Get Block Attributes in JSON REST API

Question

I am trying to make my block attributes show up in the REST API.

To start, I've added the rest_api_init hook to whitelist my block for inclusion.

add_action(
    'rest_api_init',
    function () {

        if ( ! function_exists( 'use_block_editor_for_post_type' ) ) {
            require ABSPATH . 'wp-admin/includes/post.php';
        }

        // add Location Block to the WordPress REST API
        $post_types = get_post_types_by_support( [ 'editor' ] );
        foreach ( $post_types as $post_type ) {
            if ( use_block_editor_for_post_type( $post_type ) ) {
                register_rest_field(
                    $post_type,
                    'blocks',
                    [
                        'get_callback' => function ( array $post ) {
                            $raw_blocks= parse_blocks( $post['content']['raw'] );
                            $whitelisted_blocks = [];
                            foreach ($raw_blocks as $raw_block) {
                                if( $raw_block['blockName']=='myplugin/block-map-location' ){
                                    array_push($whitelisted_blocks, $raw_block);
                                }
                            }
                            return $whitelisted_blocks;
                        },
                    ]
                );
            }
        }
    }
);

This outputs my raw block content, but the attrs array is empty.

blocks: 
  0:
  blockName:    "myplugin/block-map-location"
  attrs:    []
  innerBlocks:; []
  innerHTML:    "\n<div class=\"wp-block-myplugin-block-map-location\" aria-label=\"Interactive Map\" role=\"region\"><figure><div class=\"map-pp\" id=\"placepress-map\" data-lat=\"41.50214445\" data-lon=\"-81.6751670486689\" data-zoom=\"13\" data-basemap=\"carto_voyager\"></div><figcaption class=\"map-caption-pp\">This is the map caption.</figcaption></figure></div>\n"
  innerContent: 
    0:  "\n<div class=\"wp-block-myplugin-block-map-location\" aria-label=\"Interactive Map\" role=\"region\"><figure><div class=\"map-pp\" id=\"placepress-map\" data-lat=\"41.50214445\" data-lon=\"-81.6751670486689\" data-zoom=\"13\" data-basemap=\"carto_voyager\"></div><figcaption class=\"map-caption-pp\">This is the map caption.</figcaption></figure></div>\n"

To solve this, I've tried the following, using the example from the Gutenberg Handbook, but it doesn't seem to have any affect. (Note that in this instance, I am working with a custom "locations" post type and trying to get "lat" and "lon" attributes from my block.)

add_action( 'init', 'register_block_attributes' );
function register_block_attributes() {
    register_meta( 'post', 'lat', array(
        'object_subtype' => 'locations',
        'show_in_rest' => true,
    ) );
    register_meta( 'post', 'lon', array(
        'object_subtype' => 'locations',
        'show_in_rest' => true,
    ) );
}

I'm obviously missing something but am not finding any answers in the documentation.

Answer 1

Answering my own question. Since register_meta() actually appends to meta, not blocks, in the JSON output, I just added a new (redundant) attribute to my block using source:meta where I can manually manage outside the attributes I use to display the block.

attributes: {
  api_coordinates: {
    type: 'string',
    source: 'meta',
    meta: 'api_coordinates',
  }, //...
}