28

我在 Flutter 中使用SliverAppBar，带有背景小部件。

问题是当它展开时，标题和图标（引导和动作）应该是白色的以便正确看到，当它被折叠时，它们应该改为黑色。

关于如何从中获得布尔值的任何想法？或解决此问题的其他方法。

谢谢你。

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),
        ],
      ),
    );
  }
}
4

4 回答 4

80

您可以使用 LayoutBuilder 来获得 sliver AppBar 的最大高度。当maximum.height = MediaQuery.of(context).padding.top + kToolbarHeight 时，其实就是sliver appbar 折叠了。

这是一个完整的例子，在这个例子中 MediaQuery.of(context).padding.top + kToolbarHeight = 80.0：

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 == MediaQuery.of(context).padding.top + kToolbarHeight ? 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());
        },
      ),
    ));
  }
}

最后结果：

在此处输入图像描述

于 2018-11-19T18:28:43.447 回答
15

需要使用 ScrollController 来达到想要的效果

试试这个代码

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),
        ],
      ),
    );
  }
}

如果你不熟悉？: 符号你可以使用以下

_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');
                });
              }
            }
          }
于 2018-11-19T13:50:44.337 回答
4

您可以使用SliverLayoutBuilder获取当前SliverConstraints值并读取其值，以轻松检测发生了多少滚动。LayoutBuilder这与在 sliver-world 中运行非常相似。

如果constraints.scrollOffset > 0，则表示用户至少滚动了一点点。使用这种方法，如果你想在滚动时做一些动画/过渡，也很容易——只需获取当前scrollOffset并基于它生成你的动画帧。

例如，SliverAppBar当用户滚动时，这会改变颜色：

  SliverLayoutBuilder(
    builder: (BuildContext context, constraints) {
      final scrolled = constraints.scrollOffset > 0;
      return SliverAppBar(
        title: Text('Sliver App Bar'),
        backgroundColor: scrolled ? Colors.blue : Colors.red,
        pinned: true,
      );
    },
  )
于 2021-10-19T16:17:54.973 回答
2

使用具有 innerBoxIsScrolled 布尔标志的 NestedScrollView，这将是解决您的问题的一个不错的解决方案，如下所示

NestedScrollView(
        headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
          print("INNEER SCROLLED VI=======>$innerBoxIsScrolled");
          return <Widget>[
            SliverAppBar(), 
            ]},
body:Center(child:Text("Test IT")),

);
于 2021-09-25T12:53:26.603 回答