class AnimatableSpriteLayer: CALayer {
 private var animationValues = [CGFloat]()
 convenience init(spriteSheetImage: UIImage, spriteFrameSize: CGSize ) {
 self.init()
 //1
 masksToBounds = true
 contentsGravity = CALayerContentsGravity.left
 contents = spriteSheetImage.cgImage
 bounds.size = spriteFrameSize
 //2
 let frameCount = Int(spriteSheetImage.size.width / spriteFrameSize.width)
 for frameIndex in 0..<frameCount {
  animationValues.append(CGFloat(frameIndex) / CGFloat(frameCount))
 }
 }
 
 func play() {
 let spriteKeyframeAnimation = CAKeyframeAnimation(keyPath: "contentsRect.origin.x")
 spriteKeyframeAnimation.values = animationValues
 spriteKeyframeAnimation.duration = 2.0
 spriteKeyframeAnimation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.linear)
 //3
 spriteKeyframeAnimation.calculationMode = CAAnimationCalculationMode.discrete
 add(spriteKeyframeAnimation, forKey: "spriteKeyframeAnimation")
 }
}

class DustEffectView: UIView {
 private func createDustImages(image: UIImage) -> [UIImage] {
 var result = [UIImage]()
 guard let inputCGImage = image.cgImage else {
  return result
 }
 //1
 let colorSpace = CGColorSpaceCreateDeviceRGB()
 let width = inputCGImage.width
 let height = inputCGImage.height
 let bytesPerPixel = 4
 let bitsPerComponent = 8
 let bytesPerRow = bytesPerPixel * width
 let bitmapInfo = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Little.rawValue
 
 guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
  return result
 }
 context.draw(inputCGImage, in: CGRect(x: 0, y: 0, width: width, height: height))
 guard let buffer = context.data else {
  return result
 }
 let pixelBuffer = buffer.bindMemory(to: UInt32.self, capacity: width * height)
 //2
 let imagesCount = 32
 var framePixels = Array(repeating: Array(repeating: UInt32(0), count: width * height), count: imagesCount)
 for column in 0..<width {
  for row in 0..<height {
  let offset = row * width + column
  //3
  for _ in 0...1 { 
   let factor = Double.random(in: 0..<1) + 2 * (Double(column)/Double(width))
   let index = Int(floor(Double(imagesCount) * ( factor / 3)))
   framePixels[index][offset] = pixelBuffer[offset]
  }
  }
 }
 //4
 for frame in framePixels {
  let data = UnsafeMutablePointer(mutating: frame)
  guard let context = CGContext(data: data, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
  continue
  }
  result.append(UIImage(cgImage: context.makeImage()!, scale: image.scale, orientation: image.imageOrientation))
 }
 return result
 }
}

 let layer = CALayer()
 layer.frame = bounds
 layer.contents = image.cgImage
 self.layer.addSublayer(layer)
 let centerX = Double(layer.position.x)
 let centerY = Double(layer.position.y)
 let radian1 = Double.pi / 12 * Double.random(in: -0.5..<0.5)
 let radian2 = Double.pi / 12 * Double.random(in: -0.5..<0.5)
 let random = Double.pi * 2 * Double.random(in: -0.5..<0.5)
 let transX = 60 * cos(random)
 let transY = 30 * sin(random)
 //1: 
 // x' = x*cos(rad) - y*sin(rad)
 // y' = y*cos(rad) + x*sin(rad)
 let realTransX = transX * cos(radian1) - transY * sin(radian1)
 let realTransY = transY * cos(radian1) + transX * sin(radian1)
 let realEndPoint = CGPoint(x: centerX + realTransX, y: centerY + realTransY)
 let controlPoint = CGPoint(x: centerX + transX, y: centerY + transY)
 //2:
 let movePath = UIBezierPath()
 movePath.move(to: layer.position)
 movePath.addQuadCurve(to: realEndPoint, controlPoint: controlPoint)
 let moveAnimation = CAKeyframeAnimation(keyPath: "position")
 moveAnimation.path = movePath.cgPath
 moveAnimation.calculationMode = .paced
 //3:   
 let rotateAnimation = CABasicAnimation(keyPath: "transform.rotation")
 rotateAnimation.toValue = radian1 + radian2
 let fadeOutAnimation = CABasicAnimation(keyPath: "opacity")
 fadeOutAnimation.toValue = 0.0
 let animationGroup = CAAnimationGroup()
 animationGroup.animations = [moveAnimation, rotateAnimation, fadeOutAnimation]
 animationGroup.duration = 1
 //4:
 animationGroup.beginTime = CACurrentMediaTime() + 1.35 * Double(i) / Double(imagesCount)
 animationGroup.isRemovedOnCompletion = false
 animationGroup.fillMode = .forwards
 layer.add(animationGroup, forKey: nil)