Angular

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

On top of this, Angular has the ability to bundle component styles with our components enabling a more modular design than regular stylesheets.

In this chapter we learn how to load and apply these component styles.

Using Component Styles
Special selectors
Loading Styles into Components
Controlling View Encapsulation: Emulated, Native, and None
Appendix 1: Inspecting the generated runtime component styles
Appendix 2: Loading Styles with Relative URLs

Run the of the code shown in this chapter.

Using Component Styles

For every Angular component we write, we 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 we need.

One way to do this is to set the styles property in the component metadata. The styles property takes a list of strings that contain CSS code. Usually we give it one string as in this example:

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

Component styles differ from traditional, global styles in a couple of ways.

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

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

We can use the CSS class names and selectors that make the most sense in the context of each component.
Class names and selectors are local to the component and won't collide with classes and selectors used elsewhere in the application.
Our component's styles cannot be changed by changes to styles elsewhere in the application.
We 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.
We can change or remove component CSS code in the future without trawling through the whole application to see where else it may have been used. We just look at the component we're in.

Special selectors

Component styles have a few special selectors from the world of shadow DOM style scoping:

:host

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;
}

This is the only way we can target the host element. We cannot reach it from inside the component with other selectors, because it is not part of the component's own template. It is in a parent component's template.

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

In the next example we target the host element again, but only when it also has the active CSS class.

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

:host-context

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

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

In the following example, we apply 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/

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

We can 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 both to the view children and the content children of the component.

In this example, we target 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;
}

The /deep/ selector also has the alias >>>. We can use either of the two interchangeably.

The /deep/ and >>> selectors should only be used with emulated view encapsulation. This is the default and it is what we use most of the time. See the Controlling View Encapsulation section for more details.

Loading Styles into Components

We have several ways to add styles to a component:

inline in the template HTML
by setting styles or styleUrls metadata
with CSS imports

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

Styles in Metadata

We can add a styles list property to the @Component annotation. Each string in the list (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: const ['h1 { font-weight: normal; }'],
    directives: const [HeroAppMainComponent])
class HeroAppComponent {
/* . . . */
}

Template Inline Styles

We 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>''')
class HeroControlsComponent {
  @Input()
  Hero hero;

  void activate() {
    hero.active = true;
  }
}

Style URLs in Metadata

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

@Component(
    selector: 'hero-details',
    template: '''
      <h2>{{hero.name}}</h2>
      <hero-team [hero]=hero></hero-team>
      <ng-content></ng-content>''',
    styleUrls: const ['hero_details_component.css'],
    directives: const [HeroTeamComponent])
class HeroDetailsComponent {
/* . . . */
}

Note that the URLs in styleUrls are relative to the component.

Template Link Tags

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

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

@Component(
    selector: 'hero-team',
    template: '''
      <link rel="stylesheet" href="hero_team_component.css">
      <h3>Team</h3>
      <ul>
        <li *ngFor="let member of hero.team">
          {{member}}
        </li>
      </ul>''')
class HeroTeamComponent {
  @Input() Hero hero;
}

CSS @imports

We can also import CSS files into our CSS files by using the standard CSS @import rule.

In this case the URL is relative to the CSS file into which we are importing.

URLs are currently not interpreted in this way, see issue 8518. Until this issue is fixed, absolute package-reference style URLs must be given as is illustrated below.

lib/hero_details_component.css (excerpt)/* pub build fails on
     @ import 'hero_details_box.css';
   See https://github.com/angular/angular/issues/8518 */

@import '/packages/component_styles/hero_details_box.css';

Controlling View Encapsulation: Native, Emulated, and None

As discussed above, component CSS styles are encapsulated into the component's own view and do not affect the rest of the application.

We can control how this encapsulation happens on a per component basis by setting the view encapsulation mode in the component metadata. There are three modes to choose from:

Native view encapsulation uses the browser's native Shadow DOM implementation 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 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. See Appendix 1 for details.
None means that Angular does no view encapsulation. Angular adds the CSS to the global styles. The scoping rules, isolations, and protections discussed earlier do not apply. This is essentially the same as pasting the component's styles into the HTML.

Set the components encapsulation mode using the encapsulation property in the component metadata:

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

Native view encapsulation only works on browsers that have native support for Shadow DOM. The support is still limited, which is why Emulated view encapsulation is the default mode and recommended in most cases.

Appendix 1: Inspecting The CSS Generated in Emulated View Encapsulation

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

When we inspect the DOM of a running Angular application with emulated view encapsulation enabled, we see that 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>

We see two kinds of generated attributes:

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

The exact values of these attributes are not important. They are automatically generated and we never refer to them in application code. But they are targeted by the generated component styles, which we'll find 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; }

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

We'll likely live with emulated mode until shadow DOM gains traction.

Appendix 2: Loading Styles with Relative URLs

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

We 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.

Thankfully, this is the default interpretation of relative URLs in Angular2 for Dart:

templateUrl: 'quest_summary_component.html',
styleUrls: const ['quest_summary_component.css'])

Note that special measures must be taken in Angular2 for TypeScript, if relative URLs are to have the same interpretation. See here for details.

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