run something only once

RunOnce.swift

import Foundation

public class Once {

  var already: Bool = false

  public init() { }

  public func run(_ block: () -> Void) {
    guard !already else {
      return
    }
    block()
    already = true
  }
}

//////
let once = Once()
once.run {
  // do something
}

once.run {
  // no effect
}