Resolving Data Race in iOS Swift

I have NetworkProvider which will make api call continuously and also once I received the data i will update the userid. At the same time I will access the userid from other functions.

This is data race condition, can someone help to remove the condition.

`

class NetworkProvider  {
   public var userID: String

   func observeStateChange() {

        FIRAuth.auth()?.addStateDidChangeListener({ (auth, authenticatedUser) in
          if let user = authenticatedUser {
              userID = user.uid
          }
       }
    }

   func currentUserID() -> String {
        return self.userID
  }
}`