Flutter 初探(四)：滚动类Widgets

以下是可滚动Widgets的部分汇总：

• SingleChildScrollView
• ListView
• GridView
• CustomScrollView

简单使用

// SingleChildScrollView
class NewSingleChildScrollView extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    return new Scaffold(
      appBar: AppBar(title: Text('SingleChildScrollView学习')),
      body: Scrollbar(
          child: SingleChildScrollView(
        padding: EdgeInsets.all(16.0),
        child: Center(
          child: Column(
            // 动态创建一个 List<Widget>
            children: str
                .split('')
                // 每个字母都用一个Text显示，设置字体大小为原来的两倍
                .map((c) => Text(
                      c,
                      textScaleFactor: 2.0,
                    ))
                .toList(),
          ),
        ),
      )),
    );
  }
}

// ListView 一个无限加载实例
class InfiniteListView extends StatefulWidget {
  @override
  _InfiniteListViewState createState() => new _InfiniteListViewState();
}

class _InfiniteListViewState extends State<InfiniteListView> {
  static const loadingTag = "##loading##"; //表尾标记
  var _words = <String>[loadingTag];

  @override
  void initState() {
    _retrieveData();
  }

  void _retrieveData() {
    Future.delayed(Duration(seconds: 2)).then((e) {
      _words.insertAll(
          _words.length - 1,
          //每次生成20个单词
          generateWordPairs().take(20).map((e) => e.asPascalCase).toList());
      setState(() {
        //重新构建列表
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: AppBar(title: Text("ListView学习")),
        body: ListView.separated(
          itemCount: _words.length,
          itemBuilder: (context, index) {
            //如果到了表尾
            if (_words[index] == loadingTag) {
              //不足100条，继续获取数据
              if (_words.length - 1 < 100) {
                //获取数据
                _retrieveData();
                //加载时显示loading
                return Container(
                  padding: const EdgeInsets.all(16.0),
                  alignment: Alignment.center,
                  child: SizedBox(
                      width: 24.0,
                      height: 24.0,
                      child: CircularProgressIndicator(strokeWidth: 2.0)),
                );
              } else {
                //已经加载了100条数据，不再获取数据。
                return Container(
                    alignment: Alignment.center,
                    padding: EdgeInsets.all(16.0),
                    child: Text(
                      "没有更多了",
                      style: TextStyle(color: Colors.grey),
                    ));
              }
            }
            //显示单词列表项
            return ListTile(title: Text(_words[index]));
          },
          separatorBuilder: (context, index) => Divider(height: .0),
        ));
  }
}

// 模拟异步获取数据并利用GirdView展示
class InfiniteGridView extends StatefulWidget {
  @override
  _InfiniteGridViewState createState() => new _InfiniteGridViewState();
}

class _InfiniteGridViewState extends State<InfiniteGridView> {
  List<IconData> _icons = []; //保存Icon数据

  @override
  void initState() {
    // 初始化数据
    _retrieveIcons();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(title: Text('GridView')),
      body: GridView.builder(
          gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 3, //每行三列
              childAspectRatio: 1.0 //显示区域宽高相等
              ),
          itemCount: _icons.length,
          itemBuilder: (context, index) {
            //如果显示到最后一个并且Icon总数小于200时继续获取数据
            if (index == _icons.length - 1 && _icons.length < 200) {
              _retrieveIcons();
            }
            return Icon(_icons[index]);
          }),
    );
  }

  //模拟异步获取数据
  void _retrieveIcons() {
    Future.delayed(Duration(milliseconds: 200)).then((e) {
      setState(() {
        _icons.addAll([
          Icons.ac_unit,
          Icons.airport_shuttle,
          Icons.all_inclusive,
          Icons.beach_access,
          Icons.cake,
          Icons.free_breakfast
        ]);
      });
    });
  }
}

class CustomScrollViewTestRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    //因为本路由没有使用Scaffold，为了让子级Widget(如Text)使用
    //Material Design 默认的样式风格,我们使用Material作为本路由的根。
    return Material(
      child: CustomScrollView(
        slivers: <Widget>[
          //AppBar，包含一个导航栏
          SliverAppBar(
            pinned: true,
            expandedHeight: 250.0,
            flexibleSpace: FlexibleSpaceBar(
              title: const Text('Demo'),
              background: Image.asset(
                "./images/avatar.png",
                fit: BoxFit.cover,
              ),
            ),
          ),

          SliverPadding(
            padding: const EdgeInsets.all(8.0),
            sliver: new SliverGrid(
              //Grid
              gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
                crossAxisCount: 2, //Grid按两列显示
                mainAxisSpacing: 10.0,
                crossAxisSpacing: 10.0,
                childAspectRatio: 4.0,
              ),
              delegate: new SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  //创建子widget
                  return new Container(
                    alignment: Alignment.center,
                    color: Colors.cyan[100 * (index % 9)],
                    child: new Text('grid item $index'),
                  );
                },
                childCount: 20,
              ),
            ),
          ),
          //List
          new SliverFixedExtentList(
            itemExtent: 50.0,
            delegate: new SliverChildBuilderDelegate(
                (BuildContext context, int index) {
              //创建列表项
              return new Container(
                alignment: Alignment.center,
                color: Colors.lightBlue[100 * (index % 9)],
                child: new Text('list item $index'),
              );
            }, childCount: 50 //50个列表项
                ),
          ),
        ],
      ),
    );
  }
}

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