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无障碍性

Angular 中的无障碍功能

Accessibility in Angular

Web 会被各种各样的人使用，包括有视觉或运动障碍的人。有多种辅助技术能使这些人更轻松地和基于 Web 的软件应用进行交互。另外，将应用设计得更易于访问通常也能改善所有用户的体验。

The web is used by a wide variety of people, including those who have visual or motor impairments. A variety of assistive technologies are available that make it much easier for these groups to interact with web-based software applications. In addition, designing an application to be more accessible generally improves the user experience for all users.

关于如何设计无障碍应用的问题和技术的深入介绍，请参阅 Google 网络基础知识的无障碍功能部分。

For an in-depth introduction to issues and techniques for designing accessible applications, see the Accessibility section of the Google's Web Fundamentals.

本页讨论了设计 Angular 应用的最佳实践，这些实践对所有用户（包括依赖辅助技术的用户）都适用。

This page discusses best practices for designing Angular applications that work well for all users, including those who rely on assistive technologies.

本页所描述的范例程序，参阅现场演练 / 下载范例。

For the sample application that this page describes, see the现场演练 / 下载范例.

无障碍属性（Attribute）

Accessibility attributes

建立无障碍的 Web 体验通常会涉及设置 ARIA 属性（Attribute） 以提供可能会丢失的语义。使用 Attribute 绑定模板语法来控制与无障碍性相关的属性（Attribute）值。

Building accessible web experience often involves setting ARIA attributes to provide semantic meaning where it might otherwise be missing. Use attribute binding template syntax to control the values of accessibility-related attributes.

在 Angular 中绑定 ARIA 属性（Attribute）时，必须使用 attr. 前缀，因为 ARIA 规范针对的是 HTML 属性（Attribute），而不是 DOM 元素的属性（Property）。

When binding to ARIA attributes in Angular, you must use the attr. prefix, as the ARIA specification depends specifically on HTML attributes rather than properties of DOM elements.

      
      <!-- Use attr. when binding to an ARIA attribute -->
<button [attr.aria-label]="myActionLabel">...</button>

注意，此语法仅对于属性（Attribute）绑定是必需的。静态 ARIA 属性（Attribute）不需要额外的语法。

Note that this syntax is only necessary for attribute bindings. Static ARIA attributes require no extra syntax.

      
      <!-- Static ARIA attributes require no extra syntax -->
<button aria-label="Save document">...</button>

按照约定，HTML 属性（Attribute）使用小写名称（ tabindex ），而 Property 使用 camelCase 名称（ tabIndex ）。

By convention, HTML attributes use lowercase names (tabindex), while properties use camelCase names (tabIndex).

关于 Attribute 和 Property 之间差异的更多背景信息，请参阅模板语法一章。

See the Binding syntax guide for more background on the difference between attributes and properties.

Angular UI 组件

Angular UI components

由 Angular 团队维护的 Angular Material 库是旨在提供完全无障碍的一组可复用 UI 组件。组件开发工具包（CDK）中包括 a11y 软件包，该软件包提供了支持无障碍领域的各种工具。例如：

The Angular Material library, which is maintained by the Angular team, is a suite of reusable UI components that aims to be fully accessible. The Component Development Kit (CDK) includes the a11y package that provides tools to support various areas of accessibility. For example:

  • LiveAnnouncer 用于使用 aria-live 区域向屏幕阅读器用户朗读消息。关于 aria-live 领域的更多信息，请参阅 W3C 文档。

    LiveAnnouncer is used to announce messages for screen-reader users using an aria-live region. See the W3C documentation for more information on aria-live regions.

  • cdkTrapFocus 指令能将 Tab 键焦点捕获在元素内。使用它可为必须限制焦点的模态对话框之类的组件创建无障碍体验。

    The cdkTrapFocus directive traps Tab-key focus within an element. Use it to create accessible experience for components like modal dialogs, where focus must be constrained.

关于这些工具和其它工具的完整详细信息，请参阅 Angular CDK 无障碍功能概述。

For full details of these and other tools, see the Angular CDK accessibility overview.

增强原生元素

Augmenting native elements

原生 HTML 元素捕获了许多对无障碍性很重要的标准交互模式。在制作 Angular 组件时，应尽可能直接复用这些原生元素，而不是重新实现已获良好支持的行为。

Native HTML elements capture a number of standard interaction patterns that are important to accessibility. When authoring Angular components, you should re-use these native elements directly when possible, rather than re-implementing well-supported behaviors.

例如，你可以创建一个组件，它使用属性（Attribute）选择器指向原生 <button> 元素，而不是为各种新按钮创建自定义元素。通常这适用于 <button> 和 <a>，但也可以用于许多其它类型的元素。

For example, instead of creating a custom element for a new variety of button, you can create a component that uses an attribute selector with a native <button> element. This most commonly applies to <button> and <a>, but can be used with many other types of element.

