组件样式

Angular 应用使用标准的 CSS 来设置样式。这意味着我们可以把关于 CSS 的那些知识和技能直接用于我们的 Angular 程序中，例如：样式表、选择器、规则以及媒体查询等。

Angular applications are styled with standard CSS. That means you can apply everything you know about CSS stylesheets, selectors, rules, and media queries directly to Angular applications.

另外，Angular 还能把组件样式捆绑在我们的组件上，以实现比标准样式表更加模块化的设计。

Additionally, Angular can bundle component styles with components, enabling a more modular design than regular stylesheets.

在本章中，我们将学到如何加载和使用这些组件样式。

This page describes how to load and apply these component styles.

目录

Table Of Contents

• 使用组件样式

  Using component styles

• 特殊的选择器

  Special selectors

• 把样式加载进组件

  Loading styles into components

• 控制视图的封装模式：仿真 (Emulated)、原生 (Native) 或无 (None)

  Controlling view encapsulation: native, emulated, and none

• 附录 1：审查生成的运行时组件样式

  Appendix 1: Inspecting the CSS generated in emulated view encapsulation

• 附录 2：使用相对 URL 加载样式

  Appendix 2: Loading styles with relative URLs

你可以在Plunker上运行本章这些代码的在线例子 / 可下载的例子并下载这些代码。

You can run the live example / downloadable example in Plunker and download the code from there.

使用组件样式

Using component styles

对于我们写的每个 Angular 组件来说，除了定义 HTML 模板之外，我们还要定义用于模板的 CSS 样式、 指定任意的选择器、规则和媒体查询。

For every Angular component you write, you may define not only an HTML template, but also the CSS styles that go with that template, specifying any selectors, rules, and media queries that you need.

实现方式之一，是在组件的元数据中设置styles属性。 styles属性可以接受一个包含 CSS 代码的字符串数组。 通常我们只给它一个字符串就行了，如同下例：

One way to do this is to set the styles property in the component metadata. The styles property takes an array of strings that contain CSS code. Usually you give it one string, as in the following example:

@Component({
  selector: 'hero-app',
  template: `
    <h1>Tour of Heroes</h1>
    <hero-app-main [hero]=hero></hero-app-main>`,
  styles: ['h1 { font-weight: normal; }']
})
export class HeroAppComponent {
/* . . . */
}

我们放在组件样式中的选择器，只会应用在组件自身的模板中。上面这个例子中的h1选择器只会对 HeroAppComponent模板中的<h1>标签生效，而对应用中其它地方的<h1>元素毫无影响。

The selectors you put into a component's styles apply only within the template of that component. The h1 selector in the preceding example applies only to the <h1> tag in the template of HeroAppComponent. Any <h1> elements elsewhere in the application are unaffected.

这种模块化相对于 CSS 的传统工作方式是一个巨大的改进。

This is a big improvement in modularity compared to how CSS traditionally works.

• 可以使用对每个组件最有意义的 CSS 类名和选择器。

  You can use the CSS class names and selectors that make the most sense in the context of each component.

• 类名和选择器是仅属于组件内部的，它不会和应用中其它地方的类名和选择器出现冲突。

  Class names and selectors are local to the component and don't collide with classes and selectors used elsewhere in the application.

• 我们组件的样式不会因为别的地方修改了样式而被意外改变。

  Changes to styles elsewhere in the application don't affect the component's styles.

• 我们可以让每个组件的 CSS 代码和它的 TypeScript、HTML 代码放在一起，这将促成清爽整洁的项目结构。

  You can co-locate the CSS code of each component with the TypeScript and HTML code of the component, which leads to a neat and tidy project structure.

• 将来我们可以修改或移除组件的 CSS 代码，而不用遍历整个应用来看它有没有被别处用到，只要看看当前组件就可以了。

  You can change or remove component CSS code without searching through the whole application to find where else the code is used.

特殊的选择器

Special selectors

组件样式中有一些从影子(Shadow) DOM 样式范围领域（记录在W3C的CSS Scoping Module Level 1中） 引入的特殊选择器：

Component styles have a few special selectors from the world of shadow DOM style scoping (described in the CSS Scoping Module Level 1 page on the W3C site). The following sections describe these selectors.

