Swift UIView background color opacity

Setting alpha property of a view affects its subviews. If you want just transparent background set view's backgroundColor proprty to a color that has alpha component smaller than 1.

view.backgroundColor = UIColor.white.withAlphaComponent(0.5)

You can set background color of view to the UIColor with alpha, and not affect view.alpha:

view.backgroundColor = UIColor(white: 1, alpha: 0.5)

or

view.backgroundColor = UIColor.red.withAlphaComponent(0.5)

You can also set it from InterfaceBuilder by changing color's opacity:

For Swift 4.x and above

yourView.backgroundColor = UIColor.black.withAlphaComponent(0.5)