flutter 中的列表的性能优化前奏#yyds干货盘点# _flutter

相逢意气为君饮，系马高楼垂柳边。这篇文章主要讲述flutter 中的列表的性能优化前奏#yyds干货盘点#相关的知识，希望能为你提供帮助。
【flutter 中的列表的性能优化前奏#yyds干货盘点#】这里是坚果前端小课堂，大家喜欢的话，可以关注我的公众号“坚果前端，”，或者加我好友，获取更多精彩内容
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。