:host

使用:host伪类选择器，用来选择组件宿主元素中的元素（相对于组件模板内部的元素）。

Use the :host pseudo-class selector to target styles in the element that hosts the component (as opposed to targeting elements inside the component's template).

:host {
  display: block;
  border: 1px solid black;
}

这是我们能以宿主元素为目标的唯一方式。除此之外，我们将没办法指定它， 因为宿主不是组件自身模板的一部分，而是父组件模板的一部分。

The :host selector is the only way to target the host element. You can't reach the host element from inside the component with other selectors because it's not part of the component's own template. The host element is in a parent component's template.

要把宿主样式作为条件，就要像函数一样把其它选择器放在:host后面的括号中。

Use the function form to apply host styles conditionally by including another selector inside parentheses after :host.

在下一个例子中，我们又一次把宿主元素作为目标，但是只有当它同时带有active CSS 类的时候才会生效。

The next example targets the host element again, but only when it also has the active CSS class.

:host(.active) {
  border-width: 3px;
}

:host-context

有时候，基于某些来自组件视图外部的条件应用样式是很有用的。 例如，在文档的<body>元素上可能有一个用于表示样式主题 (theme) 的 CSS 类，而我们应当基于它来决定组件的样式。

Sometimes it's useful to apply styles based on some condition outside of a component's view. For example, a CSS theme class could be applied to the document <body> element, and you want to change how your component looks based on that.

这时可以使用:host-context()伪类选择器。它也以类似:host()形式使用。它在当前组件宿主元素的祖先节点中查找 CSS 类， 直到文档的根节点为止。在与其它选择器组合使用时，它非常有用。

Use the :host-context() pseudo-class selector, which works just like the function form of :host(). The :host-context() selector looks for a CSS class in any ancestor of the component host element, up to the document root. The :host-context() selector is useful when combined with another selector.

在下面的例子中，只有当某个祖先元素有 CSS 类theme-light时，我们才会把background-color样式应用到组件内部的所有<h2>元素中。

The following example applies a background-color style to all <h2> elements inside the component, only if some ancestor element has the CSS class theme-light.

:host-context(.theme-light) h2 {
  background-color: #eef;
}

/deep/

组件样式通常只会作用于组件自身的 HTML 上。

Component styles normally apply only to the HTML in the component's own template.

我们可以使用/deep/选择器，来强制一个样式对各级子组件的视图也生效，它不但作用于组件的子视图，也会作用于组件的内容。

Use the /deep/ selector to force a style down through the child component tree into all the child component views. The /deep/ selector works to any depth of nested components, and it applies to both the view children and content children of the component.

在这个例子中，我们以所有的<h3>元素为目标，从宿主元素到当前元素再到 DOM 中的所有子元素：

The following example targets all <h3> elements, from the host element down through this component to all of its child elements in the DOM.

:host /deep/ h3 {
  font-style: italic;
}

/deep/选择器还有一个别名>>>。我们可以任意交替使用它们。

The /deep/ selector also has the alias >>>. You can use either interchangeably.

把样式加载进组件中

Loading styles into components

有几种方式把样式加入组件：

There are several ways to add styles to a component:

• 设置styles或styleUrls元数据

  By setting styles or styleUrls metadata.

• 内联在模板的 HTML 中

  Inline in the template HTML.

• 通过 CSS 文件导入

  With CSS imports.

上述作用域规则对所有这些加载模式都适用。

The scoping rules outlined earlier apply to each of these loading patterns.

元数据中的样式

Styles in metadata

我们可以给@Component装饰器添加一个styles数组型属性。 这个数组中的每一个字符串（通常也只有一个）定义一份 CSS。

You can add a styles array property to the @Component decorator. Each string in the array (usually just one string) defines the CSS.

