Officially, this is only possible by setting the shadowImage
of the navigationBar to an empty image. However, a closer look at the documentation, it is said:
For a custom shadow image to be shown, a custom background image must also be set with the setBackgroundImage:forBarMetrics: method. If the default background image is used, then the default shadow image will be used regardless of the value of this property.
By using a custom background image, you would lose the blurred background translucency.
If you feel adventurous, the "hairline" is a UIImageView that is a subview of the navigation bar. You can find it and set it as hidden. This is what Apple does in their native calendar app, for example. Remember to show it when the current view disappears.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…