Calling substituteFontName

callSubstituteFileName.swift

class AppDelegate: UIResponder, UIApplicationDelegate {

    ...

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: 
        [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        ...
        UILabel.appearance().substituteFontName = "Gills Sans"
        ...
        return true
    }
    ...
}