Draw a half pixel horizontal line

HairlineView.swift

import UIKit

// How to use
//    Drag an UIView to storyboard, set constraints. 
//    Set the height constraint priority to less than 1000.
//    The view will override that to half pixel.
class HairlineView: UIView {
    override init(frame: CGRect) {
        super.init(frame: frame)
    }

    required init(coder decoder: NSCoder) {
        super.init(coder: decoder)
    }

    override func updateConstraints() {
        let height = (1.0 / UIScreen.mainScreen().scale)
        addConstraint(NSLayoutConstraint(item: self, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1.0, constant: height))

        super.updateConstraints()
    }
}