A segmented control without the borders. Allows for a subtle background on the unselected states, and a tint on the selected state.

ModernSegmentedControl.swift

//
//  ModernSegmentedControl.swift
//  
//
//  Created by Curtis Herbert on 3/8/18.
//

import Foundation

@objc final class ModernSegmentedControl: UIControl {

    var buttonBackgroundColor: UIColor = UIColor.buttonBackground {
        didSet {
            updateButtonColors()
        }
    }

    var selectedColor: UIColor = UIColor.primary {
        didSet {
            updateButtonColors()
        }
    }

    var buttonTitles: [String] = [] {
        didSet {
            let origSubviews = stackView.subviews
            for subview in origSubviews {
                stackView.removeArrangedSubview(subview)
                subview.removeFromSuperview()
            }

            for title in buttonTitles {
                let button = UIButton(type: .custom)
                button.translatesAutoresizingMaskIntoConstraints = false
                button.setTitle(title, for: UIControl.State.normal)
                button.titleLabel?.font = UIFontMetrics(forTextStyle: UIFont.TextStyle.callout).scaledFont(for: UIFont.systemFont(ofSize: 13.0))
                button.titleLabel?.adjustsFontForContentSizeCategory = true
                button.titleLabel?.textAlignment = .center
                button.titleLabel?.minimumScaleFactor = 0.1
                button.titleLabel?.adjustsFontSizeToFitWidth = true
                button.contentEdgeInsets = UIEdgeInsets(top: 8, left: 5, bottom: 8, right: 5)
                button.setContentHuggingPriority(contentHuggingPriority(for: NSLayoutConstraint.Axis.horizontal), for: NSLayoutConstraint.Axis.horizontal)
                button.setContentHuggingPriority(contentHuggingPriority(for: NSLayoutConstraint.Axis.vertical), for: NSLayoutConstraint.Axis.vertical)
                button.setContentCompressionResistancePriority(contentCompressionResistancePriority(for: NSLayoutConstraint.Axis.horizontal), for: NSLayoutConstraint.Axis.horizontal)
                button.setContentCompressionResistancePriority(contentCompressionResistancePriority(for: NSLayoutConstraint.Axis.vertical), for: NSLayoutConstraint.Axis.vertical)
                button.titleLabel?.setContentHuggingPriority(contentHuggingPriority(for: NSLayoutConstraint.Axis.horizontal), for: NSLayoutConstraint.Axis.horizontal)
                button.titleLabel?.setContentHuggingPriority(contentHuggingPriority(for: NSLayoutConstraint.Axis.vertical), for: NSLayoutConstraint.Axis.vertical)
                button.titleLabel?.setContentCompressionResistancePriority(contentCompressionResistancePriority(for: NSLayoutConstraint.Axis.horizontal), for: NSLayoutConstraint.Axis.horizontal)
                button.titleLabel?.setContentCompressionResistancePriority(contentCompressionResistancePriority(for: NSLayoutConstraint.Axis.vertical), for: NSLayoutConstraint.Axis.vertical)
                
                stackView.addArrangedSubview(button)

                button.addTarget(self, action: #selector(ModernSegmentedControl.buttonTapped(_:)), for: UIControl.Event.touchUpInside)
                button.addTarget(self, action: #selector(ModernSegmentedControl.buttonHighlighted(_:)), for: UIControl.Event.touchDown)
                button.addTarget(self, action: #selector(ModernSegmentedControl.buttonUnHighlighted(_:)), for: UIControl.Event.touchUpOutside)
            }

            selectedIndex = 0
        }
    }

    var selectedIndex: Int = 0 {
        didSet {
            updateButtonColors()
        }
    }
    
    func setEnabled(_ enabled: Bool, forSegmentAtIndex index: Int) {
        if stackView.arrangedSubviews.count >= index, let button = stackView.arrangedSubviews[index] as? UIButton {
            button.isEnabled = enabled
            updateButtonColors()
        }
    }

    @objc func buttonTapped(_ sender: Any?) {
        guard let sender = sender as? UIView, let index = stackView.arrangedSubviews.firstIndex(of: sender) else {
            return
        }
        let oldValue = selectedIndex
        for case let button as UIButton in stackView.subviews {
            button.isHighlighted = false
        }
        selectedIndex = index
        
        if oldValue != index {
            sendActions(for: UIControl.Event.valueChanged)
        }
    }

    @objc func buttonHighlighted(_ sender: Any?) {
        updateButtonColors()
    }

    @objc func buttonUnHighlighted(_ sender: Any?) {
        updateButtonColors()
    }

    private func updateButtonColors() {
        for (index, view) in stackView.subviews.enumerated() {
            guard let button = view as? UIButton else {
                continue
            }
            var bgColor = selectedIndex == index ? selectedColor : buttonBackgroundColor
            var textColor = selectedIndex == index ? UIColor.white : selectedColor
            if !button.isEnabled {
                bgColor = bgColor.withAlphaComponent(0.5)
                textColor = buttonBackgroundColor
            } else if tintAdjustmentMode == .dimmed {
                bgColor = bgColor.desaturated()
                textColor = textColor.desaturated()
            } else if button.isHighlighted && !button.isSelected {
                textColor = textColor.withAlphaComponent(0.4)
            }
            button.backgroundColor = bgColor
            button.setTitleColor(textColor, for: UIControl.State.normal)
        }
    }
    
    override func tintColorDidChange() {
        super.tintColorDidChange()
        updateButtonColors()
    }
    
    private weak var stackView: UIStackView!

    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
        isAccessibilityElement = true
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
        isAccessibilityElement = true
    }

    private func setup() {
        let stack = UIStackView()
        stack.alignment = .center
        stack.axis = .horizontal
        stack.distribution = .fillEqually
        stack.spacing = 2.0
        stack.translatesAutoresizingMaskIntoConstraints = false
        addSubview(stack)
        addConstraints(stack.constraint(equalTo: self))
        addConstraint(heightAnchor.constraint(greaterThanOrEqualToConstant: 30.0))
        stackView = stack

        layer.cornerRadius = 6.0
        layer.masksToBounds = true
    }
}