Singleton and init with parameter

Question

I want to use the singleton pattern in my class which has a private init with parameter. It also has a class function called setup which configures

Answer 1

A simplified version of different answers; but without any force unwraps, not having shared as a func and having the possibility to have a config. It doesn't need to be more complicated then this, which works well with Swift 5:

import UIKit

final class ParameterSingleton {

  static var shared: ParameterSingleton {
    if let initializedShared = _shared {
      return initializedShared
    }
    fatalError("Singleton not yet initialized. Run setup(withConfig:) first")
  }
  private static var _shared: ParameterSingleton? // This can only be set by the setup() func since it is private
  private var config: ParameterSingletonConfig // The configuration for the singleton. Could be a `String` if so requested

  /// The ParameterSingleton setup func. Will initialize the singleton with the config. Without a config, `shared` will cause a `fatalError`
  ///
  /// - Parameter config: The config needed for initializing the singleton
  class func setup(withConfig config: ParameterSingletonConfig) {
    _shared = ParameterSingleton(withConfig: config)
  }


  // Make the init private so this class can only be used be invoking the `setup(withConfig:)` func
  private init(withConfig config: ParameterSingletonConfig) {
    self.config = config
  }

  /// The public func to do something
  func doSomething() {
    print("Config URL: \(config.url)")
  }
}

struct ParameterSingletonConfig {
  let url: String
}

//Will cause fatalError
ParameterSingleton.shared.doSomething()

//Will not cause fatalError
ParameterSingleton.setup(withConfig: ParameterSingletonConfig(url: "http://www.google.com"))
ParameterSingleton.shared.doSomething()

Of course, if you only need to set one parameter, you could remove ParameterSingletonConfig and replace it with a String instead