VFL example

SomeView.swift

import UIKit

final class SomeView: LayoutView
{
    let view1 = UIView()
    let view2 = UIView()
    let view3 = UIView()

    override func layout() {
        super.layout()
        autoLayout.applyVisualFormatLanguage([
            "|-(30)-[view1]-(30)-[view2]-(30)-[view3]"
        ])
    }
}

class LayoutView: UIView
{
    override init(frame: CGRect) {
        super.init(frame: frame)
        layout()
    }

    @available(*, unavailable)
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        layout()
    }

    func layout() {
        autoLayout.enumerateSubViews().values.forEach { addSubview($0) }
    }
}
public extension UIView {

    var autoLayout: AutoLayout {

        AutoLayout(self)
    }
}

public struct AutoLayout {

    public let base: UIView

    public init(_ base: UIView) {
        self.base = base
    }

    public func applyVisualFormatLanguage(_ constraints: [String],
                                          options: NSLayoutConstraint.FormatOptions = [],
                                          metrics: [String: Any]? = nil) {
        base.translatesAutoresizingMaskIntoConstraints = false
        var views = enumerateSubViews()
        views["baseView"] = base
        views.values.forEach { $0.translatesAutoresizingMaskIntoConstraints = false }
        constraints.forEach {
            base.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: $0, options: options, metrics: metrics, views: views))
        }
    }

    /// Returns every non nil instance UIView variable in the given view, indexed by its variable name.
    public func enumerateSubViews() -> [String: UIView] {

        var views = [String: UIView]()
        let mirror = Mirror(reflecting: base)

        let addToViewsClosure: ((Mirror.Child) -> Void) = { child in
            child.label.flatMap { label in

                // lazy var has $__lazy_storage_$_ (swift 5.1+) and .storage (swift 5-)  in `label` property.
                #if swift(>=5.1)
                let key = label.replacingOccurrences(of: "$__lazy_storage_$_", with: "")
                #else
                let key = label.replacingOccurrences(of: ".storage", with: "")
                #endif

                views[key] = child.value as? UIView
            }
        }

        mirror.children.forEach(addToViewsClosure)
        mirror.superclassMirror?.children.forEach(addToViewsClosure)

        return views
    }
}