Add a border to a UIButton. Original code - http://stackoverflow.com/a/21881788/1077789

UIButton+Border.swift

import Foundation
import UIKit

public enum UIButtonBorderSide {
    case Top, Bottom, Left, Right
}

extension UIButton {
    
    public func addBorder(side: UIButtonBorderSide, color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.CGColor
        
        switch side {
        case .Top:
            border.frame = CGRect(x: 0, y: 0, width: frame.size.width, height: width)
        case .Bottom:
            border.frame = CGRect(x: 0, y: self.frame.size.height - width, width: self.frame.size.width, height: width)
        case .Left:
            border.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
        case .Right:
            border.frame = CGRect(x: self.frame.size.width - width, y: 0, width: width, height: self.frame.size.height)
        }
        
        self.layer.addSublayer(border)
    }
}

Swift 4 + we can add this extension to UIView so its available across more elements.

public enum UIButtonBorderSide {
    case top, bottom, left, right
}

extension UIView {
    
    public func addBorder(side: UIButtonBorderSide, color: UIColor, width: CGFloat) {
        let border = CALayer()
        border.backgroundColor = color.cgColor
        
        switch side {
        case .top:
            border.frame = CGRect(x: 0, y: 0, width: frame.size.width, height: width)
        case .bottom:
            border.frame = CGRect(x: 0, y: self.frame.size.height - width, width: self.frame.size.width, height: width)
        case .left:
            border.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
        case .right:
            border.frame = CGRect(x: self.frame.size.width - width, y: 0, width: width, height: self.frame.size.height)
        }
        
        self.layer.addSublayer(border)
    }
}

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