你可以在 Angular Material 中看到此模式的范例：MatButton，MatTabNav，MatTable。

You can see examples of this pattern in Angular Material: MatButton, MatTabNav, MatTable.

将容器用于原生元素

Using containers for native elements

有时要使用的原生元素需要一个容器元素。例如，原生 <input> 元素不能有子元素，因此任何自定义的文本输入组件都需要用其它元素来包装 <input>。尽管你可能只在自定义组件的模板中包含 <input>，但这将使该组件的用户无法为 input 元素设置任意 Property 和 Attribute。相反，你可以创建一个使用内容投影的容器组件，以将原生控件包含在组件的 API 中。

Sometimes using the appropriate native element requires a container element. For example, the native <input> element cannot have children, so any custom text entry components need to wrap an <input> with additional elements. While you might just include the <input> in your custom component's template, this makes it impossible for users of the component to set arbitrary properties and attributes to the input element. Instead, you can create a container component that uses content projection to include the native control in the component's API.

你可以把 MatFormField作为该模式的例子。

You can see MatFormFieldas an example of this pattern.

案例研究：构建自定义进度条

Case study: Building a custom progress bar

以下范例显示如何通过使用宿主（host）绑定来控制与无障碍性相关的属性（Attribute），来把进度条无障碍化。

The following example shows how to make a progress bar accessible by using host binding to control accessibility-related attributes.

  • 该组件使用标准的 HTML 属性（Attribute）role 和 ARIA 属性（Attribute）来定义要启用无障碍支持的元素。ARIA 属性（Attribute）aria-valuenow 绑定到用户的输入。

    The component defines an accessibility-enabled element with both the standard HTML attribute role, and ARIA attributes. The ARIA attribute aria-valuenow is bound to the user's input.

    src/app/progress-bar.component.ts
          
      import { Component, Input } from '@angular/core';

/**
 * Example progressbar component.
 */
@Component({
  selector: 'app-example-progressbar',
  template: `<div class="bar" [style.width.%]="value"></div>`,
  styleUrls: ['./progress-bar.component.css'],
  host: {
    // Sets the role for this component to "progressbar"
    role: 'progressbar',

    // Sets the minimum and maximum values for the progressbar role.
    'aria-valuemin': '0',
    'aria-valuemax': '100',

    // Binding that updates the current value of the progressbar.
    '[attr.aria-valuenow]': 'value',
  }
})
export class ExampleProgressbarComponent  {
  /** Current value of the progressbar. */
  @Input() value = 0;
}

  • 在模板中，aria-label 属性（Attribute）可以确保屏幕阅读器能访问该控件。

    In the template, the aria-label attribute ensures that the control is accessible to screen readers.

    src/app/app.component.html
          
      <label>
  Enter an example progress value
  <input type="number" min="0" max="100"
      [value]="progress" (input)="setProgress($event)">
</label>

<!-- The user of the progressbar sets an aria-label to communicate what the progress means. -->
<app-example-progressbar [value]="progress" aria-label="Example of a progress bar">
</app-example-progressbar>

路由和焦点管理

Routing and focus management

在设计无障碍性时，在 UI 中跟踪和控制焦点是很重要的考虑因素。使用 Angular 路由时，你需要确定页面焦点在导航上的位置。

Tracking and controlling focus in a UI is an important consideration in designing for accessibility. When using Angular routing, you should decide where page focus goes upon navigation.

为了避免仅仅依靠视觉提示，你需要确保路由代码会在页面导航之后更新焦点。使用 Router 服务中的 NavigationEnd 事件可以知道何时该更新焦点。

To avoid relying solely on visual cues, you need to make sure your routing code updates focus after page navigation. Use the NavigationEnd event from the Router service to know when to update focus.

以下范例显示了导航后如何在 DOM 中查找并把焦点移动到主体内容的头部。

The following example shows how to find and focus the main content header in the DOM after navigation.

      
      router.events.pipe(filter(e => e instanceof NavigationEnd)).subscribe(() => {
  const mainHeader = document.querySelector('#main-content-header')
  if (mainHeader) {
    mainHeader.focus();
  }
});

在实际的应用程序中，哪些元素获得焦点将取决于该应用特有的结构和布局。获得焦点的元素应使用户能够立即移动到刚刚进入视野的主要内容。你应该避免当路由变化后焦点重新回到 body 元素的情况。

In a real application, the element that receives focus will depend on your specific application structure and layout. The focused element should put users in a position to immediately move into the main content that has just been routed into view. You should avoid situations where focus returns to the body element after a route change.

额外资源

Additional resources

图书

Books

  • "A Web for Everyone: Designing Accessible User Experiences", Sarah Horton and Whitney Quesenbery

  • "Inclusive Design Patterns", Heydon Pickering