Draws a `singlePixelLine` line above or below the given point, depending on `topBias`.

Hairline.swift

import UIKit
 
func singlePixelLine(at y: CGFloat, in rect: CGRect, topBias: Bool = true) {
  let offset = pixelUnit/2.0
  let adjustedY = round(from: y - offset, fraction: pixelUnit, down: topBias) + offset
  let line = makeLine(at: adjustedY, in: rect)
  strokePath(line, width: pixelUnit)
}
 
 
var pixelUnit: CGFloat {
  return 1.0 / UIScreen.main().scale
}

 
func round(from: CGFloat, fraction: CGFloat, down: Bool = true) -> CGFloat {
  let expanded = from / fraction
  let rounded = (down ? floor : ceil)(expanded)
  return rounded * fraction
} 


func makeLine(at y: CGFloat, in rect: CGRect) -> UIBezierPath {
  precondition((rect.minY...rect.maxY).contains(y))
  let line = UIBezierPath()
  line.move(to: CGPoint(x: rect.minX, y: y))
  line.addLine(to: CGPoint(x: rect.maxX, y: y))
  return line
}
 
 
func strokePath(_ path: UIBezierPath, width: CGFloat) {
  path.lineWidth = width
  print(path)
  UIColor.blue().setStroke()
  path.stroke()
}