<?php

// Registers a REST field for the Activity Endpoints.
function example_register_activity_rest_field() {
	bp_rest_register_field(
		'activity',      // Id of the BuddyPress component the REST field is about
		'example_field', // Used into the REST response/request
		array(
			'get_callback'    => 'example_get_rest_field_callback',    // The function to use to get the value of the REST Field
			'update_callback' => 'example_update_rest_field_callback', // The function to use to update the value of the REST Field
			'schema'          => array(                                // The example_field REST schema.
				'description' => 'Example of Activity Meta Field',
				'type'        => 'string',
				'context'     => array( 'view', 'edit' ),
			),
		)
	);
}
add_action( 'bp_rest_api_init', 'example_register_activity_rest_field' );

/**
 * The function to use to get the value of the REST Field.
 *
 * @param  array  $array     The list of properties of the BuddyPress component's object.
 * @param  string $attribute The REST Field key used into the REST response.
 * @return string            The value of the REST Field to include into the REST response.
 */
function example_get_rest_field_callback( $array, $attribute ) {
	// The key of the metadata can be different from the REST Field key.
	$metadata_key = '_example_metadata_key';

	return bp_activity_get_meta( $array['id'], $metadata_key );
}

/**
 * The function to use to update the value of the REST Field.
 *
 * @param  object $object    The BuddyPress component's object that was just created/updated during the request.
 *                           (in this case the BP_Activity_Activity object).
 * @return string $value     The value of the REST Field to save.
 * @param  string $attribute The REST Field key used into the REST response.
 */
function example_update_rest_field_callback( $object, $value, $attribute ) {
	// The key of the metadata can be different from the REST Field key.
	$metadata_key = '_example_metadata_key';

	bp_activity_update_meta( $object->id, $metadata_key, $value );
}