpace: FlexibleSpaceBar(
              centerTitle: true,
              title: Text(
                'title',
                style: TextStyle(
                  fontSize: 18.0,
                  color: _theme, // Here
                ),
              ),
              // Not affecting the question.
              background: widget.backgroundWidget,
            ),
          ),
          // Not affecting the question.
          SliverToBoxAdapter(child: widget.bodyWidgets),
        ],
      ),
    );
  }
}

if you are not familiar with ? : notation you can use the following

_scrollController = ScrollController()
      ..addListener(
          (){
            if(_isAppBarExpanded){
              if(_theme != Colors.white){
                setState(() {
                  _theme = Colors.white;
                  print('setState is called with white');
                });
              }
            }else{
              if(_theme != Colors.black){
                setState(() {
                  _theme = Colors.black;
                  print('setState is called with black');
                });
              }
            }
          }

来源：https://stackoverflow.com/questions/53372276/flutter-how-to-check-if-sliver-appbar-is-expanded-or-collapsed