Response to https://www.reddit.com/r/swift/comments/5gehjj/how_can_i_refactor_this_to_be_more_dry/

RubberDuckyController.swift

import UIKit

final class RubberDuckyController : UIViewController {
    @IBOutlet weak var  leftButton : UIButton!
    @IBOutlet weak var rightButton : UIButton!
    
    @IBOutlet weak var   textLeftButtonUnderline : UIView!
    @IBOutlet weak var shapeRightButtonUnderline : UIView!
    
    enum Selection {
        case left
        case right
    }
    
    private var selection : Selection = .left {
        didSet {
            guard selection != oldValue else { return }
            refreshSelection()
        }
    }
    
    public override func viewDidLoad() {
        super.viewDidLoad()
        refreshSelection()
    }
    
    private func refreshSelection() {
        textLeftButtonUnderline  .isHidden = (selection != .left ) // Hide the left underline if left is not selected
        shapeRightButtonUnderline.isHidden = (selection != .right) // Hide the right underline if right is not selected
    }
    
    // Hook both button's touchUpInside events to this IBAction
    @IBAction func onButtonTapped(_ sender: UIButton) {
        switch sender {
        case leftButton  : selection = .left
        case rightButton : selection = .right
        default: assertionFailure()  // This can only happen if an unexpected sender
        }
    }
}