View encapsulation
In Angular, 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, set the view encapsulation mode in the component metadata. Choose from the following modes:
-
ShadowDomview encapsulation uses the browser's built-in shadow DOM implementation (see Shadow DOM) 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. -
Emulatedview 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 Inspecting generated CSS. -
Nonemeans 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 mode is essentially the same as pasting the component's styles into the HTML.
To set the component's encapsulation mode, use the encapsulation property in the component metadata:
// warning: not all browsers support shadow DOM encapsulation at this time encapsulation: ViewEncapsulation.ShadowDom
ShadowDom view encapsulation only works on browsers that have built-in support for shadow DOM (see Can I use - Shadow DOM v1). The support is still limited, which is why Emulated view encapsulation is the default mode and recommended in most cases.
Inspecting generated CSS
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 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>
There are two kinds of generated attributes:
- An element that would be a shadow DOM host in native encapsulation has a generated
_nghostattribute. This is typically the case for component host elements. - An element within a component's view has a
_ngcontentattribute that identifies to which host's emulated shadow DOM this element belongs.
The exact values of these attributes aren't important. They are automatically generated and you should 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 attribute selectors. These extra selectors enable the scoping rules described in this page.
Mixing encapsulation modes
Avoid mixing components that use different view encapsulation. Where it is necessary, you should be aware of how the component styles will interact.
-
The styles of components with
ViewEncapsulation.Emulatedare added to the<head>of the document, making them available throughout the application, but are "scoped" so they only affect elements within the component's template. -
The styles of components with
ViewEncapsulation.Noneare added to the<head>of the document, making them available throughout the application, and are not "scoped" so they can affect any element in the application. -
The styles of components with
ViewEncapsulation.ShadowDomare only added to the shadow DOM host, ensuring that they only affect elements within the component's template.
All the styles for ViewEncapsulation.Emulated and ViewEncapsulation.None components are also added to the shadow DOM host of each ViewEncapsulation.ShadowDom component.
The result is that styling for components with ViewEncapsulation.None will affect matching elements within the shadow DOM.
This approach may seem counter-intuitive at first, but without it a component with ViewEncapsulation.None could not be used within a component with ViewEncapsulation.ShadowDom, since its styles would not be available.
Examples
This section shows examples of how the styling of components with different ViewEncapsulation interact.
See the to try out these components yourself.
No encapsulation
The first example shows a component that has ViewEncapsulation.None. This component colors its template elements red.
@Component({
selector: 'app-no-encapsulation',
template: `
<h2>None</h2>
<div class="none-message">No encapsulation</div>
`,
styles: ['h2, .none-message { color: red; }'],
encapsulation: ViewEncapsulation.None,
})
export class NoEncapsulationComponent { }
Angular adds the styles for this component as global styles to the <head> of the document.
Angular also adds the styles to all shadow DOM hosts. Therefore, the styles are available throughout the application.
Emulated encapsulation
The second example shows a component that has ViewEncapsulation.Emulated. This component colors its template elements green.
@Component({
selector: 'app-emulated-encapsulation',
template: `
<h2>Emulated</h2>
<div class="emulated-message">Emulated encapsulation</div>
<app-no-encapsulation></app-no-encapsulation>
`,
styles: ['h2, .emulated-message { color: green; }'],
encapsulation: ViewEncapsulation.Emulated,
})
export class EmulatedEncapsulationComponent { }
Similar to ViewEncapsulation.None, Angular adds the styles for this component to the <head> of the document, and to all the shadow DOM hosts. But in this case, the styles are "scoped" by the attributes described in "Inspecting generated CSS".
Therefore, only the elements directly within this component's template will match its styles. Since the "scoped" styles from the EmulatedEncapsulationComponent are very specific, they override the global styles from the NoEncapsulationComponent.
In this example, the EmulatedEncapsulationComponent contains a NoEncapsulationComponent. The NoEncapsulationComponent is styled as expected because the scoped styles do not match elements in its template.
Shadow DOM encapsulation
The third example shows a component that has ViewEncapsulation.ShadowDom. This component colors its template elements blue.
@Component({
selector: 'app-shadow-dom-encapsulation',
template: `
<h2>ShadowDom</h2>
<div class="shadow-message">Shadow DOM encapsulation</div>
<app-emulated-encapsulation></app-emulated-encapsulation>
<app-no-encapsulation></app-no-encapsulation>
`,
styles: ['h2, .shadow-message { color: blue; }'],
encapsulation: ViewEncapsulation.ShadowDom,
})
export class ShadowDomEncapsulationComponent { }
Angular adds styles for this component only to the shadow DOM host, so they are not visible outside the shadow DOM.
Note that Angular also adds the global styles from the NoEncapsulationComponent and ViewEncapsulationComponent to the shadow DOM host, so those styles are still available to the elements in the template of this component.
In this example, the ShadowDomEncapsulationComponent contains both a NoEncapsulationComponent and ViewEncapsulationComponent.
The styles added by the ShadowDomEncapsulationComponent component are available throughout the shadow DOM of this component, and so to both the NoEncapsulationComponent and ViewEncapsulationComponent.
The EmulatedEncapsulationComponent has specific "scoped" styles, so the styling of this component's template is unaffected.
But since styles from ShadowDomEncapsulationComponent are added to the shadow host after the global styles, the h2 style overrides the style from the NoEncapsulationComponent. The result is that the <h2> element in the NoEncapsulationComponent is colored blue rather than red, which may not be what the component author intended.
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Licensed under the Creative Commons Attribution License 4.0.
https://v12.angular.io/guide/view-encapsulation