@Component({
  selector: 'hero-app',
  template: `
    <h1>Tour of Heroes</h1>
    <hero-app-main [hero]=hero></hero-app-main>`,
  styles: ['h1 { font-weight: normal; }']
})
export class HeroAppComponent {
/* . . . */
}

元数据中指定样式表的URL

Style URLs in metadata

通过在组件的@Component装饰器中添加styleUrls属性，我们可以从外部CSS文件中加载样式：

You can load styles from external CSS files by adding a styleUrls attribute into a component's @Component decorator:

@Component({
  selector: 'hero-details',
  template: `
    <h2>{{hero.name}}</h2>
    <hero-team [hero]=hero></hero-team>
    <ng-content></ng-content>
  `,
  styleUrls: ['app/hero-details.component.css']
})
export class HeroDetailsComponent {
/* . . . */
}

模板内联样式

Template inline styles

我们也可以在组件的 HTML 模板中嵌入<style>标签。

You can embed styles directly into the HTML template by putting them inside <style> tags.

@Component({
  selector: 'hero-controls',
  template: `
    <style>
      button {
        background-color: white;
        border: 1px solid #777;
      }
    </style>
    <h3>Controls</h3>
    <button (click)="activate()">Activate</button>
  `
})

模板中的link标签

Template link tags

我们也可以在组件的 HTML 模板中嵌入<link>标签。

You can also embed <link> tags into the component's HTML template.

像styleUrls标签一样，这个 link 标签的href指向的 URL 也是相对于应用的根目录的，而不是组件文件。

As with styleUrls, the link tag's href URL is relative to the application root, not the component file.

@Component({
  selector: 'hero-team',
  template: `
    <link rel="stylesheet" href="app/hero-team.component.css">
    <h3>Team</h3>
    <ul>
      <li *ngFor="let member of hero.team">
        {{member}}
      </li>
    </ul>`
})

CSS @imports

我们还可以利用标准的 CSS @import规则来把其它 CSS 文件导入到我们的 CSS 文件中。

You can also import CSS files into the CSS files using the standard CSS @import rule. For details, see @import on the MDN site.

在这种情况下，URL 是相对于我们执行导入操作的 CSS 文件的。

In this case, the URL is relative to the CSS file into which you're importing.

@import 'hero-details-box.css';

控制视图的封装模式：原生 (Native)、仿真 (Emulated) 和无 (None)

Controlling view encapsulation: native, emulated, and none

像上面讨论过的一样，组件的 CSS 样式被封装进了自己的视图中，而不会影响到应用程序的其它部分。

As discussed earlier, component CSS styles are encapsulated into the component's view and don't affect the rest of the application.

通过在组件的元数据上设置视图封装模式，我们可以分别控制每个组件的封装模式。 可选的封装模式一共有如下几种：

To control how this encapsulation happens on a per component basis, you can set the view encapsulation mode in the component metadata. Choose from the following modes:

• Native模式使用浏览器原生的 Shadow DOM 实现来为组件的宿主元素附加一个 Shadow DOM。组件的样式被包裹在这个 Shadow DOM 中。(译注：不进不出，没有样式能进来，组件样式出不去。)

  Native view encapsulation uses the browser's native shadow DOM implementation (see Shadow DOM on the MDN site) to attach a shadow DOM to the component's host element, and then puts the component view inside that shadow DOM. The component's styles are included within the shadow DOM.

• Emulated模式（默认值）通过预处理（并改名）CSS 代码来模拟 Shadow DOM 的行为，以达到把 CSS 样式局限在组件视图中的目的。 更多信息，见附录 1 。(译注：只进不出，全局样式能进来，组件样式出不去)

  Emulated view encapsulation (the default) emulates the behavior of shadow DOM by preprocessing (and renaming) the CSS code to effectively scope the CSS to the component's view. For details, see Appendix 1.

• None意味着 Angular 不使用视图封装。 Angular 会把 CSS 添加到全局样式中。而不会应用上前面讨论过的那些作用域规则、隔离和保护等。 从本质上来说，这跟把组件的样式直接放进 HTML 是一样的。(译注：能进能出。)

  None means that Angular does no view encapsulation. Angular adds the CSS to the global styles. The scoping rules, isolations, and protections discussed earlier don't apply. This is essentially the same as pasting the component's styles into the HTML.

