What Is a Singleton and How To Create One In Swift (https://cocoacasts.com/what-is-a-singleton-and-how-to-create-one-in-swift/)

NetworkManager.swift

import Foundation

enum API {

    static let baseURL = URL(string: "https://cocoacasts.com")!
    
}

class NetworkManager {

    // MARK: - Properties

    private static var sharedNetworkManager: NetworkManager = {
        let networkManager = NetworkManager(baseURL: API.baseURL)

        // Configuration
        // ...

        return networkManager
    }()

    // MARK: -

    let baseURL: URL

    // Initialization

    private init(baseURL: URL) {
        self.baseURL = baseURL
    }

    // MARK: - Accessors

    class func shared() -> NetworkManager {
        return sharedNetworkManager
    }
    
}