preact

@remote-dom/preact

Utilities for rendering Remote DOM elements using Preact.

Installation

npm install @remote-dom/core @remote-dom/preact --save # npm
pnpm install @remote-dom/core @remote-dom/preact --save # pnpm
yarn add @remote-dom/core @remote-dom/preact # yarn

Usage

This library provides helpers for both the “host” and “remote” environments. The @remote-dom/preact entry provides wrappers for RemoteElement subclasses that add some nice Preact-specific features on top of core Remote DOM library. The @remote-dom/preact/host provides the SignalRemoteReceiver class and RemoteRootRenderer component, which are used to render a tree of remote elements to the host environment using Preact.

Remote

createRemoteComponent()

Preact has built-in support for web components, so you can use @remote-dom/core with Preact without any additional setup:

import {h, render} from 'preact';
import {RemoteElement} from '@remote-dom/core/elements';

// Define your remote element...
// @see https://github.com/Shopify/remote-dom/tree/main/packages/core/README.md#remoteelement
class MyElement extends RemoteElement {
  static get remoteAttributes() {
    return ['label'];
  }
}

customElements.define('my-element', MyElement);

// ...and render it using Preact components

render(
  h('my-element', {label: 'Hello, world!'}),
  document.querySelector('#root'),
);

However, we can make the Preact integration a bit more seamless by using the createRemoteComponent() function to create a wrapper Preact component. This wrapper component will automatically have the TypeScript prop types it should, given the custom element definition you pass in, and will correctly assign the props on the component to either attributes, properties, or event listeners.

import {render} from 'preact';

const MyElementComponent = createRemoteComponent('my-element', MyElement);

render(
  <MyElementComponent label="Hello, world!" />,
  document.querySelector('#root'),
);

More importantly, though, this wrapper will also take care of adapting some parts of the custom element API to be feel more natural in a Preact application.

Event listener props

Custom Preact components generally expose events as callback props on the component. To support this pattern, the createRemoteComponent() wrapper can map specific props on the resulting Preact component to event listeners on underlying custom element.

Imagine a ui-card element with a clickable header. When clicked, the card will emit an expand event to the remote environment, and reveal the children of the ui-card element to the user. First, we define our custom element:

import {RemoteElement} from '@remote-dom/core/elements';

class Card extends RemoteElement {
  static get remoteEvents() {
    return ['expand'];
  }
}

customElements.define('ui-card', Card);

Then, we use the createRemoteComponent() helper function to create a wrapper Preact component with an onExpand prop, mapped to the expand event:

import {createRemoteComponent} from '@remote-dom/preact';

const Card = createRemoteComponent('ui-card', CardElement, {
  eventProps: {
    onExpand: {event: 'expand'},
  },
});

render(
  <Card
    onExpand={() => {
      console.log('Card expanded!');
    }}
  >
    This is the body of the card.
  </Card>,
  document.querySelector('#root'),
);

Slotted children to Preact elements

The createRemoteComponent helper also supports mapping slotted children to Preact elements. Each top-level slot of the element’s children will be mapped to a prop with the same name on the Preact component.

For example, our ui-card custom element could take a header slot for customizing the title of the card:

import {RemoteElement} from '@remote-dom/core/elements';

class Card extends RemoteElement {
  static get remoteSlots() {
    return ['header'];
  }
}

class Text extends RemoteElement {}

customElements.define('ui-card', Card);
customElements.define('ui-text', Text);

The createRemoteComponent() wrapper will allow you to pass a header prop to the resulting Preact component, which can be any other Preact element:

import {render} from 'preact';
import {createRemoteComponent} from '@remote-dom/preact';

const Card = createRemoteComponent('ui-card', CardElement);
const Text = createRemoteComponent('ui-text', TextElement);

render(
  <Card header={<Text>Hello, world!</Text>}>
    This is the body of the card.
  </Card>,
  document.querySelector('#root'),
);

To make this work, the wrapper component will wrap the Preact element prop in a <remote-fragment> element, and place it as a child of the parent element. So, the above example could have been written manually like this:

import {render} from 'preact';

render(
  <ui-card>
    This is the body of the card.
    <remote-fragment slot="header">
      <ui-text>Hello, world!</ui-text>
    </remote-fragment>
  </ui-card>,
  document.querySelector('#root'),
);

To disable this behavior, you can pass {slotProps: {wrapper: false}} option to createRemoteComponent(). This will cause any element props to be cloned with a slot prop, instead of wrapping them in a <remote-fragment> element.

import {render} from 'preact';
import {createRemoteComponent} from '@remote-dom/preact';

const Card = createRemoteComponent('ui-card', CardElement, {
  slotProps: {
    wrapper: false,
  },
});

const Text = createRemoteComponent('ui-text', TextElement);

render(
  <Card header={<Text>Hello, world!</Text>}>
    This is the body of the card.
  </Card>,
  document.querySelector('#root'),
);

