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Development: Scoped Elements

Scope element tag names avoiding naming collision and allowing to use different versions of the same web component in your code.

Installation

npm i --save @open-wc/scoped-elements@next

Version 2 of Scoped Elements only supports lit with lit-element 3.x. If you need to support lit-element 2.x be sure to use version 1 of Scoped Elements.

Usage

  1. Import ScopedElementsMixin from @open-wc/scoped-elements.

    import { ScopedElementsMixin } from '@open-wc/scoped-elements';
    
  2. Import the classes of the components you want to use.

    import { MyButton } from './MyButton.js';
    import { MyPanel } from './MyPanel.js';
    
  3. Apply ScopedElementsMixin and define the tags you want to use for your components.

    class MyElement extends ScopedElementsMixin(LitElement) {
      static get scopedElements() {
        return {
          'my-button': MyButton,
          'my-panel': MyPanel,
        };
      }
    }
    

    If you are going to use elements that are globally defined you have to declare them in scopedElements as well. This is required because we are trying to work as close as possible to the future Scoped Custom Element Registries feature and, by the moment, there is not going to be inheritance between registries. You can declare them like in the following example:

    static get scopedElements() {
      return {
        'old-button': customElements.get('old-button'),
        'my-panel': MyPanel,
      };
    }
    
  4. Use your components in your html.

    render() {
      return html`
        <my-panel class="panel">
          <my-button>${this.text}</my-button>
        </my-panel>
      `;
    }
    

ScopedElements loads a polyfill of the scoped registry for you . Why do we do this? The spec is quite mature but not yet implemented in a browser and the polyfill is pretty new so we want to control it to be able to patch things if needed to stay backward compatible. The polyfill we use is @webcomponents/scoped-custom-element-registry.

Complete example

import { css, LitElement } from 'lit';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyButton } from './MyButton.js';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-button': MyButton,
      'my-panel': MyPanel,
    };
  }

  static get styles() {
    return css`
      .panel {
        padding: 10px;
        background-color: grey;
      }
    `;
  }

  static get properties() {
    return {
      text: String,
    };
  }

  render() {
    return html`
      <my-panel class="panel">
        <my-button>${this.text}</my-button>
      </my-panel}>
    `;
  }
}

Lazy scoped components

In some situations may happen that you want to use a component in your templates that is not already loaded at the moment of defining the scoped elements map. The ScopedElementsMixin provides the defineScopedElement method to define scoped elements at any time.

import { LitElement } from 'lit';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-panel': MyPanel,
    };
  }

  constructor() {
    super();

    import('./MyButton.js').then(({ MyButton }) => this.defineScopedElement('my-button', MyButton));
  }

  render() {
    return html`
      <my-panel class="panel">
        <my-button>${this.text}</my-button>
      </my-panel>
    `;
  }
}

Using a registry per component instance

By default, ScopedElementsMixin shares the same CustomElementsRegistry instance between all the instances of the same component class. There are some use cases in which you need to have just one registry instance per component instance. For those cases, you can override the get and set methods for the registry assigning and retrieving it from the component instance.

import { LitElement } from 'lit';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-panel': MyPanel,
    };
  }

  get registry() {
    return this.__registry;
  }

  set registry(registry) {
    this.__registry = registry;
  }

  constructor() {
    super();

    import('./MyButton.js').then(({ MyButton }) => this.defineScopedElement('my-button', MyButton));
  }

  render() {
    return html`
      <my-panel class="panel">
        <my-button>${this.text}</my-button>
      </my-panel>
    `;
  }
}

Motivation

Complex Web Component applications are often developed by several teams across organizations. In that scenario it is common that shared component libraries are used by teams to create a homogeneous look and feel or just to avoid creating the same components multiple times, but as those libraries evolve problems between different versions of the same library may appear, as teams may not be able to evolve and update their code at the same velocity. This causes bottlenecks in software delivery that should be managed by the teams and complex build systems, to try to alleviate the problem.

Scoped Custom Element Registries is a proposal that will solve this problem, but until it is ready, or a polyfill becomes available, we have to scope custom element tag names if we want to use different versions of those custom elements in our code. This package allows you to forget about how custom elements are defined, registered and scopes their tag names if it is necessary, and avoids the name collision problem.

Use case and demos

Consider the following setup

  • Team Blue owns Page A
  • Team Green owns Page B
  • Team Black owns Feature A & B
  1. Everything is good and your app is live [code] with both pages.
  2. Team Black releases a new version (2.x) of Feature B which unfortunately needs to be breaking in order to support new use-cases.
  3. Team Blue (on Page A) does not use any of those new use cases, and they have a tight deadline to meet, so they cannot update right now.
  4. Team Green (on Page B) has to deliver an important functionality to your end users, but they need to upgrade to Feature B 2.x since it can only be solved with this new version.
  5. Since Feature A 1.x & 2.x are both used in the same app, this will lead to nested dependencies, which then will lead to catastrophic failure, and errors [code].

Two possible solutions come to mind:

  1. Temporarily (!) allow shipping similar source code (most breaking releases are not a total rewrite) and scope them via @open-wc/scoped-elements; see the "fixed" example with-scope [code] running with nested dependencies.
  2. Synchronizing updates of shared dependencies - e.g. make sure Team Blue & Team Green always use the same version when releasing. This can be a viable solution however it comes with a high organizational overhead and is hard to scale up (for 10+ teams)

Technical explanation of scenario

The simplified app has the following dependencies

  • app
    • page-a
      • feature-a 1.x
      • feature-b 1.x
    • page-b
      • feature-a 2.x
      • feature-b 1.x

which leads to the following node_modules tree

├── node_modules
│   ├── feature-a
│   ├── feature-b
│   ├── page-a
│   └── page-b
│       └── node_modules
│           └── feature-a
├── demo-app.js
└── index.html

To demonstrate, we made three demos:

  1. before-nesting [code] In this demo, everything works fine as Page A and B both are using the same version of Feature A

  2. no-scope [code] Feature A version 1.x and 2.x are imported via self registering entry points which leads to the following error message, because the feature-a component tries to register multiple times:

    Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': the name "feature-a" has already been used with this registry
      at [...]/node_modules/page-b/node_modules/feature-a/feature-a.js:3:16
    
  3. with-scope [code] This example successfully fixes the problem by using ScopedElementsMixin on both Page A and Page B.

Limitations

  1. Components imported via npm SHOULD NOT be self registering components. If a shared component (installed from npm) does not offer an export to the class alone, without the registration side effect, then this component may not be used. E.g. every component that calls customElement.define

    export class MyEl { ... }
    customElement.define('my-el', MyEl);
    

    Or uses the customElement typescript decorator

    @customElement('my-el')
    export class MyEl { ... }
    

    Only side effects free class exports may be used

    export class MyEl { ... }
    
  2. Every component that uses sub components should use scoped-elements. Any import to a self registering component can potentially result in a browser exception - completely breaking the whole application

  3. Imported elements should be fully side effect free (not only element registration)

  4. Using the scoped registry polyfill may result in a small performance degradation

  5. Loading of duplicate/similar source code (most breaking releases are not a total rewrite) should always be a temporary solution

  6. Often, temporary solutions tend to become more permanent. Be sure to focus on keeping the lifecycle of nested dependencies short