Understanding ViewEncapsulation in Angular

Dhananjay Kumar / Tuesday, June 05, 2018

To understand ViewEncapsulation in Angular, first we should understand about Shadow DOM.  You can learn in detail about Shadow DOM here. Putting it in simple words, Shadow DOM brings Encapsulation in HTML Elements. Using the Shadow DOM , markup, styles, and behaviors are scoped to the element and do not clash with other nodes of the DOM. Shadow DOM is part of Web Components, which encapsulates styles and login of element.

Angular Components are made up of three things:

  1. Component class
  2. Template
  3. Style

Combination of these three makes an Angular component reusable across application. Theoretically, when you create a component, in some way you create a web component (Theoretically, Angular Components are not web components) to take advantage of Shadow DOM. You can also use Angular with browsers, which does not support Shadow DOM because Angular has its own emulation and it can emulate Shadow DOM.

To emulate Shadow DOM and encapsulate styles, Angular provides there types of ViewEncapsulation. They are as follows:

Let us try to understand it using an example. I have created a component, as shown below:

app.component.ts

import { Component, ViewEncapsulation } from '@angular/core';
 
@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css'],
    encapsulation: ViewEncapsulation.None
})
export class AppComponent {
    title = 'parent component';
}

app.component.html

<div>
    <h1>
        Welcome to {{ title }}!
    </h1>
</div>
<app-child></app-child>

app.component.css

h1 {
        backgroundred;
        colorwhite;
        text-transformuppercase;
        text-aligncenter;
    }

We are setting the style of h1 in the component CSS. We have also created another component:

import { Component } from '@angular/core';
 
@Component({
    selector: 'app-child',
    template: `
  <h1>{{title}}</h1>
  `
})
export class AppChildComponent {
    title = 'child app';
}

 

In AppChildComponent, we are also using h1 tag. To understand different ViewEncapsulation options, we will change the metadata of AppComponent.

Let us start with ViewEncapsulation.None, in this option,

  1. There is no shadow DOM
  2. Style is not scoped to component

 

As you run the application, you will find h1 style has applied to both components, even though we set style only in AppComponent. It happened because in AppComponent we have set encapsulation property to ViewEncapsulation.None.

@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css'],
    encapsulation: ViewEncapsulation.None
})
export class AppComponent {
    title = 'parent component';
}
 

In the browser when you examine source code, you will find h1 style has been declared in the head section of the DOM.

Therefore, in ViewEncapsulation.None, style gets move to the DOM head section and is not scoped to the component.  There is no Shadow DOM for the component and component style can affect all nodes of the DOM.

 

Next, let us explore ViewEncapsulation.Native, in this option:

  1. Angular will create Shadow DOM for the component.
  2. Style is scoped to component

 

As you run the application, you will find h1 style has applied to both components, even though we set style only in AppComponent. It happened because in AppComponent we have set encapsulation property to ViewEncapsulation.Native, and we are using AppChildComponnet as child inside template of AppComponent.

import { Component, ViewEncapsulation } from '@angular/core';
 
@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css'],
    encapsulation: ViewEncapsulation.Native
})
export class AppComponent {
    title = 'parent component';
}

In the browser when you examine source code, you will Shadow DOM has created for the AppComponent and style is scoped to that.

Therefore, in ViewEncapsulation.Native Angular creates a Shadow DOM and style is scoped to that Shadow DOM.

Next, let us explore ViewEncapsulation.Emulated, in this option:

  1. Angular will not create Shadow DOM for the component.
  2. Style will be scoped to the component.
  3. This is default value for encapsulation.
import { Component, ViewEncapsulation } from '@angular/core';

@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css'],
    encapsulation: ViewEncapsulation.Emulated
})
export class AppComponent {
    title = 'parent component';
}

 As you run the application, you will find that h1 style from AppComponent is not applied to h1 of AppChildComponent. It is due to emulated scoping. In this style is scoped only to the component. In this option, Angular only emulates to Shadow DOM and does not create a real shadow DOM. Hence, the application that runs in browsers does not support Shadow DOM also and styles are scoped to the component as well.

Let us see how Angular achieves this? In the browser when you examine source code, you will find answer.

Angular has created style in the head section of the DOM and given an arbitrary id to the component. On basis of ID, selector style is scoped to the component.

 These are three ViewEncapsulation options are available in Angular.   I hope you find this post useful. Thanks for reading.  If you like this post, please share it. Also, if you have not checked out Infragistics Ignite UI for Angular Components, be sure to do so! They have 50+ Material-based Angular components to help you code web apps faster.