Invoice.swift

struct Invoice {
   private var invoiceId = 0
   private var date = Date()
   private var items = [Item]()
   private var name = ""
   private var paid = false
   private var paymentData: Date? 
   
   func paid() {
       paid = true
       paymentDate = Date()
   }
}

class Payment {
    func pay(invoice: Invoice) -> Invoice {
        // do something with invoice here
        invoice.paid()
        return invoice
    }
}