Widgets 介绍

Hello world
Basic widgets
基础 widgets
Using Material Components
使用 Material 组件
Handling gestures
处理手势
Changing widgets in response to input
根据用户输入改变 widget
Bringing it all together
整合在一起
Responding to widget lifecycle events
响应 widget 的生命周期事件
Keys
Global keys
全局 key

Flutter 从 React 中吸取灵感，通过现代化框架创建出精美的组件。它的核心思想是用 widget 来构建你的 UI 界面。 Widget 描述了在当前的配置和状态下视图所应该呈现的样子。当 widget 的状态改变时，它会重新构建其描述（展示的 UI），框架则会对比前后变化的不同，以确定底层渲染树从一个状态转换到下一个状态所需的最小更改。

Flutter widgets are built using a modern framework that takes inspiration from React. The central idea is that you build your UI out of widgets. Widgets describe what their view should look like given their current configuration and state. When a widget’s state changes, the widget rebuilds its description, which the framework diffs against the previous description in order to determine the minimal changes needed in the underlying render tree to transition from one state to the next.

Hello world

创建一个最小的 Flutter 应用简单到仅需调用 runApp() 方法并传入一个 widget 即可：

The minimal Flutter app simply calls the runApp() function with a widget:

import 'package:flutter/material.dart';

void main() {
  runApp(
    Center(
      child: Text(
        'Hello, world!',
        textDirection: TextDirection.ltr,
      ),
    ),
  );
}

runApp() 函数会持有传入的 Widget，并且使它成为 widget 树中的根节点。在这个例子中，Widget 树有两个 widgets， Center widget 及其子 widget ——Text 。框架会强制让根 widget 铺满整个屏幕，也就是说“Hello World”会在屏幕上居中显示。在这个例子我们需要指定文字的方向，当使用 MaterialApp widget 时，你就无需考虑这一点，之后我们会进一步的描述。

The runApp() function takes the given Widget and makes it the root of the widget tree. In this example, the widget tree consists of two widgets, the Center widget and its child, the Text widget. The framework forces the root widget to cover the screen, which means the text “Hello, world” ends up centered on screen. The text direction needs to be specified in this instance; when the MaterialApp widget is used, this is taken care of for you, as demonstrated later.

在写应用的过程中，取决于是否需要管理状态，你通常会创建一个新的组件继承 StatelessWidget 或 StatefulWidget。 Widget 的主要工作是实现 build方法，该方法根据其它较低级别的 widget 来描述这个 widget。框架会逐一构建这些 widget，直到最底层的描述 widget 几何形状的 RenderObject。

When writing an app, you’ll commonly author new widgets that are subclasses of either StatelessWidget or StatefulWidget, depending on whether your widget manages any state. A widget’s main job is to implement a build() function, which describes the widget in terms of other, lower-level widgets. The framework builds those widgets in turn until the process bottoms out in widgets that represent the underlying RenderObject, which computes and describes the geometry of the widget.

基础 widgets

Basic widgets

Flutter 自带了一套强大的基础 widgets，下面列出了一些常用的：

Flutter comes with a suite of powerful basic widgets, of which the following are commonly used:

Text
Text widget 可以用来在应用内创建带样式的文本。

Text
The Text widget lets you create a run of styled text within your application.

Row, Column
这两个 flex widgets 可以让你在水平 (Row) 和垂直(Column) 方向创建灵活的布局。它是基于 web 的 flexbox 布局模型设计的。

Row, Column
These flex widgets let you create flexible layouts in both the horizontal (Row) and vertical (Column) directions. The design of these objects is based on the web’s flexbox layout model.

Stack
Stack widget 不是线性（水平或垂直）定位的，而是按照绘制顺序将 widget 堆叠在一起。你可以用 Positioned widget 作为Stack 的子 widget，以相对于 Stack 的上，右，下，左来定位它们。 Stack 是基于 Web 中的绝对位置布局模型设计的。

Stack
Instead of being linearly oriented (either horizontally or vertically), a Stack widget lets you place widgets on top of each other in paint order. You can then use the Positioned widget on children of a Stack to position them relative to the top, right, bottom, or left edge of the stack. Stacks are based on the web’s absolute positioning layout model.

