[译] 深入了解 Flutter，阿里P7深入Binder原理讲解

m0_65145685

583人浏览 · 2021-12-17 19:49:04

m0_65145685 · 2021-12-17 19:49:04 发布

),
),
],
),
Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(Icons.share, color: Theme.of(context).primaryColor),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
‘SHARE’,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: Theme.of(context).primaryColor,
),
),
),
],
),
],
),
),
Container(
padding: const EdgeInsets.all(32.0),
child: Text(
'Lake Oeschinen lies at the foot of the Blüemlisalp in the ’
'Bernese Alps. Situated 1,578 meters above sea level, it ’
'is one of the larger Alpine Lakes. A gondola ride from ’
'Kandersteg, followed by a half-hour walk through pastures ’
'and pine forest, leads you to the lake, which warms to ’
'20 degrees Celsius in the summer. Activities enjoyed here ’
‘include rowing, and riding the summer toboggan run.’,
softWrap: true,
),
),
],
),
),
);
}
}

这只是一个静态组件树，没有实现任何行为。但是将视图逻辑直接嵌入到这样的树中估计不会是一次愉快的体验。

接受挑战。

重新审视代码编写 UI

使用 Flutter 的 UI-as-code 方法时，组件树就是代码。因此，我们可以使用所有常用的代码组织工具来改善这种情况。工具箱中最简单的工具之一就是命名子表达式。这会在语法上将组件树翻出来。而不是

return A(B(C(D(), E())), F());

我们可以命名每个子表达式并得到

final Widget d = D();
final Widget e = E();
final Widget c = C(d, e);
final Widget b = B©;
final Widget f = F();
return A(b, f);

我们的湖泊应用可以重写成下面这样：

import ‘package:flutter/material.dart’;

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
final Widget imageSection = Image.asset(
‘images/lake.jpg’,
width: 600.0,
height: 240.0,
fit: BoxFit.cover,
);
final Widget titles = Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Container(
padding: const EdgeInsets.only(bottom: 8.0),
child: Text(
‘Oeschinen Lake Campground’,
style: TextStyle(fontWeight: FontWeight.bold),
),
),
Text(
‘Kandersteg, Switzerland’,
style: TextStyle(color: Colors.grey[500]),
),
],
);
final Widget stars = Row(
children: [
Icon(Icons.star, color: Colors.red[500]),
Text(‘41’),
],
);
final Widget titleSection = Container(
padding: const EdgeInsets.all(32.0),
child: Row(
children: [
Expanded(child: titles),
stars,
],
),
);
final Widget callAction = Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(Icons.call, color

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: Theme.of(context).primaryColor),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
‘CALL’,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: Theme.of(context).primaryColor,
),
),
),
],
);
final Widget routeAction = Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(Icons.near_me, color: Theme.of(context).primaryColor),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
‘ROUTE’,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: Theme.of(context).primaryColor,
),
),
),
],
);
final Widget shareAction = Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(Icons.share, color: Theme.of(context).primaryColor),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
‘SHARE’,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: Theme.of(context).primaryColor,
),
),
),
],
);
final Widget actionSection = Container(
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
callAction,
routeAction,
shareAction,
],
),
);
final Widget textSection = Container(
padding: const EdgeInsets.all(32.0),
child: Text(
'Lake Oeschinen lies at the foot of the Blüemlisalp in the ’
'Bernese Alps. Situated 1,578 meters above sea level, it ’
'is one of the larger Alpine Lakes. A gondola ride from ’
'Kandersteg, followed by a half-hour walk through pastures ’
'and pine forest, leads you to the lake, which warms to ’
'20 degrees Celsius in the summer. Activities enjoyed here ’
‘include rowing, and riding the summer toboggan run.’,
softWrap: true,
),
);
final Widget scaffold = Scaffold(
appBar: AppBar(title: Text(‘Top Lakes’)),
body: ListView(
children: [
imageSection,
titleSection,
actionSection,
textSection,
],
),
);
return MaterialApp(
title: ‘Flutter Demo’,
home: scaffold,
);
}
}

