how to call the function in class

No, that is not necessary (hopefully).

struct Weather{
    var city :String?
    var temp :String?
    var weather :String?
}
class ViewController: UIViewController {
    @IBOutlet weak var labelWeather: UILabel!
    @IBOutlet weak var labelCity: UILabel!
    @IBOutlet weak var labelTemp: UILabel!

    var weatherData :Weather?{
        didSet{
            confView()
        }
    }

    func confView(){
       labelCity.text = self.weatherData?.city
    }
}

Somehow, it works like this (may be different, with multi pass of compiler, but that's essentially the mechanisms)

- The compiler reads the code, keep track of all declared calls to func, like line 13.

- Then, when it meets the declaration line 17, it completes the missing information.

Note that would be different in playground, which is not compiled, but somehow interpreted (with optimization).