Flutter：折叠FlexibleSpaceBar 时更改文本

Question

我查看了 Flutter 文档，试图找到一个事件、回调，甚至是在 FlexibleSpaceBar 折叠或展开时可以挂钩的状态。

return new FlexibleSpaceBar(
  title: new Column(
    crossAxisAlignment: CrossAxisAlignment.end,
    mainAxisAlignment: MainAxisAlignment.end,
    children: <Widget>[
        new Text(_name, style: textTheme.headline),
        new Text(_caption, style: textTheme.caption)
    ]),
  centerTitle: false,
  background: getImage());`

当 FlexibleSpaceBar 被对齐（折叠）时，我想隐藏_caption文本并只显示_name文本。当它完全展开时，我显然想同时显示_name和_caption。

我该怎么做？

我不熟悉颤振，所以我对此有些迷失。

也报道在https://github.com/flutter/flutter/issues/18567

Answer 1

可以这样做：

在您的initState方法中添加滚动侦听器，如下所示：

  ScrollController _controller;
  bool silverCollapsed = false;
  String myTitle = "default title";

  @override
  void initState() {
    super.initState();

    _controller = ScrollController();

    _controller.addListener(() {
       if (_controller.offset > 220 && !_controller.position.outOfRange) {
          if(!silverCollapsed){

            // do what ever you want when silver is collapsing !

            myTitle = "silver collapsed !";
            silverCollapsed = true;
            setState(() {});
          }
       }
       if (_controller.offset <= 220 && !_controller.position.outOfRange) {
         if(silverCollapsed){

            // do what ever you want when silver is expanding !

            myTitle = "silver expanded !";
            silverCollapsed = false;
            setState(() {});
         }
       }
    });
 }

然后将您的 silverAppBar 包裹在里面CustomScrollView并将控制器添加到这样的CustomScrollView：

 @override
 Widget build(BuildContext context) {
    return Scaffold(
       backgroundColor: Colors.white,
       body: CustomScrollView(
          controller: _controller,
          slivers: <Widget>[
             SliverAppBar(
                expandedHeight: 300,
                title: myTitle,
                flexibleSpace: FlexibleSpaceBar(),
             ),
             SliverList(
                delegate: SliverChildListDelegate(<Widget>[
                   // your widgets inside here !
                ]),
             ),
          ],
       ),
    );
}

最后更改条件值_controller.offset > 220以满足您的需要！

Answer 2

创建自己的 FlexibleSpaceBar 并不难。

import 'dart:math' as math;

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: SafeArea(
          child: MyHomePage(),
        ),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  ScrollController controller = ScrollController();
  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      physics: ClampingScrollPhysics(),
      controller: controller,
      slivers: [
        SliverAppBar(
          expandedHeight: 220.0,
          floating: true,
          pinned: true,
          elevation: 50,
          backgroundColor: Colors.pink,
          leading: IconButton(
            icon: Icon(Icons.menu),
            onPressed: () {},
          ),
          flexibleSpace: _MyAppSpace(),
        ),
        SliverList(
          delegate: SliverChildListDelegate(
            List.generate(
              200,
              (index) => Card(
                child: Padding(
                  padding: EdgeInsets.all(10),
                  child: Text('text $index'),
                ),
              ),
            ),
          ),
        )
      ],
    );
  }
}

class _MyAppSpace extends StatelessWidget {
  const _MyAppSpace({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, c) {
        final settings = context
            .dependOnInheritedWidgetOfExactType<FlexibleSpaceBarSettings>();
        final deltaExtent = settings.maxExtent - settings.minExtent;
        final t =
            (1.0 - (settings.currentExtent - settings.minExtent) / deltaExtent)
                .clamp(0.0, 1.0) as double;
        final fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
        const fadeEnd = 1.0;
        final opacity = 1.0 - Interval(fadeStart, fadeEnd).transform(t);

        return Stack(
          children: [
            Center(
              child: Opacity(
                  opacity: 1 - opacity,
                  child: getTitle(
                    'Collapsed Title',
                  )),
            ),
            Opacity(
              opacity: opacity,
              child: Stack(
                alignment: Alignment.bottomCenter,
                children: [
                  getImage(),
                  getTitle(
                    'Expended Title',
                  )
                ],
              ),
            ),
          ],
        );
      },
    );
  }

  Widget getImage() {
    return Container(
      width: double.infinity,
      child: Image.network(
        'https://source.unsplash.com/daily?code',
        fit: BoxFit.cover,
      ),
    );
  }

  Widget getTitle(String text) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Text(
        text,
        textAlign: TextAlign.center,
        style: TextStyle(
          color: Colors.white,
          fontSize: 26.0,
          fontWeight: FontWeight.bold,
        ),
      ),
    );
  }
}

Answer 3

您可以使用 AnimatedOpacity 类。

 flexibleSpace:  LayoutBuilder(
                    builder: (BuildContext context, BoxConstraints constraints) {
                     var top = constraints.biggest.height;
                      return FlexibleSpaceBar(
                          title: AnimatedOpacity(
                              duration: Duration(milliseconds: 300),
                               //opacity: top > 71 && top < 91 ? 1.0 : 0.0,                           
                              child: Text(
                               top > 71 && top < 91 ? "Collapse" : "Expanded",
                                style: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
                              )),
                          background: Image.network(
                            "https://images.ctfassets.net/pjshm78m9jt4/383122_header/d79a41045d07d114941f7641c83eea6d/importedImage383122_header",
                            fit: BoxFit.cover,
                          ));
                    }),

可以从此链接查看原始答案
/sf/answers/3736644131/

Answer 4

FlexibleSpaceBar本身还不够。您需要将其包装到CustomScrollView和中SliverAppBar。这些小部件必须由 a 进行控制器ScrollController，每当滚动偏移发生变化时就会触发一个事件。基于它，您可以了解应用栏是否折叠或展开，并相应地更改内容。在这里 您将找到一个工作示例。