Demonstrates how to add a bottom border to a UITextField.

TextFieldBorderPlayground.swift

import UIKit
import PlaygroundSupport

//-- Container view
let container = UIView(frame: CGRect(x: 0, y: 0, width: 400, height: 500))
container.backgroundColor = .darkGray

//-- Textfield
let textField = UITextField(frame: CGRect(x: 0, y: 0, width: container.bounds.width, height: 50))
textField.text = "There once was a man from Nantucket..."
textField.borderStyle = .none
textField.backgroundColor = .blue
textField.textColor = .white
textField.layer.masksToBounds = true

//-- Border
let borderWidth: CGFloat = 3
let borderLayer = CALayer();
borderLayer.frame = CGRect(
    x: 0,
    y: textField.frame.height - borderWidth,
    width: textField.frame.width,
    height: borderWidth
)
borderLayer.borderWidth = borderWidth
borderLayer.borderColor = UIColor.yellow.cgColor

//-- Add layers and subviews
textField.layer.addSublayer(borderLayer)
container.addSubview(textField)

//-- Playground boilerplate
PlaygroundPage.current.liveView = container
PlaygroundPage.current.needsIndefiniteExecution = true