How do I create a 1px line in Interface Builder?

Just in case someone else comes here wanting to know how it can be done programmatically, heres how you do it:

Interface Builder

Make a height constraint in IB to the desired view and set the constant to 1.

Then you will need to CTRL+Drag from the constraint into your custom view or ViewController.

Whenever the Xib is loaded, be it in awakeFromNib or viewDidLoad, you are going to set the constant of the constraint to the scale of the display:

Swift

onePixelViewHeightConstraint.constant = 1/UIScreen.main.scale

Objective-C

self.onePixelViewHeightConstraint.constant = 1.f/[UIScreen mainScreen].scale;

Enjoy