Q:

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

Note, I'm looking to make a 1px line, not a 1pt line.  Meaning it should be 1px regardless of screen scale (so 0.5pt on Retina devices).
I can do this programmatically, but can I do it in the Interface Builder?  For example I cannot set a UIView to have a height of less than 1.
If I can do it in IB then I don't have to declare an outlet and manually set the frame in awakeFromNib.

A:

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//enforces it to be a true 1 pixel line

Objective-C
self.onePixelViewHeightConstraint.constant = 1.f/[UIScreen mainScreen].scale;//enforces it to be a true 1 pixel line

Enjoy

A:

I created NSLayoutConstraint subclass:
class HairlineConstraint: NSLayoutConstraint {
    override func awakeFromNib() {
        super.awakeFromNib()

        self.constant = 1.0 / UIScreen.main.scale
    }
}

Then simply create your view in interface builder, add height constraint

and set its class to HairlineConstraint.

Done.

A:

By creating this tiny subclass of NSLayoutConstraint I'm now able to add 1px lines in IB:
@implementation NSLayoutConstraintHairline

-(void)awakeFromNib
{
    [super awakeFromNib];
    if ( self.constant == 1 ) self.constant = 1/[UIScreen mainScreen].scale;
}

@end

Any constraint with a value of 1 can be set to class NSLayoutConstraintHairline to make the constant effectively 1px instead of 1pt.
If you ever decide to change the constant to another value, it will just work as any other constraint.