通过组件元数据中的encapsulation属性来设置组件封装模式：

To set the components encapsulation mode, use the encapsulation property in the component metadata:

// warning: few browsers support shadow DOM encapsulation at this time
encapsulation: ViewEncapsulation.Native

原生(Native)模式只适用于有原生 Shadow DOM 支持的浏览器。 因此仍然受到很多限制，这就是为什么我们会把仿真 (Emulated) 模式作为默认选项，并建议将其用于大多数情况。

Native view encapsulation only works on browsers that have native support for shadow DOM (see Shadow DOM v0 on the Can I use site). The support is still limited, which is why Emulated view encapsulation is the default mode and recommended in most cases.

附录 1：查看仿真 (Emulated) 模式下生成的 CSS

Appendix 1: Inspecting the CSS generated in emulated view encapsulation

当使用默认的仿真模式时，Angular 会对组件的所有样式进行预处理，让它们模仿出标准的 Shadow CSS 作用域规则。

When using emulated view encapsulation, Angular preprocesses all component styles so that they approximate the standard shadow CSS scoping rules.

当我们查看启用了仿真模式的 Angular 应用时，我们看到每个 DOM 元素都被加上了一些额外的属性。

In the DOM of a running Angular application with emulated view encapsulation enabled, each DOM element has some extra attributes attached to it:

<hero-details _nghost-pmm-5>
  <h2 _ngcontent-pmm-5>Mister Fantastic</h2>
  <hero-team _ngcontent-pmm-5 _nghost-pmm-6>
    <h3 _ngcontent-pmm-6>Team</h3>
  </hero-team>
</hero-detail>

我们看到了两种被生成的属性：

There are two kinds of generated attributes:

• 一个元素在原生封装方式下可能是 Shadow DOM 的宿主，在这里被自动添加上一个_nghost属性。 这是组件宿主元素的典型情况。

  An element that would be a shadow DOM host in native encapsulation has a generated _nghost attribute. This is typically the case for component host elements.

• 组件视图中的每一个元素，都有一个_ngcontent属性，它会标记出该元素是哪个宿主的模拟 Shadow DOM。

  An element within a component's view has a _ngcontent attribute that identifies to which host's emulated shadow DOM this element belongs.

这些属性的具体值并不重要。它们是自动生成的，并且我们永远不会在程序代码中直接引用到它们。 但它们会作为生成的组件样式的目标，就像我们在 DOM 的<head>区所看到的：

The exact values of these attributes aren't important. They are automatically generated and you never refer to them in application code. But they are targeted by the generated component styles, which are in the <head> section of the DOM:

[_nghost-pmm-5] {
  display: block;
  border: 1px solid black;
}

h3[_ngcontent-pmm-6] {
  background-color: white;
  border: 1px solid #777;
}

这些就是我们写的那些样式被处理后的结果，于是每个选择器都被增加了_nghost或_ngcontent属性选择器。 在这些附加选择器的帮助下，我们实现了本指南中所描述的这些作用域规则。

These styles are post-processed so that each selector is augmented with _nghost or _ngcontent attribute selectors. These extra selectors enable the scoping rules described in this page.

附录 2：使用相对 URL 加载样式

Appendix 2: Loading styles with relative URLs

把组件的代码 (ts/js)、HTML 和 CSS 分别放到同一个目录下的三个不同文件，是一个常用的实践：

It's common practice to split a component's code, HTML, and CSS into three separate files in the same directory:

quest-summary.component.ts
quest-summary.component.html
quest-summary.component.css

我们会通过设置元数据的templateUrl和styleUrls属性把模板和 CSS 文件包含进来。 既然这些文件都与组件（代码）文件放在一起，那么通过名字，而不是到应用程序根目录的全路径来指定它，就会是一个漂亮的方式。

You include the template and CSS files by setting the templateUrl and styleUrls metadata properties respectively. Because these files are co-located with the component, it would be nice to refer to them by name without also having to specify a path back to the root of the application.

我们也可以通过为文件名加上./前缀来使用相对URL：

You can use a relative URL by prefixing your filenames with ./:

@Component({
  selector: 'quest-summary',
  templateUrl: './quest-summary.component.html',
  styleUrls:  ['./quest-summary.component.css']
})
export class QuestSummaryComponent { }