[AddOn Development for e-invoice for Aruba]

Hi, 

i am developing an addon for e-invoice to send the invoice data to the Aruba e-invoice service.
I am searching for some methods to retrive custom fields of single user.

 // $invoice is an array with all the information about the invoice

 $user_data      = $invoice["user_data"];
 $items          = Invoices::get_items($invoice["id"]); // array of items inside the invoice
 $user_id 		 = $user_data["id"];

 // $user_custom_fields // method to get the user custom fields by $user_id

Is there a documentation with all the classes and methods?

Thx