override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
  let pos = touches.first!.location(in: view)
  let center = view!.center
  let xDist = pos.x - center.x
  let yDist = pos.y - center.y
  let distance = sqrt((xDist * xDist) + (yDist * yDist)) * 0.02
  camera?.position.x += xDist * distance * 0.02
  camera?.position.y -= yDist * distance * 0.02
}

private var difference = CGVector()

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
  let pos = touches.first!.location(in: view)
  let center = view!.center
  difference = CGVector(dx: pos.x - center.x, dy: pos.y - center.y)
}

override func update(_ currentTime: TimeInterval) {
  let distance = sqrt((difference.dx * difference.dx) + (difference.dy * difference.dy)) * 0.02
  camera?.position.x += difference.dx * distance * 0.02
  camera?.position.y -= difference.dy * distance * 0.02
}

private var difference = CGVector()
private var touching: Bool = false
private var fade: CGFloat = 0

private func touched(_ touch: UITouch) {
  let pos = touch.location(in: view)
  let center = view!.center
  difference = CGVector(dx: pos.x - center.x, dy: pos.y - center.y)
}

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
  touching = true
  touched(touches.first!)
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
  touched(touches.first!)
}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
  touching = false
  touched(touches.first!)
}

override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent?) {
  touching = false
}

override func update(_ currentTime: TimeInterval) {
  if touching {
    if fade < 0.02 { fade += 0.0005 }
  } else {
    if fade > 0 { fade -= 0.0005 }
  }
  let distance = sqrt((difference.dx * difference.dx) + (difference.dy * difference.dy)) * 0.01
  camera?.position.x += difference.dx * distance * fade
  camera?.position.y -= difference.dy * distance * fade
}