Example of extending Co-Authors Plus blocks to include custom data

block.json

{
	"$schema": "https://schemas.wp.org/trunk/block.json",
	"apiVersion": 3,
	"name": "myplugin/myblock",
	"version": "0.1.0",
	"title": "My Block",
	"category": "widgets",
	"icon": "smiley",
	"description": "Example block scaffolded with Create Block tool.",
	"example": {},
	"supports": {},
	"usesContext": ["co-authors-plus/author"],
	"textdomain": "myplugin",
	"editorScript": "file:./index.js"
}

edit.js

/**
 * Retrieves the translation of text.
 *
 * @see https://developer.wordpress.org/block-editor/reference-guides/packages/packages-i18n/
 */
import { __ } from '@wordpress/i18n';

/**
 * React hook that is used to mark the block wrapper element.
 * It provides all the necessary props like the class name.
 *
 * @see https://developer.wordpress.org/block-editor/reference-guides/packages/packages-block-editor/#useblockprops
 */
import { useBlockProps } from '@wordpress/block-editor';
import { useSelect } from '@wordpress/data';

/**
 * The edit function describes the structure of your block in the context of the
 * editor. This represents what the editor will render when the block is used.
 *
 * @see https://developer.wordpress.org/block-editor/reference-guides/block-api/block-edit-save/#edit
 *
 * @return {WPElement} Element to render.
 */
export default function Edit( { context } ) {
	const authorPlaceholder = useSelect( select => select( 'co-authors-plus/blocks' ).getAuthorPlaceholder(), []);
	const author = context['co-authors-plus/author'] || authorPlaceholder;
	const { example_field } = author;
	return (
		<div { ...useBlockProps() }>{ example_field }</div>
	);
}

index.js

/**
 * Registers a new block provided a unique name and an object defining its behavior.
 *
 * @see https://developer.wordpress.org/block-editor/reference-guides/block-api/block-registration/
 */
import { registerBlockType } from '@wordpress/blocks';

/**
 * Internal dependencies
 */
import Edit from './edit';
import metadata from './block.json';

/**
 * Every block starts by registering a new block type definition.
 *
 * @see https://developer.wordpress.org/block-editor/reference-guides/block-api/block-registration/
 */
registerBlockType( metadata.name, {
	/**
	 * @see ./edit.js
	 */
	edit: Edit
} );

myplugin.php

<?php
/**
 * Plugin Name:       MyPlugin
 * Description:       Example block scaffolded with Create Block tool.
 * Requires at least: 6.1
 * Requires PHP:      7.0
 * Version:           0.1.0
 * Author:            The WordPress Contributors
 * License:           GPL-2.0-or-later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 * Text Domain:       myplugin
 *
 * @package           myplugin
 */

/**
 * MyPlugin MyBlock Block Init
 */
function myplugin_myblock_block_init() {
	register_block_type(
		__DIR__ . '/build',
		array(
			'render_callback' => 'myplugin_myblock_render'
		)
	);
}
add_action( 'init', 'myplugin_myblock_block_init' );

/**
 * MyPlugin MyBlock Render
 * 
 * @param array    $attributes Any attributes saved with our block, currently empty array.
 * @param string   $content The block as HTML, currently empty.
 * @param WP_Block $block The block instance. We will use the context.
 * @return string The rendered content of our block.
 */
function myplugin_myblock_render( array $attributes, string $content, WP_Block $block ): string {

	$author = $block->context['co-authors-plus/author'] ?? array();

	if ( empty( $author ) ) {
		return '';
	}

	$example = $author['example_field'] ?? '';

	if ( '' === $example ) {
		return '';
	}

	$attributes  = get_block_wrapper_attributes();
	$new_content = esc_html( $example );

	return "<div {$attributes}>{$new_content}</div>";
}

/**
 * MyPlugin MyBlock Register REST Field
 */
function myplugin_myblock_register_rest_field(): void {
	register_rest_field(
		'coauthor',
		'example_field',
		array(
			'get_callback' => 'myplugin_myblock_get_example_field',
			'schema'       => array(
				'context'     => array( 'view' ),
				'description' => 'This is an example value for an example REST field.',
				'readonly'    => true,
				'type'        => 'string'
			),
		)
	);
}
add_action( 'rest_api_init', 'myplugin_myblock_register_rest_field' );

/**
 * MyPlugin MyBlock Get Example Field
 * 
 * @param array           $data The existing data about this object.
 * @param string          $field_name The name of the field we are adding: `example_field`.
 * @param WP_REST_Request $request The request that expects we will want to add this data.
 * @param string          $object_type What the REST API object type was named, in this case `coauthor`. 
 * @return string The value of our example field.
 */
function myplugin_myblock_get_example_field( array $data, string $field_name, WP_REST_Request $request, string $object_type ): string{
	return 'Example value';
}

/**
 * MyPlugin MyBlock CoAuthor Blocks Store Data
 * 
 * @param array $block_store_data Data we'll pass to the `co-authors-plus/blocks` store for use in the editor.
 * @return array Updated data with placeholder text for our example field.
 */
function myplugin_myblock_coauthor_blocks_store_data( array $block_store_data ): array {
	return array_merge(
		$block_store_data,
		array(
			'authorPlaceholder' => array_merge(
				$block_store_data['authorPlaceholder'],
				array(
					'example_field' => 'This is an example placeholder for an example field.'
				)
			)
		)
	);
}
add_filter( 'coauthors_blocks_store_data', 'myplugin_myblock_coauthor_blocks_store_data' );