Create a view equal to exactly one real pixel in height, even on Retina and Retina HD displays.

HairlineView.swift

//
//  HairlineView.swift
//
//  Created by Ayden Panhuyzen on 2018-04-02.
//  Copyright © 2018 Ayden Panhuyzen. All rights reserved.
//  https://gist.github.com/aydenp
//

import UIKit

class HairlineView: UIView {
    private var constraint: NSLayoutConstraint?
    
    convenience init(axis: NSLayoutConstraint.Axis) {
        self.init(frame: .zero)
        defer { self.axis = axis }
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }
    
    private func setup() {
        translatesAutoresizingMaskIntoConstraints = false
        constraint?.isActive = false
        constraint = (axis == .horizontal ? heightAnchor : widthAnchor).constraint(equalToConstant: desiredSize)
        constraint!.isActive = true
    }
    
    override func didMoveToWindow() {
        super.didMoveToWindow()
        constraint?.constant = desiredSize
    }
    
    var axis: NSLayoutConstraint.Axis = .horizontal {
        didSet { setup() }
    }
    
    private var desiredSize: CGFloat {
        return 1 / (window?.screen.scale ?? 1)
    }
}