// Now, renders this tree of HTML elements:
// <ui-card>
//   This is the body of the card.
//   <ui-text slot="header">Hello, world!</ui-text>
// </ui-card>

Host

SignalRemoteReceiver

The @remote-dom/preact/host package re-exports the SignalRemoteReceiver class from @remote-dom/signals. This object will store the state of the remote tree of elements, and the RemoteRootRenderer component expects to receive an instance of this class in order to map the remote tree to Preact components.

createRemoteComponentRenderer()

The RemoteRootRenderer component needs a map of which Preact components to render for each remote element. These components will receive a description of the remote element, but not much more. The createRemoteComponentRenderer() function can be used to create a wrapper Preact component that will automatically update whenever the properties or children of the associated remote element change. The props passed to your Preact component will be the combined result of:

• The properties of the remote element
• The attributes of the remote element
• The eventListeners of the remote element, with each event listener being mapped to a prop named in the format onEventName
• The children of the remote element, where any children with a slot attribute are mapped to a prop with the same name

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

// Imagine we are implementing the host version of our `ui-card` custom element above,
// which allows a `header` slot and `expand` event. We’ll also have it accept a `subdued` property to
// customize its appearance.

const Card = createRemoteComponentRenderer(function Card({
  header,
  subdued,
  onExpand,
  children,
}) {
  const isExpanded = useIsExpanded();

  return (
    <div
      class={[
        'Card',
        isExpanded.value && 'Card--expanded',
        subdued && 'Card--subdued',
      ]
        .filter(Boolean)
        .join(' ')}
    >
      {header && (
        <button
          class="Card__Header"
          onClick={() => {
            isExpanded.value = !isExpanded.value;
            if (isExpanded.value && onExpand) onExpand();
          }}
        >
          {header}
        </button>
      )}
      {children}
    </div>
  );
});

Host event listener props

Like with creating a Preact wrapper in the remote environment with createRemoteComponent(), a host using Preact likely wants to map event listeners to conventional Preact callback props. Like with creating a Preact wrapper for the remote environment, let’s return to our ui-card example. To refresh, we are imagining a collapsible card element that will emit an expand event to the remote environment when its contents are revealed. In the remote environment, our custom element would be defined like this:

import {RemoteElement} from '@remote-dom/core/elements';

class Card extends RemoteElement {
  static get remoteEvents() {
    return ['expand'];
  }
}

customElements.define('ui-card', Card);

const card = document.createElement('ui-card');
card.textContent = 'This is the body of the card.';

card.addEventListener('expand', (event) => {
  console.log('Card expanded!', event.detail);
});

By default, Remote DOM will map these an event listener to a conventionally-named Preact prop — that is, on, followed by the pascal-case name of the event, like onExpand. So, when using the createRemoteComponentRenderer() function, you can automatically invoke the event listeners by calling the conventional prop, with the first argument you pass being set as the detail of the resulting remote event:

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(function Card({children, onExpand}) {
  return (
    <div>
      {/* when clicked, will emit the `expand` remote event with this object as the `detail` field */}
      <button onClick={() => onExpand({timestamp: Date.now()})}>Expand</button>
      {children}
    </div>
  );
});

The default logic also allows you to pass an Event object to the Preact callback. When you do so, the detail field of the event will be passed to the remote environment directly. This can be useful when, for example, you have an existing web component being rendered by Preact that already emits a custom event with the necessary data.

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(function Card({children, onExpand}) {
  return (
    // Can pass the callback directly as an event listener to create a corresponding remote event automatically.
    <ui-card onexpand={onExpand}>{children}</ui-card>
  );
});

If this default behavior does not work for you, you can customize which events get mapped to properties, and what prop name that event maps to. This is done by passing a second argument to createRemoteComponentRenderer(), an object with an eventProps key. Each key in this eventProps option should be the name of a Preact property to create on the wrapper component, and the value is an object that describes which event listener to read for that property.

