Exemplo de organização e uso de constantes

Constants.swift

struct Constants {
    
    struct UserDefaults {
        static let keyForUID             = "uid"
        static let keyForOwnMessages     = "ownMessages"
        static let keyForAllMessages     = "allMessages"
        static let keyForNewMessages     = "newMessages"
        static let showNSFW              = "showNSFW"
        static let hideOwnMessages       = "hideOwnMessages"
        static let hideCollectedMessages = "hideCollectedMessages"
    }
    
    struct GeoFire {
        let path: String = "geofire"
        let visibleRadius: Double = 0.2   // in kilometers
        let collectRadius: Double = 0.025 // in kilometers
    }
}

ExampleViewController.swift

import UIKit

class ExampleViewController: UIViewController {

    func viewDidLoad() {
        super.viewDidLoad()
        
        print(Constants.UserDefaults.keyForOwnMessages)
        print(Constants.GeoFire.visibleaRadius)
    }
}