Container
Container widget 可以用来创建一个可见的矩形元素。 Container 可以使用 BoxDecoration 来进行装饰，如背景，边框，或阴影等。 Container 还可以设置外边距、内边距和尺寸的约束条件等。另外，Container可以使用矩阵在三维空间进行转换。

Container
The Container widget lets you create a rectangular visual element. A container can be decorated with a BoxDecoration, such as a background, a border, or a shadow. A Container can also have margins, padding, and constraints applied to its size. In addition, a Container can be transformed in three dimensional space using a matrix.

下面是一些简单的 widget，它们结合了上面提到的 widget 和一些其他的 widget：

Below are some simple widgets that combine these and other widgets:

import 'package:flutter/material.dart';

class MyAppBar extends StatelessWidget {
  MyAppBar({this.title});

  // Fields in a Widget subclass are always marked "final".

  final Widget title;

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 56.0, // in logical pixels
      padding: const EdgeInsets.symmetric(horizontal: 8.0),
      decoration: BoxDecoration(color: Colors.blue[500]),
      // Row is a horizontal, linear layout.
      child: Row(
        // <Widget> is the type of items in the list.
        children: <Widget>[
          IconButton(
            icon: Icon(Icons.menu),
            tooltip: 'Navigation menu',
            onPressed: null, // null disables the button
          ),
          // Expanded expands its child to fill the available space.
          Expanded(
            child: title,
          ),
          IconButton(
            icon: Icon(Icons.search),
            tooltip: 'Search',
            onPressed: null,
          ),
        ],
      ),
    );
  }
}

class MyScaffold extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Material is a conceptual piece of paper on which the UI appears.
    return Material(
      // Column is a vertical, linear layout.
      child: Column(
        children: <Widget>[
          MyAppBar(
            title: Text(
              'Example title',
              style: Theme.of(context).primaryTextTheme.headline6,
            ),
          ),
          Expanded(
            child: Center(
              child: Text('Hello, world!'),
            ),
          ),
        ],
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    title: 'My app', // used by the OS task switcher
    home: MyScaffold(),
  ));
}

请确认在 pubspec.yaml 文件中 flutter 部分有 uses-material-design: true 这条，它能让你使用预置的 Material icons。

Be sure to have a uses-material-design: true entry in the flutter section of your pubspec.yaml file. It allows you to use the predefined set of Material icons.

name: my_app
flutter:
  uses-material-design: true

为了获得(MaterialApp)主题的数据，许多 Material Design 的 widget 需要在 MaterialApp 中才能显现正常。因此，请使用 MaterialApp 运行应用。

Many Material Design widgets need to be inside of a MaterialApp to display properly, in order to inherit theme data. Therefore, run the application with a MaterialApp.

MyAppBar widget 创建了一个高 56 独立像素，左右内边距 8 像素的 Container。在容器内，MyAppBar 以 Row 布局来组织它的子元素。中间的子 widget（title widget），被标记为 Expanded，这意味着它会扩展以填充其它子 widget 未使用的可用空间。你可以定义多个Expanded 子 widget，并使用 flex 参数确定它们占用可用空间的比例。

The MyAppBar widget creates a Container with a height of 56 device-independent pixels with an internal padding of 8 pixels, both on the left and the right. Inside the container, MyAppBar uses a Row layout to organize its children. The middle child, the title widget, is marked as Expanded, which means it expands to fill any remaining available space that hasn’t been consumed by the other children. You can have multiple Expanded children and determine the ratio in which they consume the available space using the flex argument to Expanded.

MyScaffold widget 将其子 widget 组织在垂直列中。在列的顶部，它放置一个 MyAppBar 实例，并把 Text widget 传给它来作为应用的标题。把 widget 作为参数传递给其他 widget 是一个很强大的技术，它可以让你以各种方式创建一些可重用的通用组件。最后，MyScaffold 使用 Expanded 来填充剩余空间，其中包含一个居中的消息。

The MyScaffold widget organizes its children in a vertical column. At the top of the column it places an instance of MyAppBar, passing the app bar a Text widget to use as its title. Passing widgets as arguments to other widgets is a powerful technique that lets you create generic widgets that can be reused in a wide variety of ways. Finally, MyScaffold uses an Expanded to fill the remaining space with its body, which consists of a centered message.

