static.swift

import UIKit

class VC: UIViewController {
  func formatter() -> DateFormatter {
    struct Formatter {
      static let f: DateFormatter = {
        let f = DateFormatter()
        f.dateStyle = .medium
        print("I am called just once")
        return f
      }()
    }
    
    return Formatter.f
  }
}

VC().formatter() === VC().formatter() // true, and printing just once.