tbusainvoicegen/api/v3/Invoicegen/Generate.php

<?php
use CRM_Tbusainvoicegen_ExtensionUtil as E;

/**
 * Invoicegen.Generate API specification (optional)
 * This is used for documentation and validation.
 *
 * @param array $spec description of fields supported by this API call
 * @return void
 * @see http://wiki.civicrm.org/confluence/display/CRMDOC/API+Architecture+Standards
 */
function _civicrm_api3_invoicegen_Generate_spec(&$spec) {
  $spec['contact_id']['api.required'] = 0;
  $spec['billing_period']['api.required'] = 1;
}

/**
 * Invoicegen.Generate API
 *
 * @param array $params
 * @return array API result descriptor
 * @see civicrm_api3_create_success
 * @see civicrm_api3_create_error
 * @throws API_Exception
 */
function civicrm_api3_invoicegen_Generate($params) {
  $returnValues = CRM_Tbusainvoicegen_Timebank::generate($params['contact_id'], $params['billing_period']);
  return civicrm_api3_create_success($returnValues, $params, 'Invoicegen', 'generate');
}