有关更多信息，请参阅布局。

For more information, see Layouts.

使用 Material 组件

Using Material Components

Flutter 提供了许多 widget，可帮助你构建遵循 Material Design 的应用。 Material 应用以 MaterialApp widget 开始，它在你的应用的底层下构建了许多有用的 widget。这其中包括 Navigator，它管理由字符串标识的 widget 栈，也称为“routes”。 Navigator 可以让你在应用的页面中平滑的切换。使用 MaterialApp widget 不是必须的，但这是一个很好的做法。

Flutter provides a number of widgets that help you build apps that follow Material Design. A Material app starts with the MaterialApp widget, which builds a number of useful widgets at the root of your app, including a Navigator, which manages a stack of widgets identified by strings, also known as “routes”. The Navigator lets you transition smoothly between screens of your application. Using the MaterialApp widget is entirely optional but a good practice.

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Flutter Tutorial',
    home: TutorialHome(),
  ));
}

class TutorialHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Scaffold is a layout for the major Material Components.
    return Scaffold(
      appBar: AppBar(
        leading: IconButton(
          icon: Icon(Icons.menu),
          tooltip: 'Navigation menu',
          onPressed: null,
        ),
        title: Text('Example title'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.search),
            tooltip: 'Search',
            onPressed: null,
          ),
        ],
      ),
      // body is the majority of the screen.
      body: Center(
        child: Text('Hello, world!'),
      ),
      floatingActionButton: FloatingActionButton(
        tooltip: 'Add', // used by assistive technologies
        child: Icon(Icons.add),
        onPressed: null,
      ),
    );
  }
}

现在我们已经从 MyAppBar 和 MyScaffold 切换到了 material.dart 中的 AppBar 和 Scaffold widget，我们的应用更“Material”了一些。例如，标题栏有了阴影，标题文本会自动继承正确的样式，此外还添加了一个浮动操作按钮。

Now that the code has switched from MyAppBar and MyScaffold to the AppBar and Scaffold widgets, and from material.dart, the app is starting to look at bit more Material. For example, the app bar has a shadow and the title text inherits the correct styling automatically. A floating action button is also added.

注意，widget 作为参数传递给了另外的 widget。 Scaffold widget 将许多不同的 widget 作为命名参数，每个 widget 都放在了 Scofford 布局中的合适位置。同样的，AppBar widget 允许我们给 leading、title widget 的 actions 传递 widget。这种模式在整个框架会中重复出现，在设计自己的 widget 时可以考虑这种模式。

Notice that widgets are passed as arguments to other widgets. The Scaffold widget takes a number of different widgets as named arguments, each of which are placed in the Scaffold layout in the appropriate place. Similarly, the AppBar widget lets you pass in widgets for the leading widget, and the actions of the title widget. This pattern recurs throughout the framework and is something you might consider when designing your own widgets.

有关更多信息，请参阅 Material 组件。

For more information, see Material Components widgets.

处理手势

Handling gestures

大多数应用都需要通过系统来处理一些用户交互。构建交互式应用程序的第一步是检测输入手势，这里通过创建一个简单的按钮来了解其工作原理：

Most applications include some form of user interaction with the system. The first step in building an interactive application is to detect input gestures. See how that works by creating a simple button:

class MyButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        print('MyButton was tapped!');
      },
      child: Container(
        height: 36.0,
        padding: const EdgeInsets.all(8.0),
        margin: const EdgeInsets.symmetric(horizontal: 8.0),
        decoration: BoxDecoration(
          borderRadius: BorderRadius.circular(5.0),
          color: Colors.lightGreen[500],
        ),
        child: Center(
          child: Text('Engage'),
        ),
      ),
    );
  }
}

GestureDetector widget 没有可视化的展现，但它能识别用户的手势。当用户点击 Container 时， GestureDetector 会调用其 onTap() 回调，在这里会向控制台打印一条消息。你可以使用 GestureDetector 检测各种输入的手势，包括点击，拖动和缩放。

The GestureDetector widget doesn’t have a visual representation but instead detects gestures made by the user. When the user taps the Container, the GestureDetector calls its onTap() callback, in this case printing a message to the console. You can use GestureDetector to detect a variety of input gestures, including taps, drags, and scales.

