关于flutter列表的性能优化,你必须要了解的
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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<ListView>
更改为 List<SliverList>
。
// 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);
Lazy building!
上面的代码已经应用了这些更改。运行应用程序并注意 Flutter 不再需要立即渲染 100 个 ColorRow 小部件。当您滚动时,会动态构建更多小部件,正如您所期望的那样。更好的是,一直滚动到下一个列表也不会产生任何特殊费用。
Flutter 会根据需要重新构建小部件,而且很快。
这节课对你来说怎么样,可以的话,支持一下吧
你快速的滑动的时候会发现,这个时候的列表没有抖动!