How to execute an action only once in Swift

Issue #10

There is time we want to execute an action only once. We can surely introduce a flag, but it will be nicer if we abstract that out using composition. Then we have

class Once {

  var already: Bool = false

  func run(@noescape block: () -> Void) {
    guard !already else { 
      return 
    }

    block()
    already = true
  }
}

Usage

class ViewController: UIViewController {
  let once = Once()

  override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(animated)

    once.run {
      cameraMan.setup()
    }
  }
}

In the same way, we can check to run a closure when a value changes

final class WhenChange<T: Equatable> {
    private(set) var value: T

    init(value: T) {
        self.value = value
    }

    func run(newValue: T, closure: (T) -> Void) {
        if newValue != value {
            value = newValue
            closure(value)
        }
    }
}