使用 Themes 统一颜色和字体风格

通过定义 Theme，我们可以更好地复用颜色和字体样式，从而让整个 app 的设计看起来更一致。全局 Theme 会在整个 app 范围内生效，而局部 Theme 只作用于特定元素。其实所谓的全局 Theme 和局部 Theme 的区别只在于，全局 Theme 定义在了 app 的 root 处而已。而 MaterialApp 已经事先为你预设了一个全局的 Theme Widget。

To share colors and font styles throughout an app, use themes. You can either define app-wide themes, or use Theme widgets that define the colors and font styles for a particular part of the application. In fact, app-wide themes are just Theme widgets created at the root of an app by the MaterialApp.

在定义一个 Theme 之后，我们可以让它在指定的 widgets （包括 Flutter 自带的 Material widgets，例如 AppBars、Buttons、Checkboxes 等等）中生效。

After defining a Theme, use it within your own widgets. Flutter’s Material widgets also use your Theme to set the background colors and font styles for AppBars, Buttons, Checkboxes, and more.

定义一个全局 theme

Creating an app theme

全局 Theme 会影响整个 app 的颜色和字体样式。只需要向 MaterialApp 构造器传入 ThemeData 即可。

To share a Theme across an entire app, provide a ThemeData to the MaterialApp constructor.

如果没有放置 Theme，Flutter 将会使用预设的样式。

If no theme is provided, Flutter creates a default theme for you.

MaterialApp(
  title: title,
  theme: ThemeData(
    // Define the default brightness and colors.
    brightness: Brightness.dark,
    primaryColor: Colors.lightBlue[800],
    accentColor: Colors.cyan[600],

    // Define the default font family.
    fontFamily: 'Georgia',

    // Define the default TextTheme. Use this to specify the default
    // text styling for headlines, titles, bodies of text, and more.
    textTheme: TextTheme(
      headline: TextStyle(fontSize: 72.0, fontWeight: FontWeight.bold),
      title: TextStyle(fontSize: 36.0, fontStyle: FontStyle.italic),
      body1: TextStyle(fontSize: 14.0, fontFamily: 'Hind'),
    ),
  )
);

在 ThemeData 查看所有可自定义的颜色和字体样式。

See the ThemeData documentation to see all of the colors and fonts you can define.

定义一个局部 Theme

Themes for part of an application

如果我们只想对局部进行样式修改，可以创建一个 Theme Widget。

To override the app-wide theme in part of an application, wrap a section of the app in a Theme widget.

有以下两种方式：定义一个独立的 ThemeData，或者从父级 Theme 扩展。下面为你分别介绍。

There are two ways to approach this: creating a unique ThemeData, or extending the parent theme.

定义一个独立的 ThemeData

Creating unique ThemeData

如果不想从任何全局 Theme 继承样式，我们可以创建一个 ThemeData() 实例，然后把它传给 Theme widget：

If you don’t want to inherit any application colors or font styles, create a ThemeData() instance and pass that to the Theme widget.

Theme(
  // Create a unique theme with "ThemeData"
  data: ThemeData(
    accentColor: Colors.yellow,
  ),
  child: FloatingActionButton(
    onPressed: () {},
    child: Icon(Icons.add),
  ),
);

从父级 Theme 扩展

Extending the parent theme

相比从头开始定义一套样式，从父级 Theme 扩展可能更常规一些，使用 copyWith 方法即可。

Rather than overriding everything, it often makes sense to extend the parent theme. You can handle this by using the copyWith() method.

Theme(
  // Find and extend the parent theme using "copyWith". See the next
  // section for more info on `Theme.of`.
  data: Theme.of(context).copyWith(accentColor: Colors.yellow),
  child: FloatingActionButton(
    onPressed: null,
    child: Icon(Icons.add),
  ),
);

使用定义好的 Theme

Using a Theme

现在我们定义好了一个 theme，接下来我们该使用它了！在我们 widget 的 build 方法中调用 Theme.of(context) 函数，可以让这些主题样式生效。

Now that you’ve defined a theme, use it within the widgets’ build() methods by using the Theme.of(context) method.

Theme.of(context) 会查询 widget 树，并返回其中最近的 Theme。所以他会优先返回我们之前定义过的一个独立的 Theme，如果找不到，它会返回全局 theme。

The Theme.of(context) method looks up the widget tree and returns the nearest Theme in the tree. If you have a standalone Theme defined above your widget, that’s returned. If not, the app’s theme is returned.

实际上，FloatingActionButton 就是使用这种方式来定义自己的 accentColor 的。

In fact, the FloatingActionButton uses this technique to find the accentColor.

Container(
  color: Theme.of(context).accentColor,
  child: Text(
    'Text with a background color',
    style: Theme.of(context).textTheme.title,
  ),
);

交互式样例

Interactive example

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final appName = 'Custom Themes';

    return MaterialApp(
      title: appName,
      theme: ThemeData(
        // Define the default brightness and colors.
        brightness: Brightness.dark,
        primaryColor: Colors.lightBlue[800],
        accentColor: Colors.cyan[600],

        // Define the default font family.
        fontFamily: 'Georgia',

        // Define the default TextTheme. Use this to specify the default
        // text styling for headlines, titles, bodies of text, and more.
        textTheme: TextTheme(
          headline: TextStyle(fontSize: 72.0, fontWeight: FontWeight.bold),
          title: TextStyle(fontSize: 36.0, fontStyle: FontStyle.italic),
          body1: TextStyle(fontSize: 14.0, fontFamily: 'Hind'),
        ),
      ),
      home: MyHomePage(
        title: appName,
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;

  MyHomePage({Key key, @required this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Container(
          color: Theme.of(context).accentColor,
          child: Text(
            'Text with a background color',
            style: Theme.of(context).textTheme.title,
          ),
        ),
      ),
      floatingActionButton: Theme(
        data: Theme.of(context).copyWith(
          colorScheme:
              Theme.of(context).colorScheme.copyWith(secondary: Colors.yellow),
        ),
        child: FloatingActionButton(
          onPressed: null,
          child: Icon(Icons.add),
        ),
      ),
    );
  }
}