Answer Playground for https://stackoverflow.com/questions/49309402/placing-labels-above-uislider-using-uistackview

UISliderWithCenteredThumbAndLabels.swift

import UIKit
import PlaygroundSupport

class MySlider: UISlider {
	
	// indent the tracking so the thumb does not hang outside the view
	private let padding = CGFloat(18)
	
	// the default rect for tracking is indented by a few pixels; keep note of that value here
	private var trackIndent = CGFloat(0)
	
	// how far the thumb is indented from the edge of the control; expose this so things outside the control can use it, e.g. laying out labels
	var thumbIndent: CGFloat { return padding - trackIndent}
	
	// take the default track-rect and pad it left and right, and keep note of the few pixels of base implementation indent in trackIndent
	override func trackRect(forBounds bounds: CGRect) -> CGRect {
		let superTrackRect = super.trackRect(forBounds: bounds)
		trackIndent = superTrackRect.origin.x
		return CGRect(
			x: superTrackRect.origin.x + padding,
			y: superTrackRect.origin.y,
			width: superTrackRect.size.width - padding*2,
			height: superTrackRect.size.height)
	}
	
	// take the default thumb-rect and allow it to overhang the left and right of the track-rect by half of its width
	override func thumbRect(forBounds bounds: CGRect, trackRect rect: CGRect, value: Float) -> CGRect {
		let superThumbRect = super.thumbRect(forBounds: bounds, trackRect: rect, value: value)
		let valueAsDecimalFraction = value == minimumValue ? 0 : (value - minimumValue)/(maximumValue - minimumValue)
		let valueAsDecimalFractionOfMinusOneToOne = (valueAsDecimalFraction*2 - 1)/2
		let xOffset = CGFloat(valueAsDecimalFractionOfMinusOneToOne) * superThumbRect.width
		return CGRect(
			x: superThumbRect.origin.x + xOffset,
			y: superThumbRect.origin.y,
			width: superThumbRect.size.width,
			height: superThumbRect.size.height)
	}
}


class MyViewController : UIViewController {
	
	static let range = 1...7
	
	let labels: [UILabel] = {
		range.map { (i: Int) -> (UILabel) in
			let l = UILabel()
			l.text = "\(i)"
			l.textAlignment = .center
			return l
		}
	}()
	
	let slider: MySlider = {
		let s = MySlider()
		s.minimumValue = Float(range.lowerBound)
		s.maximumValue = Float(range.upperBound)
		return s
	}()
	
	override func loadView() {
		let view = UIView()
		view.backgroundColor = .white
		slider.backgroundColor = .orange
		labels.forEach { $0.backgroundColor = .magenta }
		let subviews: [UIView] = [slider] + labels
		subviews.forEach {
			$0.translatesAutoresizingMaskIntoConstraints = false
			view.addSubview($0)
		}
		self.view = view
	}
	
	override func viewWillLayoutSubviews() {
		
		let sliderConstraints = [
			slider.widthAnchor.constraint(equalTo: view.widthAnchor),
			slider.centerYAnchor.constraint(equalTo: view.centerYAnchor),
			]
		
		let labelConstraints = labels.enumerated().map { (i, label) -> [NSLayoutConstraint] in
			let pct = CGFloat(i)/CGFloat(labels.count-1)
			let centerX = (view.bounds.size.width-2*self.slider.thumbIndent) * pct + self.slider.thumbIndent
			return [
				label.centerXAnchor.constraint(equalTo: slider.leftAnchor, constant: centerX),
				label.bottomAnchor.constraint(equalTo: slider.topAnchor, constant: 0),
			]
			}.flatMap { $0 }
		
		view.removeConstraints(view.constraints)
		
		(sliderConstraints + labelConstraints).forEach {
			view.addConstraint($0)
			$0.isActive = true
		}
	}
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()