缩进级别现在更合理，我们可以通过引入更多名称使子树的缩进级别变得像我们希望的那样浅。更好的是，通过为各个子树提供有意义的名称，我们可以表示每个子树的作用。所以我们现在可以谈谈 xxxAction 子树…并观察到我们在这里面有很多重复的代码！另一个基本的代码组织工具——功能抽象——负责这部分内容：

import ‘package:flutter/material.dart’;

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
final Widget imageSection = …
final Widget titles = …
final Widget stars = …
final Widget titleSection = …

Widget action(String label, IconData icon) {
final Color color = Theme.of(context).primaryColor;
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(icon, color: color),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
label,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: color,
),
),
),
],
);
}

final Widget actionSection = Container(
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
action(‘CALL’, Icons.call),
action(‘ROUTE’, Icons.near_me),
action(‘SHARE’, Icons.share),
],
),
);
final Widget textSection = …
final Widget scaffold = …
return MaterialApp(
title: ‘Flutter Demo’,
home: scaffold,
);
}
}

我们将看到一个简单功能抽象的替代，它会更具有更 Flutter 风格的。

重新审视组成

接下来是什么？好吧，build 方法依然很长。也许我们可以提取一些有意义的作品…片断？组件！Flutter 的组件都是关于组合和重用的。我们用框架提供的简单组件组成了一个复杂的组件但是发现结果过于复杂，我们可以选择把它分解成不太复杂的自定义组件。定制组件是 Flutter 世界中的一等公民，而明确定义的组件具有很大的潜力被重用。让我们将 action 函数转换为 Action 组件类型并将其放在自己的文件中：

main.dart:

import ‘package:flutter/material.dart’;
import ‘src/widgets.dart’;

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
final Widget imageSection = …
final Widget titles = …
final Widget titleSection = …
final Widget actionSection = Container(
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Action(label: ‘CALL’, icon: Icons.call),
Action(label: ‘ROUTE’, icon: Icons.near_me),
Action(label: ‘SHARE’, icon: Icons.share),
],
),
);
final Widget textSection = …
final Widget scaffold = …
return MaterialApp(
title: ‘Flutter Demo’,
home: scaffold,
);
}
}

widgets.dart:

import ‘package:flutter/material.dart’;

class Action extends StatelessWidget {
Action({Key key, this.label, this.icon}) : super(key: key);

final String label;
final IconData icon;

@override
Widget build(BuildContext context) {
final Color color = Theme.of(context).primaryColor;
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(icon, color: color),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
label,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: color,
),
),
),
],
);
}
}

现在我们可以在应用程序的任何位置重用 Action 组件，就像它是由 Flutter 框架定义的一样。

但是，嘿，顶级的 action 功能不能满足同样的需求吗？

一般来说，不能。

许多组件是由其他组件构造的；它们的构造函数有 Widget 与 List<Widget> 类型的 child 和 children 参数。所以 action 函数不能传递给任何一个函数。当然，调用 action 的结果可以。但是，你将通过在当前构建环境中预先构造的组件树，而不是 StatelessWidget，它只在必要时才构建子树，并且是最后在整个树中定义的上下文中定义的。注意到表达式中在 Action.build 开头的 Theme.of(context).primaryColor 了吗？它从父链上最近的 Theme 组件中检索主颜色——在调用 action 时，它很可能与最近的 Theme 不同。
Action is defined as a StatelessWidget which is little more than a build function turned into an instance method. But there are other kinds of widget with more elaborate behavior. Clients of Action shouldn’t care what kind of widget Action is. As an example, if we wanted to endow Action with an intrinsic animation, we might have to turn it into a StatefulWidget to manage the animation state. The rest of the app should be unaffected by such a change.

重新审视响应式编程

状态管理是开始利用 Flutter 响应式编程模型，并让我们的静态视图生动起来的暗示。让我们定义应用程序的状态。我们将尽量保持简单，先假设一个 Lake 业务逻辑类，其唯一可变状态是用户是否已加星标：

abstract class Lake {
String get imageAsset;
String get name;
String get locationName;
String get description;

int get starCount;
bool get isStarred;
void toggleStarring();

void call();
void route();
void share();
}

然后，我们可以从 Lake 实例动态地构造我们的组件树，并且同时还可以设置事件处理程序以调用其方法。响应式编程模型的优点在于我们只需在代码库中执行一次。只要 Lake 实例发生变化，Flutter 框架就会重建我们的组件树——前提是我们告诉框架。这需要使 MyApp 成为一个 StatefulWidget，这反过来又涉及将组件构建委托给一个相关的 State 对象，然后每当我们在 Lake 上加星标时调用 State.setState 方法。

main.dart:

import ‘package:flutter/material.dart’;
import ‘src/lake.dart’;
import ‘src/widgets.dart’;

void main() {
// 假装我们从业务逻辑中获取 Lake 实例。
final Lake lake = Lake();
runApp(MyApp(lake));
}

class MyApp extends StatefulWidget {
final Lake lake;

