ios 导航栏

将导航栏设置为透明

   //背景设置为空
self.navigationController?.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: .Default)
   //去除横线
   self.navigationController?.navigationBar.shadowImage = UIImage()
   //设置为透明
   self.navigationController?.navigationBar.isTranslucent = true

导航栏的颜色随着滚动条上翻渐渐从透明变成不透明
此时注意，一定需要将istranslucent设为true，不然不管怎么改alpha值，导航栏都会有背景色。

func scrollViewDidScroll(_ scrollView: UIScrollView) {

    let offset = scrollView.contentOffset.y
    //大标题模式下的数值
    if offset <= -88 {
        naviAlpha = 0
    } else if offset < -22 {
        naviAlpha = (offset+88)/64
    } else {
        naviAlpha = 1
    }
    let image = UIImage.imageWithColor(Colors.naviBackground.withAlphaComponent(naviAlpha))
    navigationController?.navigationBar.setBackgroundImage(image, for: .default)
}