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嵌套列表 - ShrinkWrap 与 Slivers

使用 ShrinkWrap 的列表列表

下面是一些使用ListView对象呈现列表列表的代码，内部列表的shrinkWrap值设置为 true。shrinkWrap强行评估整个内部列表，允许它请求有限的高度，而不是通常的ListView对象高度，即无穷大！

下面是基本的代码结构：

ListView(
  // Setting `shrinkWrap` to `true` here is both unnecessary and expensive.
  children: <Widget>[
    ListView.builder(
      itemCount: list1Children.length,
      itemBuilder: (BuildContext context, int index) {
        return list1Children[index];
      },
      // This forces the `ListView` to build all of its children up front,
      // negating much of the benefit of using `ListView.builder`.
      shrinkWrap: true,
    ),
    ListView.builder(
      itemCount: list2Children.length,
      itemBuilder: (BuildContext context, int index) {
        return list2Children[index];
      },
      // This forces the `ListView` to build all of its children up front,
      // negating much of the benefit of using `ListView.builder`.
      shrinkWrap: true,
    ),
    ...
  ],
)

注意：观察外部ListView没有将其shrinkWrap 值设置为true。只有内部列表需要设置shrinkWrap。

另请注意：虽然ListView.builder（默认情况下）有效地构建其子项，为您节省构建屏幕外小部件的不必要成本，但设置 shrinkWrap为true覆盖此默认行为！

import 'package:flutter/material.dart';
import 'dart:math' as math;
void main() {
  runApp(ShrinkWrApp());
}
class ShrinkWrApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'ShrinkWrap vs Slivers',
      home: Scaffold(
        appBar: AppBar(
          title: const Text("ShrinkWrap, Street Rat, I don't, Buy that!"),
        ),
        body: const ShrinkWrapSlivers(),
      ),
    );
  }
}
class ShrinkWrapSlivers extends StatefulWidget {
  const ShrinkWrapSlivers({
    Key? key,
  }) : super(key: key);
  @override
  _ShrinkWrapSliversState createState() => _ShrinkWrapSliversState();
}
class _ShrinkWrapSliversState extends State<ShrinkWrapSlivers> {
  List<ListView> innerLists = [];
  final numLists = 15;
  final numberOfItemsPerList = 100;
  @override
  void initState() {
    super.initState();
    for (int i = 0; i < numLists; i++) {
      final _innerList = <ColorRow>[];
      for (int j = 0; j < numberOfItemsPerList; j++) {
        _innerList.add(const ColorRow());
      }
      innerLists.add(
        ListView.builder(
          itemCount: numberOfItemsPerList,
          itemBuilder: (BuildContext context, int index) => _innerList[index],
          shrinkWrap: true,
          physics: const NeverScrollableScrollPhysics(),
        ),
      );
    }
  }
  @override
  Widget build(BuildContext context) {
    return ListView.builder(
        itemCount: numLists,
        itemBuilder: (context, index) => innerLists[index]);
  }
}
@immutable
class ColorRow extends StatefulWidget {
  const ColorRow({Key? key}) : super(key: key);
  @override
  State createState() => ColorRowState();
}
class ColorRowState extends State<ColorRow> {
  Color? color;
  @override
  void initState() {
    super.initState();
    color = randomColor();
  }
  @override
  Widget build(BuildContext context) {
    print('Building ColorRowState');
    return Container(
      decoration: BoxDecoration(
        gradient: LinearGradient(
          begin: Alignment.topLeft,
          end: Alignment.bottomRight,
          colors: [
            randomColor(),
            randomColor(),
          ],
        ),
      ),
      child: Row(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: Container(height: 50, width: 50, color: Colors.white),
          ),
          Flexible(
            child: Column(
              children: const <Widget>[
                Padding(
                  padding: EdgeInsets.all(8),
                  child: Text('这里是 坚果前端小课堂!',
                      style: TextStyle(color: Colors.white)),
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }
}
Color randomColor() =>
    Color((math.Random().nextDouble() * 0xFFFFFF).toInt()).withOpacity(1.0);

一切都建立起来！

当您滚动浏览此 UI 并注意该ColorBarState.build方法的调用方式时，会出现可怕的部分。每个内部列表包含 100 个元素，因此当 UI 加载时，您会立即看到 100 个“Building ColorBarState”的实例打印到控制台，

更糟糕的是，一旦向下滚动大约一百行，就会再生成一百行。😱😱😱

而且你滑动的快的时候列表会抖动！

重新构建嵌套列表

要了解如何使您的用户免受卡顿威胁，请等待我的第二节，下一节将使用 Slivers 而不是 ListViews 重建相同的 UI。

使用 Slivers 的列表列表

下面的代码构建了与之前相同的 UI，但这次它使用Slivers 而不是收缩包装ListView对象。本页的其余部分将引导您逐步完成更改。

如何将嵌套列表迁移到 Slivers

第1步

首先，将最外面的 ListView 更改为SliverList.

