A SwiftUI-like function builder approach for UIStackView.

SwiftUIStackView.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

@_functionBuilder
struct UIViewBuilder {
    static func buildBlock(_ views: UIView...) -> [UIView] {
        return views
    }
}

extension UIStackView {

    convenience init(alignment: UIStackView.Alignment = .fill,
                     axis: NSLayoutConstraint.Axis = .horizontal,
                     isBaselineRelativeArrangement: Bool = false,
                     distribution: UIStackView.Distribution = .fill,
                     isLayoutMarginsRelativeArrangement: Bool = false,
                     spacing: CGFloat = 0,
                     @UIViewBuilder arrangedSubviewsBuilder: () -> [UIView]) {
        self.init(arrangedSubviews: arrangedSubviewsBuilder())

        self.alignment = alignment
        self.axis = axis
        self.isBaselineRelativeArrangement = isBaselineRelativeArrangement
        self.distribution = distribution
        self.isLayoutMarginsRelativeArrangement = isLayoutMarginsRelativeArrangement
        self.spacing = spacing
    }

}

extension UILabel {

    convenience init(_ text: String) {
        self.init()
        self.text = text
    }

    func font(_ font: UIFont) -> UILabel {
        self.font = font
        return self
    }

    func font(_ fontStyle: UIFont.TextStyle) -> UILabel {
        return font(.preferredFont(forTextStyle: fontStyle))
    }

}

class MyViewController : UIViewController {

    var stackView: UIStackView!

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        stackView = UIStackView(axis: .vertical) {
            UILabel("Hello")
                .font(.title1)
            UILabel("World")
                .font(.title2)
        }

        stackView.translatesAutoresizingMaskIntoConstraints = false
        
        view.addSubview(stackView)
        self.view = view
    }

    override func viewDidLayoutSubviews() {
        stackView.frame = view.bounds.inset(
            by: UIEdgeInsets(top: 8, left: 8, bottom: 8, right: 8)
        )
    }

}

PlaygroundPage.current.liveView = MyViewController()