flutter 中的列表的性能优化前奏#yyds干货盘点#

Posted 2021-11-22 坚果前端

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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 dont, 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。