许多 widget 使用 GestureDetector 为其他 widget 提供可选的回调。例如，IconButton、RaisedButton 和 FloatingActionButton widget 都有 onPressed() 回调，当用户点击 widget 时就会触发这些回调。

Many widgets use a GestureDetector to provide optional callbacks for other widgets. For example, the IconButton, RaisedButton, and FloatingActionButton widgets have onPressed() callbacks that are triggered when the user taps the widget.

有关更多信息，请参阅 Flutter 中的手势。

For more information, see Gestures in Flutter.

根据用户输入改变 widget

Changing widgets in response to input

到目前为止，这个页面仅使用了无状态的 widget。无状态 widget 接收的参数来自于它的父 widget，它们储存在 final 成员变量中。当 widget 需要被 build() 时，就是用这些存储的变量为创建的 widget 生成新的参数。

So far, this page has used only stateless widgets. Stateless widgets receive arguments from their parent widget, which they store in final member variables. When a widget is asked to build(), it uses these stored values to derive new arguments for the widgets it creates.

为了构建更复杂的体验，例如，以更有趣的方式对用户输入做出反应—应用通常带有一些状态。 Flutter 使用 StatefulWidgets 来实现这一想法。 StatefulWidgets 是一种特殊的 widget，它会生成 State 对象，用于保存状态。看看这个基本的例子，它使用了前面提到的RaisedButton：

In order to build more complex experiences—for example, to react in more interesting ways to user input—applications typically carry some state. Flutter uses StatefulWidgets to capture this idea. StatefulWidgets are special widgets that know how to generate State objects, which are then used to hold state. Consider this basic example, using the RaisedButton mentioned earlier:

class Counter extends StatefulWidget {
  // This class is the configuration for the state. It holds the
  // values (in this case nothing) provided by the parent and used
  // by the build  method of the State. Fields in a Widget
  // subclass are always marked "final".

  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      // This call to setState tells the Flutter framework that
      // something has changed in this State, which causes it to rerun
      // the build method below so that the display can reflect the
      // updated values. If you change _counter without calling
      // setState(), then the build method won't be called again,
      // and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called,
    // for instance, as done by the _increment method above.
    // The Flutter framework has been optimized to make rerunning
    // build methods fast, so that you can just rebuild anything that
    // needs updating rather than having to individually change
    // instances of widgets.
    return Row(
      children: <Widget>[
        RaisedButton(
          onPressed: _increment,
          child: Text('Increment'),
        ),
        Text('Count: $_counter'),
      ],
    );
  }
}

您可能想知道为什么 StatefulWidget 和 State 是独立的对象。在 Flutter 中，这两种类型的对象具有不同的生命周期。 Widget 是临时对象，用于构造应用当前状态的展示。而 State 对象在调用 build() 之间是持久的，以此来存储信息。

You might wonder why StatefulWidget and State are separate objects. In Flutter, these two types of objects have different life cycles. Widgets are temporary objects, used to construct a presentation of the application in its current state. State objects, on the other hand, are persistent between calls to build(), allowing them to remember information.

上面的示例接受用户输入并直接在其 build() 方法中直接使用结果。在更复杂的应用中，widget 层次不同的部分可能负责不同的关注点；例如，一个 widget 可能呈现复杂的用户界面，来收集像日期或位置这样特定的信息，而另一个 widget 可能使用该信息来改变整体的展现。

The example above accepts user input and directly uses the result in its build() method. In more complex applications, different parts of the widget hierarchy might be responsible for different concerns; for example, one widget might present a complex user interface with the goal of gathering specific information, such as a date or location, while another widget might use that information to change the overall presentation.

在 Flutter 中，widget 通过回调得到状态改变的通知，同时当前状态通知给其他 widget 用于显示。重定向这一流程的共同父级是 State，下面稍微复杂的示例显示了它在实践中的工作原理：

In Flutter, change notifications flow “up” the widget hierarchy by way of callbacks, while current state flows “down” to the stateless widgets that do presentation. The common parent that redirects this flow is the State. The following slightly more complex example shows how this works in practice:

class CounterDisplay extends StatelessWidget {
  CounterDisplay({this.count});

  final int count;

  @override
  Widget build(BuildContext context) {
    return Text('Count: $count');
  }
}

class CounterIncrementor extends StatelessWidget {
  CounterIncrementor({this.onPressed});

  final VoidCallback onPressed;

