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Lit 2.0 Upgrade Guide

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Kevin Schaaf edited this page Apr 20, 2021 · 5 revisions

Lit 2.0 is designed to work with most code written for LitElement 2.x and lit-html 1.x. There are a small number of changes required to migrate your code to Lit 2.0. The high-level changes required include:

  1. Updating npm packages and import paths.
  2. Loading polyfill-support script when loading the webcomponents polyfills.
  3. Updating any custom directive implementations to use new class-based API and associated helpers
  4. Updating code to renamed APIs.
  5. Adapting to minor breaking changes, mostly in uncommon cases.

The following sections will go through each of these changes in detail.

Update packages and import paths

Use the lit package

Lit 2.0 ships with a one-stop-shop lit package, which consolidates lit-html and lit-element into an easy-to-use package. Use the following commands to upgrade:

npm uninstall lit-element lit-html
npm install lit

And re-write your module imports appropriately:

From:

import {LitElement, html} from `lit-element`;

To:

import {LitElement, html} from `lit`;

Although the lit-element@^3 and lit-html@^2 packages should be largely backward-compatible, we recommend updating to the lit package as the other packages are moving towards eventual deprecation.

Update decorator imports

The previous version of lit-element exported all TypeScript decorators from the main module. In Lit 2.0, these have been moved to a separate module, to enable smaller bundle sizes when the decorators are unused.

From:

import {property, customElement} from `lit-element`;

To:

import {property, customElement} from `lit/decorators.js`;

Update directive imports

Built-in lit-html directives are also now exported from the lit package.

From:

import {repeat} from `lit-html/directives/repeat.js`;

To:

import {repeat} from `lit/directives/repeat.js`;

Load polyfill-support when using webcomponents polyfills

Lit 2.0 still supports the same browsers down to IE11. However, given the broad adoption of Web Components APIs in modern browsers, we have taken the opportunity to move all of the code required for interfacing with the webcomponents polyfills out of the core libraries and into an opt-in support file, so that the tax for supporting older browsers is only paid when required.

In general, any time you use the webcomponents polyfills, you should also load the lit/polyfill-support.js support file once on the page, similar to a polyfill. For example:

<script src="node_modules/@webcomponents/webcomponentsjs/webcomponents-loader.js"></script>
<script src="node_modules/lit/platform-support.js"></script>

If using @web/test-runner or @web/dev-server with the legacyPlugin for development, adding the following configuration to your web-test-runner.config.js or web-dev-server.config.js file will configure it to automatically inject the support file when needed:

export default {
  ...
  plugins: [
    legacyPlugin({
      polyfills: {
        webcomponents: true,
        custom: [
          {
            name: 'lit-polyfill-support',
            path: 'node_modules/lit/polyfill-support.js',
            test: "!('attachShadow' in Element.prototype)",
            module: false,
          },
        ],
      },
    }),
  ],
};

Update to renamed APIs

The following advanced API's have been renamed in Lit 2.0. It should be safe to simply rename these across your codebase if used:

Previous name New name Notes
UpdatingElement ReactiveElement The base class underpinning LitElement. Naming now aligns with terminology we use to describe its reactive lifecycle.
@internalProperty @state Decorator for LitElement / ReactiveElement used to denote private state that trigger updates, as opposed to public properties on the element settable by the user which use the @property decorator.
static getStyles() static finalizeStyles(styles) Method on LitElement and ReactiveElement class used for overriding style processing. Note it now also takes an argument reflecting the static styles for the class.
_getUpdateComplete() getUpdateComplete() Method on LitElement and ReactiveElement class used for overriding the updateComplete promise
NodePart ChildPart Typically only used in directive code; see below.

Update custom directive implementations

While the API for using directives should be 100% backward-compatible with 1.x, there is a breaking change to how custom directives are authored. The API change improves ergonomics around making stateful directives while providing a clear pattern for SSR-compatible directives: only render will be called on the server, while update will not be.

Overview of directive API changes

Concept Previous API New API
Code idiom Function that takes directive arguments, and returns function that takes part and returns value Class that extends Directive with update & render methods which accept directive arguments
Declarative rendering Pass value to part.setValue() Return value from render() method
DOM manipulation Implement in directive function Implement in update() method
State Stored in WeakMap keyed on part Stored in class instance fields
Part validation instanceof check on part in every render part.type check in constructor
Async updates part.setValue(v);
part.commit(); 		Extend AsyncDirective instead of Directive and call this.setValue(v)

Example directive migration

Below is an example of a lit-html 1.x directive, and how to migrate it to the new API:

1.x Directive API:

import {html, directive, Part, NodePart} from 'lit-html';

// State stored in WeakMap
const previousState: WeakMap<Part, number> = new WeakMap();

