A simple, @IBDesignable view which draws a pixel wide hairline of chosen color in chosen direction

HairlineView

import Foundation
import UIKit
import QuartzCore

enum HairlineDirection: Int {
    case Left, Right, Top, Bottom, None
    
    init(point: CGPoint) {
        if point.x < 0 {
            self = Left
        } else if point.x > 0 {
            self = Right
        } else if point.y > 0 {
            self = Top
        } else if point.y < 0 {
            self = Bottom
        } else {
            self = None
        }
    }
}

@IBDesignable
class HairlineView: UIView {
    
    @lazy var hairlineLayer: CALayer = {
        return CALayer()
    }()
    
    @IBInspectable var hairlineColor: UIColor = UIColor.blackColor() {
        didSet { hairlineLayer.backgroundColor = hairlineColor.CGColor }
    }
    
    @IBInspectable var direction: CGPoint = CGPointZero {
        didSet { setNeedsLayout() }
    }
    
    let Thickness = 1.0/UIScreen.mainScreen().scale
    
    override func layoutSubviews() {
    
        super.layoutSubviews()
        
        if hairlineLayer.superlayer != layer {
            layer.addSublayer(hairlineLayer)
        }
        
        var hairlineFrame = CGRectZero
        let hairlineDirection = HairlineDirection(point: direction)
        
        switch hairlineDirection {
        case .Left:
            hairlineFrame = CGRect(x: 0, y: 0, width: Thickness, height: bounds.height)
        case .Right:
            hairlineFrame = CGRect(x: bounds.width-Thickness, y: 0, width: Thickness, height: bounds.height)
        case .Top:
            hairlineFrame = CGRect(x: 0, y: 0, width: bounds.width, height: Thickness)
        case .Bottom:
            hairlineFrame = CGRect(x: 0, y: bounds.height-Thickness, width: bounds.width, height: Thickness)
        default:
            hairlineFrame = CGRectZero
        }
        
        hairlineLayer.frame = hairlineFrame
    }
}