  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      onPressed: onPressed,
      child: Text('Increment'),
    );
  }
}

class Counter extends StatefulWidget {
  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      ++_counter;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Row(children: <Widget>[
      CounterIncrementor(onPressed: _increment),
      CounterDisplay(count: _counter),
    ]);
  }
}

注意创建两个新的无状态 widget 的方式，它清楚地分离了显示计数器（CounterDisplay）和改变计数器（CounterIncrementor）。尽管最终结果与前面的示例相同，但是责任的分离将更大的复杂性封装在各个 widget 中，保证了父级的简单性。

Notice the creation of two new stateless widgets, cleanly separating the concerns of displaying the counter (CounterDisplay) and changing the counter (CounterIncrementor). Although the net result is the same as the previous example, the separation of responsibility allows greater complexity to be encapsulated in the individual widgets, while maintaining simplicity in the parent.

有关更多信息，请参阅：

For more information, see:

整合在一起

Bringing it all together

下面是一个更完整的示例，汇集了上面介绍的概念：假定一个购物应用显示各种出售的产品，并在购物车中维护想购买的物品。首先定义一个用于展示的类，ShoppingListItem：

What follows is a more complete example that brings together these concepts: A hypothetical shopping application displays various products offered for sale, and maintains a shopping cart for intended purchases. Start by defining the presentation class, ShoppingListItem:

class Product {
  const Product({this.name});
  final String name;
}

typedef void CartChangedCallback(Product product, bool inCart);

class ShoppingListItem extends StatelessWidget {
  ShoppingListItem({this.product, this.inCart, this.onCartChanged})
      : super(key: ObjectKey(product));

  final Product product;
  final bool inCart;
  final CartChangedCallback onCartChanged;

  Color _getColor(BuildContext context) {
    // The theme depends on the BuildContext because different parts
    // of the tree can have different themes.
    // The BuildContext indicates where the build is
    // taking place and therefore which theme to use.

    return inCart ? Colors.black54 : Theme.of(context).primaryColor;
  }

  TextStyle _getTextStyle(BuildContext context) {
    if (!inCart) return null;

    return TextStyle(
      color: Colors.black54,
      decoration: TextDecoration.lineThrough,
    );
  }

  @override
  Widget build(BuildContext context) {
    return ListTile(
      onTap: () {
        onCartChanged(product, inCart);
      },
      leading: CircleAvatar(
        backgroundColor: _getColor(context),
        child: Text(product.name[0]),
      ),
      title: Text(product.name, style: _getTextStyle(context)),
    );
  }
}

ShoppingListItem widget 遵循无状态 widget 的通用模式。它将构造函数中接受到的值存储在 final 成员变量中，然后在 build() 函数中使用它们。例如，inCart 布尔值使两种样式进行切换：一个使用当前主题的主要颜色，另一个使用灰色。

The ShoppingListItem widget follows a common pattern for stateless widgets. It stores the values it receives in its constructor in final member variables, which it then uses during its build() function. For example, the inCart boolean toggles between two visual appearances: one that uses the primary color from the current theme, and another that uses gray.

当用户点击列表中的一项，widget 不会直接改变 inCart 的值，而是通过调用从父 widget 接收到的 onCartChanged 函数。这种方式可以在组件的生命周期中存储状态更长久，从而使状态持久化。甚至，widget 传给 runApp() 的状态可以持久到整个应用的生命周期。

When the user taps the list item, the widget doesn’t modify its inCart value directly. Instead, the widget calls the onCartChanged function it received from its parent widget. This pattern lets you store state higher in the widget hierarchy, which causes the state to persist for longer periods of time. In the extreme, the state stored on the widget passed to runApp() persists for the lifetime of the application.

当父级接收到 onCartChanged 回调时，父级会更新其内部状态，从而触发父级重建并使用新的 inCart 值来创建新的 ShoppingListItem 实例。尽管父级在重建时会创建 ShoppingListItem 的新实例，但是由于框架会将新构建的 widget 与先前构建的 widget 进行比较，仅将差异应用于底层的 RenderObject，这种代价是很小的。

When the parent receives the onCartChanged callback, the parent updates its internal state, which triggers the parent to rebuild and create a new instance of ShoppingListItem with the new inCart value. Although the parent creates a new instance of ShoppingListItem when it rebuilds, that operation is cheap because the framework compares the newly built widgets with the previously built widgets and applies only the differences to the underlying RenderObject.