MyApp(this.lake);

@override
MyAppState createState() => new MyAppState();
}

class MyAppState extends State {
@override
Widget build(BuildContext context) {
final Lake lake = widget.lake;
final Widget imageSection = Image.asset(
lake.imageAsset,
width: 600.0,
height: 240.0,
fit: BoxFit.cover,
);
final Widget titles = Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Container(
padding: const EdgeInsets.only(bottom: 8.0),
child: Text(
lake.name,
style: TextStyle(fontWeight: FontWeight.bold),
),
),
Text(
lake.locationName,
style: TextStyle(color: Colors.grey[500]),
),
],
);
final Widget stars = GestureDetector(
child: Row(
children: [
Icon(
lake.isStarred ? Icons.star : Icons.star_border,
color: Colors.red[500],
),
Text(’${lake.starCount}’),
],
),
onTap: () {
setState(() {
lake.toggleStarring();
});
},
);
final Widget titleSection = Container(
padding: const EdgeInsets.all(32.0),
child: Row(
children: [
Expanded(child: titles),
stars,
],
),
);
final Widget actionSection = Container(
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Action(label: ‘CALL’, icon: Icons.call, handler: lake.call),
Action(label: ‘ROUTE’, icon: Icons.near_me, handler: lake.route),
Action(label: ‘SHARE’, icon: Icons.share, handler: lake.share),
],
),
);
final Widget textSection = Container(
padding: const EdgeInsets.all(32.0),
child: Text(
lake.description,
softWrap: true,
),
);
final Widget scaffold = Scaffold(
appBar: AppBar(title: Text(‘Top Lakes’)),
body: ListView(
children: [
imageSection,
titleSection,
actionSection,
textSection,
],
),
);
return MaterialApp(
title: ‘Flutter Demo’,
home: scaffold,
);
}
}

widgets.dart:

import ‘package:flutter/material.dart’;

class Action extends StatelessWidget {
Action({Key key, this.label, this.icon, this.handler}) : super(key: key);

final String label;
final IconData icon;
final VoidCallback handler;

@override
Widget build(BuildContext context) {
final Color color = Theme.of(context).primaryColor;
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
IconButton(
icon: Icon(icon, color: color),
onPressed: handler,
),
Text(
label,
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: color,
),
),
],
);
}
}

这有用，但效率不高。最初的挑战是深度嵌套的组件树。那个树仍然在那里，如果不在我们的代码中，那么就在运行时。重建所有这些只是为了切换切换星标完全是一种浪费。当然，Dart 的实现可以非常有效地处理短寿命对象，但如果你反复重建，Dart 也会耗尽你的电池——特别是涉及动画的地方。一般来说，我们应该将重建限制在实际改变的子树上。

你有没有抓住这个矛盾？组件树是用户界面的不可变描述。如何在不从根重构的情况下重建其中的一部分？实际上，组件树不是具有从父组件到子组件，从根到叶的引用的物化树结构。特别是 StatelessWidget 和 StatefulWidget，它们没有子引用。他们提供的是 build 方法（在有状态的情况下，通过相关的 State 实例）。Flutter 框架递归地调用那些 build 方法，同时生成或更新实际的运行时树结构，不是组件，而是引用组件的 Element 实例。元素树是可变的，并由 Flutter 框架管理。

那么当你在 State 实例 s 上调用 setState 时会发生什么？Flutter 框架标记了以 s 对应元素为根的子树，用于重建。当下一帧到期时，该子树将根据 s 的 build 方法返回的组件树进行更新，而后者依赖于当前的应用程序状态。

我们对代码的最终尝试提取了一个有状态的 LakeStars 组件，将重建限制在一个非常小的子树中。而 MyApp又变回无状态。