func captureScreen() -> UIImage {
 let layer: CALayer = UIApplication.sharedApplication().keyWindow!.layer
 let scale: CGFloat = UIScreen.mainScreen().scale
 UIGraphicsBeginImageContextWithOptions(layer.frame.size, layer.opaque, scale)
 layer.renderInContext(UIGraphicsGetCurrentContext())
 let image = UIGraphicsGetImageFromCurrentImageContext()
 UIGraphicsEndImageContext()
 return image
}