这里有一个示例展示父组件是如何存储可变状态：

Here’s an example parent widget that stores mutable state:

class ShoppingList extends StatefulWidget {
  ShoppingList({Key key, this.products}) : super(key: key);

  final List<Product> products;

  // The framework calls createState the first time a widget
  // appears at a given location in the tree.
  // If the parent rebuilds and uses the same type of
  // widget (with the same key), the framework re-uses the State object
  // instead of creating a new State object.

  @override
  _ShoppingListState createState() => _ShoppingListState();
}

class _ShoppingListState extends State<ShoppingList> {
  Set<Product> _shoppingCart = Set<Product>();

  void _handleCartChanged(Product product, bool inCart) {
    setState(() {
      // When a user changes what's in the cart, you need to change
      // _shoppingCart inside a setState call to trigger a rebuild.
      // The framework then calls build, below,
      // which updates the visual appearance of the app.

      if (!inCart)
        _shoppingCart.add(product);
      else
        _shoppingCart.remove(product);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Shopping List'),
      ),
      body: ListView(
        padding: EdgeInsets.symmetric(vertical: 8.0),
        children: widget.products.map((Product product) {
          return ShoppingListItem(
            product: product,
            inCart: _shoppingCart.contains(product),
            onCartChanged: _handleCartChanged,
          );
        }).toList(),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    title: 'Shopping App',
    home: ShoppingList(
      products: <Product>[
        Product(name: 'Eggs'),
        Product(name: 'Flour'),
        Product(name: 'Chocolate chips'),
      ],
    ),
  ));
}

ShoppingList 类继承自 StatefulWidget，这意味着这个 widget 存储着可变状态。当 ShoppingList 首次插入到 widget 树中时，框架调用 createState() 函数来创建 _ShoppingListState 的新实例，以与树中的该位置相关联。（注意，State 的子类通常以下划线开头进行命名，表示它们的实现细节是私有的）当该 widget 的父 widget 重建时，父 widget 首先会创建一个 ShoppingList 的实例，但是框架会复用之前创建的 _ShoppingListState，而不会重新调用 createState。

The ShoppingList class extends StatefulWidget, which means this widget stores mutable state. When the ShoppingList widget is first inserted into the tree, the framework calls the createState() function to create a fresh instance of _ShoppingListState to associate with that location in the tree. (Notice that subclasses of State are typically named with leading underscores to indicate that they are private implementation details.) When this widget’s parent rebuilds, the parent creates a new instance of ShoppingList, but the framework reuses the _ShoppingListState instance that is already in the tree rather than calling createState again.

为了访问当前 ShoppingList 的属性，_ShoppingListState 可以使用它的 widget 属性。当父组件重建一个新的 ShoppingList 时，_ShoppingListState 会使用新的 widget 值来创建。如果希望在 widget 属性更改时收到通知，则可以重写 didUpdateWidget() 函数，该函数将 oldWidget 作为参数传递，以便将 oldWidget 与当前 widget 进行比较。

To access properties of the current ShoppingList, the _ShoppingListState can use its widget property. If the parent rebuilds and creates a new ShoppingList, the _ShoppingListState rebuilds with the new widget value. If you wish to be notified when the widget property changes, override the didUpdateWidget() function, which is passed an oldWidget to let you compare the old widget with the current widget.

当处理 onCartChanged 回调时，_ShoppingListState 通过增加或删除 _shoppingCart 中的产品来改变其内部状态。为了通知框架它改变了它的内部状态，需要调用 setState()。调用 setState() 会将该 widget 标记为“dirty”（脏的），并且计划在下次应用需要更新屏幕时重新构建它。如果在修改 widget 的内部状态后忘记调用 setState，框架将不知道这个 widget 是“dirty”(脏的)，并且可能不会调用 widget 的 build() 方法，这意味着用户界面可能不会更新以展示新的状态。通过以这种方式管理状态，你不需要编写用于创建和更新子 widget 的单独代码。相反，你只需实现 build 函数，它可以处理这两种情况。

When handling the onCartChanged callback, the _ShoppingListState mutates its internal state by either adding or removing a product from _shoppingCart. To signal to the framework that it changed its internal state, it wraps those calls in a setState() call. Calling setState marks this widget as dirty and schedules it to be rebuilt the next time your app needs to update the screen. If you forget to call setState when modifying the internal state of a widget, the framework won’t know your widget is dirty and might not call the widget’s build() function, which means the user interface might not update to reflect the changed state. By managing state in this way, you don’t need to write separate code for creating and updating child widgets. Instead, you simply implement the build function, which handles both situations.