// Functional-based directive API
export const renderCounter = directive((initialValue: number) => (part: Part) => {
  // When necessary, validate part type each render using `instanceof`
  if (!(part instanceof NodePart)) {
    throw new Error('renderCounter only supports NodePart');
  }
  // Retrieve value from previous state
  let value = previousState.get(part);
  // Update state
  if (value === undefined) {
    value = initialValue;
  } else {
    value++;
  }
  // Store state
  previousState.set(part, value);
  // Update part with new rendering
  part.setValue(html`<p>${value}</p>`);
});

2.0 Directive API:

import {html} from 'lit';
import {directive, Directive, Part, PartInfo, PartType} from 'lit/directive.js';

// Class-based directive API
export class RenderCounter extends Directive {
  // State stored in class field
  value: number | undefined;
  constructor(partInfo: PartInfo) {
    super(partInfo);
    // When necessary, validate part in constructor using `part.type`
    if (partInfo.type !== PartType.CHILD) {
      throw new Error('renderCounter only supports child expressions');
    }
  }
  // Optional: override update to perform any direct DOM manipulation
  update(part: Part, [initialValue]: DirectiveParameters<this>) {
    /* Any imperative updates to DOM/parts would go here */
    return this.render(initialValue);
  }
  // Do SSR-compatible rendering (arguments are passed from call site)
  render(initialValue: number) {
    // Previous state available on class field
    if (this.value === undefined) {
      this.value = initialValue;
    } else {
      this.value++;
    }
    return html`<p>${this.value}</p>`;
  }
}
export const renderCounter = directive(RenderCounter);

Adapt to minor breaking changes

For completeness, the following is a list of minor but notable breaking changes that you may need to adapt your code to. We expect these changes to affect relatively few users.

LitElement

  • The update and render callbacks will only be called when the element is connected to the document. If an element is disconnected while an update is pending, or if an update is requested while the element is disconnected, update callbacks will be called if/when the element is re-connected.
  • For simplicity, requestUpdate no longer returns a Promise. Instead await the updateComplete Promise.
  • Errors that occur during the update cycle were previously squelched to allow subsequent updates to proceed normally. Now errors are re-fired asynchronously so they can be detected. Errors can be observed via an unhandledrejection event handler on window.
  • Creation of shadowRoot via createRenderRoot and support for applying static styles to the shadowRoot has moved from LitElement to ReactiveElement.
  • The createRenderRoot method is now called just before the first update rather than in the constructor. Element code can not assume the renderRoot exists before the element hasUpdated. This change was made for compatibility with server-side rendering.
  • ReactiveElement's initialize method has been removed. This work is now done in the element constructor.
  • The static render method on the LitElement base class has been removed. This was primarily used for implementing ShadyDOM integration, and was not intended as a user-overridable method. ShadyDOM integration is now achieved via the polyfill-support module.
  • When a property declaration is reflect: true and its toAttribute function returns undefined the attribute is now removed where previously it was left unchanged (#872).
  • The dirty check in attributeChangedCallback has been removed. While technically breaking, in practice it should very rarely be (#699).
  • LitElement's adoptStyles method has been removed. Styling is now adopted in createRenderRoot. This method may be overridden to customize this behavior.
  • Removed requestUpdateInternal. The requestUpdate method is now identical to this method and should be used instead.
  • The type of the css function has been changed to CSSResultGroup and is now the same as LitElement.styles. This avoids the need to cast the styles property to any when a subclass sets styles to an Array and its super class set a single value (or visa versa).

lit-html

  • render() no longer clears the container it's rendered to on first render. It now appends to the container by default.
  • Expressions in comments are not rendered or updated.
  • Template caching happens per callsite, not per template-tag/callsize pair. This means some rare forms of highly dynamic template tags are no longer supported.
  • Arrays and other iterables passed to attribute bindings are not specially handled. Arrays will be rendered with their default toString representation. This means that html`<div class=${['a', 'b']}> will render <div class="a,b"> instead of <div class="a b">. To get the old behavior, use array.join(' ').
  • The templateFactory option of RenderOptions has been removed.
  • TemplateProcessor has been removed.
  • Symbols are not converted to a string before mutating DOM, so passing a Symbol to an attribute or text binding will result in an exception.

Forward-compatible migration

lit-html 1.4.0 adds a number of forward-compatible modules to assist migration from 1.x to 2.x, so that many of the changes to prepare for Lit 2.0 can be done incrementally in the 1.x codebase before upgrading the entire app.

  • lit-html/directive.js - Class-based directive API
  • lit-html/async-directive.js - Class based async directive API (note disconnected and reconnected callbacks will not be called in 1.x)
  • lit-html/directive-helpers.js - Provides forward-compatible implementation of isTemplateResult

In addition, imports from lit-html 1.x and lit-element 2.x will continue to work after upgrading to the Lit 2.0 versions (lit-html 2.x and lit-element 3.x). However, once all components are upgraded in an app, we recommend switching imports to the lit package as described above, as the separate packages may be deprecated in future major releases.

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