Component Styles

3.1

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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.

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

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

Run the of the code shown in this page.

Using component styles

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.

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 you give it one string, as in the following example:

lib/app_component.dart

@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 AppComponent { /* . . . */ }

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.

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

  • You 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 don’t collide with classes and selectors used elsewhere in the application.
  • Changes to styles elsewhere in the application don’t affect the component’s styles.
  • You can co-locate the CSS code of each component with the Dart and HTML code of the component, which leads to a neat and tidy project structure.
  • You can change or remove component CSS code without searching through the whole application to find where else the code is used.

Special selectors

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

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).

lib/src/hero_details_component.css (host)

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

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.

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

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

lib/src/hero_details_component.css (host function)

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

:host-context

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.

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.

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.

lib/src/hero_details_component.css (host-context)

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

/deep/

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

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.

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

lib/src/hero_details_component.css (deep)

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

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

Use the /deep/ and >>> selectors only with emulated view encapsulation. Emulated is the default and most commonly used view encapsulation. For more information, see the Controlling view encapsulation section.

Loading styles into components

There are several ways to add styles to a component:

  • By setting styles or styleUrls metadata.
  • Inline in the template HTML.
  • With CSS imports.

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

Styles in metadata

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

lib/app_component.dart

@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 AppComponent { /* . . . */ }

Style URLs in metadata

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

lib/src/hero_details_component.dart (styleUrls)

@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 inline styles

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

lib/src/hero_controls_component.dart (inline styles)

@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; } }

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

The link tag’s href URL is relative to the application root, not the component file.

lib/src/hero_team_component.dart (stylesheet link)

@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>''', directives: const [CORE_DIRECTIVES], ) class HeroTeamComponent { @Input() Hero hero; }

CSS @imports

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

CSS @imports cannot currently be used — see issue #39. It will be fixed in AngularDart 4.0.

Controlling view encapsulation: native, emulated, and none

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

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 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 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 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.

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

lib/src/quest_summary_component.dart (native encapsulation)

// 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 (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.

Appendix 1: Inspecting the CSS generated in emulated view encapsulation

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

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

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

There are two kinds of generated classes:

  • An element that would be a shadow DOM host in native encapsulation has a generated _nghost class. This is typically the case for component host elements.
  • An element within a component’s view has a _ngcontent class that identifies to which host’s emulated shadow DOM this element belongs.

The exact values of these classes 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; }

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

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.dart
  • quest_summary_component.html
  • quest_summary_component.css

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.

Thankfully, this is the default interpretation of relative URLs in AngularDart:

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