Why can't I see UIStackView in UIScrollView?

Question

I'm going to put UIStackView in UIScrollView. To put it easier, I'm going to make a mind map. The UIStackView I put into UIScrollView is an object of mind map. Thus, The embedded UIStackView will appear in all zones of UIScrollView.

However, although I have specified the location of UIStackView for UIScrollView, UIStackView does not appear in UIScrollView.

My interface builder(Attaches an interface builder image to understand the My View hierarchy.):

import UIKit

class ViewController: UIViewController {
    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var createObject: UIBarButtonItem!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        let redView = UIView()
        redView.backgroundColor = .red

        let blueView = UIView()
        blueView.backgroundColor = .blue

        let stackView = UIStackView(arrangedSubviews: [redView, blueView])
        stackView.axis = .vertical
        stackView.distribution = .fillEqually

        stackView.center = scrollView.center

        scrollView.addSubview(stackView)
    }
}

Answer 1

Neither UIView nor UIStackView have intrinsic sizes.

So you end up adding a stack view of size 0, 0 containing two views, each sized 0, 0, to your scroll view.

You need to apply some sizing somewhere.

This will give you a stack view that is equal width to your scroll view, and twice as tall (so you have a "full-height" blue view and a a "full-height" red view):

class ViewController: UIViewController {

    @IBOutlet weak var scrollView: UIScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        let redView = UIView()
        redView.backgroundColor = .red

        let blueView = UIView()
        blueView.backgroundColor = .blue

        let stackView = UIStackView(arrangedSubviews: [redView, blueView])
        stackView.axis = .vertical
        stackView.distribution = .fillEqually

        scrollView.addSubview(stackView)

        stackView.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            stackView.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 0.0),
            stackView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: 0.0),
            stackView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor, constant: 0.0),
            stackView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor, constant: 0.0),

            stackView.widthAnchor.constraint(equalTo: scrollView.widthAnchor, constant: 0.0),
            stackView.heightAnchor.constraint(equalTo: scrollView.heightAnchor, multiplier: 2.0),
            ])

    }
}

---

Edit:

Here is another example. It adds 6 subviews, with specified widths and heights. The stack view properties are changed to:

.alignment = .center
.distribution = .fill
.spacing = 8

The result:

and scrolled down:

and the code:

class ViewController: UIViewController {

    @IBOutlet weak var scrollView: UIScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()

        let colors: [UIColor] = [
            .red,
            .blue,
            .green,
            .orange,
            .yellow,
            .cyan,
        ]

        let sizes: [CGSize] = [
            CGSize(width: 100, height: 150),
            CGSize(width: 250, height: 100),
            CGSize(width: 200, height: 120),
            CGSize(width: 160, height: 200),
            CGSize(width: 220, height: 180),
            CGSize(width:  60, height:  80),
        ]

        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.distribution = .fill
        stackView.alignment = .center
        stackView.spacing = 8

        for (color, size) in zip(colors, sizes) {
            let v = UIView()
            v.translatesAutoresizingMaskIntoConstraints = false
            v.widthAnchor.constraint(equalToConstant: size.width).isActive = true
            v.heightAnchor.constraint(equalToConstant: size.height).isActive = true
            v.backgroundColor = color
            stackView.addArrangedSubview(v)
        }

        scrollView.addSubview(stackView)

        stackView.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            stackView.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 0.0),
            stackView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: 0.0),
            stackView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor, constant: 0.0),
            stackView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor, constant: 0.0),

            stackView.widthAnchor.constraint(equalTo: scrollView.widthAnchor, constant: 0.0),

            // do NOT set a heightAnchor ... let the subviews define the height
//          stackView.heightAnchor.constraint(equalTo: scrollView.heightAnchor, multiplier: 2.0),
            ])

    }
}