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(
  function Card({children, onexpand}) {
    return <ui-card onexpand={onexpand}>{children}</ui-card>;
  },
  {
    eventProps: {
      // Convert the `expand` event listener into an all-lowercase `onexpand` prop, which allows the prop to be passed
      // directly as a web component event listener.
      onexpand: {event: 'expand'},
    },
  },
);

If your event bubbles, be careful not to call the callback unless the matching remote element for this component is actually the target of the emitted event. For example, if you had two components that both support click events in the remote environment, implementing the event listener like this would cause a single click on the host version of the button to trigger two separate events:

// Remote environment: two elements that can emit the same event, each with
// an event listener attached:

import {RemoteElement} from '@remote-dom/core/elements';

class Card extends RemoteElement {
  static get remoteEvents() {
    return {
      click: {
        bubbles: true,
      },
    };
  }
}

class Button extends RemoteElement {
  static get remoteEvents() {
    return {
      click: {
        bubbles: true,
      },
    };
  }
}

customElements.define('ui-card', Card);
customElements.define('ui-button', Button);

const card = document.createElement('ui-card');
card.addEventListener('click', (event) => {
  console.log('Click event in card', event.target);
});

const button = document.createElement('ui-button');
button.textContent = 'Click me!';
button.addEventListener('click', (event) => {
  console.log('Click event in button', event.target);
});

card.append(button);

// Host: both are implemented to call their respective event listener props when clicked,
// leads to `card.addEventListener()` callback being called twice (once from the bubbling
// click on `ui-button` in the remote, and once from the bubbling in the host).

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(function Card({children, onClick}) {
  // This `onclick` will be called, even if the `Button` was the actual target
  return <ui-card onClick={() => onClick?.()}>{children}</ui-card>;
});

const Button = createRemoteComponentRenderer(function Button({
  children,
  onClick,
}) {
  return <ui-button onClick={() => onClick?.()}>{children}</ui-button>;
});

To avoid this issue, you should manually check that the target of the event is the current element before calling the callback, which will prevent the event from being dispatched on ancestor elements in the remote environment:

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(function Card({children, onClick}) {
  // This `onclick` will be called, even if the `Button` was the actual target
  return (
    <ui-card
      onClick={(event) => {
        if (event.target === event.currentTarget) {
          onClick?.();
        }
      }}
    >
      {children}
    </ui-card>
  );
});

const Button = createRemoteComponentRenderer(function Button({
  children,
  onClick,
}) {
  return (
    <ui-button
      onClick={(event) => {
        if (event.target === event.currentTarget) {
          onClick?.();
        }
      }}
    >
      {children}
    </ui-button>
  );
});

If you pass the event object directly to the callback, Remote DOM will automatically apply this protection for you, so you do not need to protect against this case manually:

import {createRemoteComponentRenderer} from '@remote-dom/preact/host';

const Card = createRemoteComponentRenderer(function Card({children, onClick}) {
  // Pass the callback directly through as an event listener.
  return <ui-card onClick={onClick}>{children}</ui-card>;
});

const Button = createRemoteComponentRenderer(function Button({
  children,
  onClick,
}) {
  return <ui-button onClick={onClick}>{children}</ui-button>;
});

RemoteRootRenderer

The RemoteRootRenderer component is used to render a tree of remote elements to the host environment using Preact. It expects a receiver prop, a SignalRemoteReceiver instance that will store the state of the remote tree of elements. It also accepts a components prop, which provides the mapping of which Preact component to render for each remote element.

The following component shows an example of how you could render a tree of remote elements using Preact. You’ll need to hand the receiver object’s connection property to the remote environment; some examples of how to do this are shown in the runnable Remote DOM examples.

import {render} from 'preact';
import {
  createRemoteComponentRenderer,
  RemoteRootRenderer,
  SignalRemoteReceiver,
} from '@remote-dom/preact/host';

// Create wrapper elements to render our actual UI components in response
// to remote elements. See the `createRemoteComponentRenderer()` section above.
const Card = createRemoteComponentRenderer(UICard);

const receiver = new SignalRemoteReceiver();
// TODO: send the `receiver.connection` object to the remote environment,
// so it can send us updates about the tree of remote elements.

render(
  <RemoteRootRenderer
    receiver={receiver}
    components={new Map([['ui-card', Card]])}
  />,
  document.querySelector('#root'),
);

RemoteFragmentRenderer

As noted above, Remote DOM may render a <remote-fragment> element to wrap a Preact element that is passed as a prop to a remote element. The RemoteFragmentRenderer component is used to render these fragments — it simply renders each child element inside of a Preact wrapper, which prevents any unnecessary wrapper elements from being introduced in the final DOM output.

import {render} from 'preact';
import {
  createRemoteComponentRenderer,
  RemoteRootRenderer,
  RemoteFragmentRenderer,
  SignalRemoteReceiver,
} from '@remote-dom/preact/host';

// Same setup as above...

render(
  <RemoteRootRenderer
    receiver={receiver}
    components={
      new Map([
        ['ui-card', Card],
        // We allow `remote-fragment` elements to be rendered, which is the
        // name Remote DOM gives these wrapper elements by default. If you changed
        // the name using the `slotProps.wrapper` option, match that name here.
        ['remote-fragment', RemoteFragmentRenderer],
      ])
    }
  />,
  document.querySelector('#root'),
);