TKTokenPasswordAuthOperation get password

Question

I am developing Crypto Token kit extension in iOS application and using pin base authentication. iOS provides TKTokenPasswordAuthOperation class which has variable password. When "beginAuthFor operation" delegate method call i am returning TKTokenPasswordAuthOperation() and i got enter pin screen but when i enter pin how i can get that pin. every time TKTokenPasswordAuthOperation class password variable is nil. Any idea how i can get user input?. This is delegate method which show Enter Pin screen

func tokenSession(_ session: TKTokenSession, beginAuthFor operation: TKTokenOperation, constraint: Any) throws -> TKTokenAuthOperation {        
     return TKTokenPasswordAuthOperation()
}