Hello, Iam trying this to call onchange in account invoice state field

from openerp import models, fields, api 
import logging
_logger = logging.getLogger(__name__)

class invoice(models.Model):
_inherit = 'account.invoice'

@api.onchange('state')
 def _check_onchange_invoice_state(self): 
_logger.info('=======================================================')
_logger.info(self.id)
_logger.info('=======================================================')

I now this field have "track_visibility" and I don't know if I can used or not but I really need it. Any help?

I also try @api.onchange('write_date') but nothing happens :(.