// Before
@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: numberOfLists,
    itemBuilder: (context, index) => innerLists[index],
  );
}

变成：

// After
@override
Widget build(BuildContext context) {
  return CustomScrollView(slivers: innerLists);
}

第2步

其次，将内部列表的类型从List更改为 List。

// Before
List<ListView> innerLists = [];

变成：

// After
List<SliverList> innerLists = [];

第 3 步

现在是时候重建内部列表了。的SliverList类是比原始略有不同ListView的类，与主要差异是的外观delegate。

原始版本ListView对所有内容都使用对象，不知道内部构建器构造函数将被shrinkWrap.

// Before
@override
void initState() {
  super.initState();
  for (int i = 0; i < numberOfLists; i++) {
    final _innerList = <ColorRow>[];
    for (int j = 0; j < numberOfItemsPerList; j++) {
      _innerList.add(const ColorRow());
    }
    innerLists.add(
      ListView.builder(
        itemCount: numberOfItemsPerList,
        itemBuilder: (BuildContext context, int index) => _innerList[index],
        shrinkWrap: true,
        physics: const NeverScrollableScrollPhysics(),
      ),
    );
  }
}

更改后，ListView对象被替换为SliverList对象，每个对象都使用一个SliverChildBuilderDelegate来提供高效的按需构建。

// After
@override
void initState() {
  super.initState();
  for (int i = 0; i < numLists; i++) {
    final _innerList = <ColorRow>[];
    for (int j = 0; j < numberOfItemsPerList; j++) {
      _innerList.add(const ColorRow());
    }
    innerLists.add(
      SliverList(
        delegate: SliverChildBuilderDelegate(
          (BuildContext context, int index) => _innerList[index],
          childCount: numberOfItemsPerList,
        ),
      ),
    );
  }
}

完整代码：

import 'package:flutter/material.dart';
import 'dart:math' as math;
void main() {
  runApp(SliversApp());
}
class SliversApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'ShrinkWrap vs Slivers',
      home: Scaffold(
        appBar: AppBar(
          title: const Text("Revenge of the Slivers"),
        ),
        body: const ShrinkWrapSlivers(),
      ),
    );
  }
}
class ShrinkWrapSlivers extends StatefulWidget {
  const ShrinkWrapSlivers({
    Key? key,
  }) : super(key: key);
  @override
  _ShrinkWrapSliversState createState() => _ShrinkWrapSliversState();
}
class _ShrinkWrapSliversState extends State<ShrinkWrapSlivers> {
  List<SliverList> innerLists = [];
  final numLists = 15;
  final numberOfItemsPerList = 100;
  @override
  void initState() {
    super.initState();
    for (int i = 0; i < numLists; i++) {
      final _innerList = <ColorRow>[];
      for (int j = 0; j < numberOfItemsPerList; j++) {
        _innerList.add(const ColorRow());
      }
      innerLists.add(
        SliverList(
          delegate: SliverChildBuilderDelegate(
            (BuildContext context, int index) => _innerList[index],
            childCount: numberOfItemsPerList,
          ),
        ),
      );
    }
  }
  @override
  Widget build(BuildContext context) {
    return CustomScrollView(slivers: innerLists);
  }
}
@immutable
class ColorRow extends StatefulWidget {
  const ColorRow({Key? key}) : super(key: key);
  @override
  State createState() => ColorRowState();
}
class ColorRowState extends State<ColorRow> {
  Color? color;
  @override
  void initState() {
    super.initState();
    color = randomColor();
  }
  @override
  Widget build(BuildContext context) {
    print('Building ColorRowState');
    return Container(
      decoration: BoxDecoration(
        gradient: LinearGradient(
          begin: Alignment.topLeft,
          end: Alignment.bottomRight,
          colors: [
            randomColor(),
            randomColor(),
          ],
        ),
      ),
      child: Row(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: Container(height: 50, width: 50, color: Colors.white),
          ),
          Flexible(
            child: Column(
              children: const <Widget>[
                Padding(
                  padding: EdgeInsets.all(8),
                  child: Text('这里是坚果前端小课堂!',
                      style: TextStyle(color: Colors.white)),
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }
}
Color randomColor() =>
    Color((math.Random().nextDouble() * 0xFFFFFF).toInt()).withOpacity(1.0);