// Register our routes.
function prefix_register_my_comment_route() {
    register_rest_route( 'my-namespace/v1', '/comments', array(
        // Notice how we are registering multiple endpoints the 'schema' equates to an OPTIONS request.
        array(
            'methods'  => 'GET',
            'callback' => 'prefix_get_comment_sample',
        ),
        // Register our schema callback.
        'schema' => 'prefix_get_comment_schema',
    ) );
}

add_action( 'rest_api_init', 'prefix_register_my_comment_route' );

/**
 * Grabs the five most recent comments and outputs them as a rest response.
 *
 * @param WP_REST_Request $request Current request.
 */
function prefix_get_comment_sample( $request ) {
    $args = array(
        'post_per_page' => 5,
    );
    $comments = get_comments( $args );

    $data = array();

    if ( empty( $comments ) ) {
        return rest_ensure_response( $data );
    }

    foreach ( $comments as $comment ) {
        $response = prefix_rest_prepare_comment( $comment, $request );
        $data[] = prefix_prepare_for_collection( $response );
    }

    // Return all of our comment response data.
    return rest_ensure_response( $data );
}

/**
 * Matches the comment data to the schema we want.
 *
 * @param WP_Comment $comment The comment object whose response is being prepared.
 */
function prefix_rest_prepare_comment( $comment, $request ) {
    $comment_data = array();

    $schema = prefix_get_comment_schema();

    // We are also renaming the fields to more understandable names.
    if ( isset( $schema['properties']['id'] ) ) {
        $comment_data['id'] = (int) $comment->comment_id;
    }

    if ( isset( $schema['properties']['author'] ) ) {
        $comment_data['author'] = (int) $comment->user_id;
    }

    if ( isset( $schema['properties']['content'] ) ) {
        $comment_data['content'] = apply_filters( 'comment_text', $comment->comment_content, $comment );
    }

    return rest_ensure_response( $comment_data );
}

/**
 * Prepare a response for inserting into a collection of responses.
 *
 * This is copied from WP_REST_Controller class in the WP REST API v2 plugin.
 *
 * @param WP_REST_Response $response Response object.
 * @return array Response data, ready for insertion into collection data.
 */
function prefix_prepare_for_collection( $response ) {
    if ( ! ( $response instanceof WP_REST_Response ) ) {
        return $response;
    }

    $data = (array) $response->get_data();
    $server = rest_get_server();

    if ( method_exists( $server, 'get_compact_response_links' ) ) {
        $links = call_user_func( array( $server, 'get_compact_response_links' ), $response );
    } else {
        $links = call_user_func( array( $server, 'get_response_links' ), $response );
    }

    if ( ! empty( $links ) ) {
        $data['_links'] = $links;
    }

    return $data;
}

/**
 * Get our sample schema for comments.
 */
function prefix_get_comment_schema() {
    $schema = array(
        // This tells the spec of JSON Schema we are using which is draft 4.
        '$schema'              => 'http://json-schema.org/draft-04/schema#',
        // The title property marks the identity of the resource.
        'title'                => 'comment',
        'type'                 => 'object',
        // In JSON Schema you can specify object properties in the properties attribute.
        'properties'           => array(
            'id' => array(
                'description'  => esc_html__( 'Unique identifier for the object.', 'my-textdomain' ),
                'type'         => 'integer',
                'context'      => array( 'view', 'edit', 'embed' ),
                'readonly'     => true,
            ),
            'author' => array(
                'description'  => esc_html__( 'The id of the user object, if author was a user.', 'my-textdomain' ),
                'type'         => 'integer',
            ),
            'content' => array(
                'description'  => esc_html__( 'The content for the object.', 'my-textdomain' ),
                'type'         => 'string',
            ),
        ),
    );

    return $schema;
}

// Register our routes.
function prefix_register_my_arg_route() {
    register_rest_route( 'my-namespace/v1', '/schema-arg', array(
        // Here we register our endpoint.
        array(
            'methods'  => 'GET',
            'callback' => 'prefix_get_item',
            'args' => prefix_get_endpoint_args(),
        ),
    ) );
}

// Hook registration into 'rest_api_init' hook.
add_action( 'rest_api_init', 'prefix_register_my_arg_route' );

/**
 * Returns the request argument `my-arg` as a rest response.
 *
 * @param WP_REST_Request $request Current request.
 */
function prefix_get_item( $request ) {
    // If we didn't use required in the schema this would throw an error when my arg is not set.
    return rest_ensure_response( $request['my-arg'] );
}

/**
 * Get the argument schema for this example endpoint.
 */
function prefix_get_endpoint_args() {
    $args = array();

    // Here we add our PHP representation of JSON Schema.
    $args['my-arg'] = array(
        'description'       => esc_html__( 'This is the argument our endpoint returns.', 'my-textdomain' ),
        'type'              => 'string',
        'validate_callback' => 'prefix_validate_my_arg',
        'sanitize_callback' => 'prefix_sanitize_my_arg',
        'required'          => true,
    );

    return $args;
}

/**
 * Our validation callback for `my-arg` parameter.
 *
 * @param mixed           $value   Value of the my-arg parameter.
 * @param WP_REST_Request $request Current request object.
 * @param string          $param   The name of the parameter in this case, 'my-arg'.
 */
function prefix_validate_my_arg( $value, $request, $param ) {
    $attributes = $request->get_attributes();

    if ( isset( $attributes['args'][ $param ] ) ) {
        $argument = $attributes['args'][ $param ];
        // Check to make sure our argument is a string.
        if ( 'string' === $argument['type'] && ! is_string( $value ) ) {
            return new WP_Error( 'rest_invalid_param', sprintf( esc_html__( '%1$s is not of type %2$s', 'my-textdomain' ), $param, 'string' ), array( 'status' => 400 ) );
        }
    } else {
        // This code won't execute because we have specified this argument as required.
        // If we reused this validation callback and did not have required args then this would fire.
        return new WP_Error( 'rest_invalid_param', sprintf( esc_html__( '%s was not registered as a request argument.', 'my-textdomain' ), $param ), array( 'status' => 400 ) );
    }

    // If we got this far then the data is valid.
    return true;
}

/**
 * Our santization callback for `my-arg` parameter.
 *
 * @param mixed           $value   Value of the my-arg parameter.
 * @param WP_REST_Request $request Current request object.
 * @param string          $param   The name of the parameter in this case, 'my-arg'.
 */
function prefix_sanitize_my_arg( $value, $request, $param ) {
    $attributes = $request->get_attributes();

    if ( isset( $attributes['args'][ $param ] ) ) {
        $argument = $attributes['args'][ $param ];
        // Check to make sure our argument is a string.
        if ( 'string' === $argument['type'] ) {
            return sanitize_text_field( $value );
        }
    } else {
        // This code won't execute because we have specified this argument as required.
        // If we reused this validation callback and did not have required args then this would fire.
        return new WP_Error( 'rest_invalid_param', sprintf( esc_html__( '%s was not registered as a request argument.', 'my-textdomain' ), $param ), array( 'status' => 400 ) );
    }

    // If we got this far then something went wrong don't use user input.
    return new WP_Error( 'rest_api_sad', esc_html__( 'Something went terribly wrong.', 'my-textdomain' ), array( 'status' => 500 ) );
}