In v8, instead of fields.function, we use the compute field and @api.depends if the field depends on some other fields and that can be dotted paths when using sub-fields.

A simple example:

name = fields.Char(string="Name")
upper = fields.Char(compute='_compute_upper', inverse='_inverse_upper')

@api.depends('name')
def _compute_upper(self):
   for rec in self:
      self.upper = self.name.upper() if self.name else False

def _inverse_upper(self):
   for rec in self:
      self.name = self.upper.lower() if self.upper else False 

I can help you better if you tell me what exactly you want to do, like which are the fields or paste your code.