Simple Scrolling UIStackView

//
//  SimpleScrollingStack.swift
//  A super-simple demo of a scrolling UIStackView in iOS 9
//
//  Created by Paul Hudson on 10/06/2015.
//  Learn Swift at www.hackingwithswift.com
//  @twostraws
//

import UIKit

class ViewController: UIViewController {
	var scrollView: UIScrollView!
	var stackView: UIStackView!

	override func viewDidLoad() {
		super.viewDidLoad()

		scrollView = UIScrollView()
		scrollView.translatesAutoresizingMaskIntoConstraints = false
		view.addSubview(scrollView)

		view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[scrollView]|", options: .AlignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))
		view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[scrollView]|", options: .AlignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))


		stackView = UIStackView()
		stackView.translatesAutoresizingMaskIntoConstraints = false
		stackView.axis = .Vertical
		scrollView.addSubview(stackView)

		scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[stackView]|", options: NSLayoutFormatOptions.AlignAllCenterX, metrics: nil, views: ["stackView": stackView]))
		scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[stackView]", options: NSLayoutFormatOptions.AlignAllCenterX, metrics: nil, views: ["stackView": stackView]))

		for _ in 1 ..< 100 {
			let vw = UIButton(type: UIButtonType.System)
			vw.setTitle("Button", forState: .Normal)
			stackView.addArrangedSubview(vw)
		}
	}

	override func viewDidLayoutSubviews() {
		super.viewDidLayoutSubviews()
		scrollView.contentSize = CGSize(width: stackView.frame.width, height: stackView.frame.height)		
	}
}

Why is it .AlignAllCenterX for both "H" and "V"? Shouldn't it be .AlignAllCenterY on each second line?

Tap action is not working for StackView objects, how we can add tap gestures for this scroll of stack view.