func startTimer() {
    if imagePaths.count <= 1 { return }

    stopTimer()
    let dispatchTimer = DispatchSource.makeTimerSource()
    dispatchTimer.schedule(wallDeadline: .now()+duration, repeating: duration)
    dispatchTimer.setEventHandler { [weak self] in
        DispatchQueue.main.async {
            self?.scroll()
        }
    }
    dispatchTimer.resume()
    timer = dispatchTimer
}

func stopTimer() {
    timer?.cancel()
    timer = nil
}

fileprivate func scroll() {
    //帶動畫改變 scrollView 的偏移量實現自動滾動
    self.scrollView?.setContentOffset(CGPoint(x: self.scrollView!.bounds.size.width * 2, y: 0), animated: true)
}