Flashes screen with white rectangle. Taken from: https://realm.io/news/tryswift-adam-bell-prototyping-magic/

UIWindow+Flash.swift

import UIKit

public extension UIWindow {
  public func flash() {
    let flashView = UIView(frame: bounds)
    flashView.userInteractionEnabled = false
    flashView.backgroundColor = UIColor.whiteColor()
    addSubview(flashView)
    
    UIView.animateWithDuration(0.75, delay: 0.0, options: .CurveEaseOut, animations: {
        flashView.alpha = CGFloat(0.0)
    }, completion: { (finished) in
      flashView.removeFromSuperview()
    })
  }
}