Responding to widget lifecycle events

在 StatefulWidget 上调用 createState() 之后，框架将新的状态对象插入到树中，然后在状态对象上调用 initState()。 State 的子类可以重写 initState 来完成只需要发生一次的工作。例如，重写 initState 来配置动画或订阅平台服务。实现 initState 需要调用父类的 super.initState 方法来开始。

After calling createState() on the StatefulWidget, the framework inserts the new state object into the tree and then calls initState() on the state object. A subclass of State can override initState to do work that needs to happen just once. For example, override initState to configure animations or to subscribe to platform services. Implementations of initState are required to start by calling super.initState.

当不再需要状态对象时，框架会调用状态对象上的 dispose() 方法。可以重写dispose 方法来清理状态。例如，重写 dispose 以取消计时器或取消订阅平台服务。实现 dispose 时通常通过调用 super.dispose 来结束。

When a state object is no longer needed, the framework calls dispose() on the state object. Override the dispose function to do cleanup work. For example, override dispose to cancel timers or to unsubscribe from platform services. Implementations of dispose typically end by calling super.dispose.

有关更多信息，请参阅 State。

For more information, see State.

Keys

使用 key 可以控制框架在 widget 重建时与哪些其他 widget 进行匹配。默认情况下，框架根据它们的 runtimeType 以及它们的显示顺序来匹配。使用 key 时，框架要求两个 widget 具有相同的 key 和 runtimeType。

Use keys to control which widgets the framework matches up with other widgets when a widget rebuilds. By default, the framework matches widgets in the current and previous build according to their runtimeType and the order in which they appear. With keys, the framework requires that the two widgets have the same key as well as the same runtimeType.

Key 在构建相同类型 widget 的多个实例时很有用。例如，ShoppingList widget，它只构建刚刚好足够的 ShoppingListItem 实例来填充其可见区域：

Keys are most useful in widgets that build many instances of the same type of widget. For example, the ShoppingList widget, which builds just enough ShoppingListItem instances to fill its visible region:

如果没有 key，当前构建中的第一个条目将始终与前一个构建中的第一个条目同步，在语义上，列表中的第一个条目如果滚动出屏幕，那么它应该不会再在窗口中可见。
Without keys, the first entry in the current build would always sync with the first entry in the previous build, even if, semantically, the first entry in the list just scrolled off screen and is no longer visible in the viewport.
通过给列表中的每个条目分配为“语义” key，无限列表可以更高效，因为框架将通过相匹配的语义 key 来同步条目，并因此具有相似（或相同）的可视外观。此外，语义上同步条目意味着在有状态子 widget 中，保留的状态将附加到相同的语义条目上，而不是附加到相同数字位置上的条目。
By assigning each entry in the list a “semantic” key, the infinite list can be more efficient because the framework syncs entries with matching semantic keys and therefore similar (or identical) visual appearances. Moreover, syncing the entries semantically means that state retained in stateful child widgets remains attached to the same semantic entry rather than the entry in the same numerical position in the viewport.

有关更多信息，请参阅 Key API。

For more information, see the Key API.

全局 key

Global keys

全局 key 可以用来标识唯一子 widget。全局 key 在整个 widget 结构中必须是全局唯一的，而不像本地 key 只需要在兄弟 widget 中唯一。由于它们是全局唯一的，因此可以使用全局 key 来检索与 widget 关联的状态。

Use global keys to uniquely identify child widgets. Global keys must be globally unique across the entire widget hierarchy, unlike local keys which need only be unique among siblings. Because they are globally unique, a global key can be used to retrieve the state associated with a widget.

有关更多信息，请参阅 GlobalKey API。

For more information, see the GlobalKey API.