KVOSubheadingReLayout.swift

import UIKit

class PossibleView: UIView {

    let header: UILabel = {
        let label = UILabel(frame: .zero)
        label.translatesAutoresizingMaskIntoConstraints = false
        label.backgroundColor = .brown
        label.numberOfLines = 0

        return label
    }()

    dynamic let subHeading: UILabel = {
        let label = UILabel(frame: .zero)
        label.translatesAutoresizingMaskIntoConstraints = false
        label.backgroundColor = .blue
        label.numberOfLines = 0

        return label
    }()

    let button: UIButton = {
        let button = UIButton(type: .system)
        button.translatesAutoresizingMaskIntoConstraints = false
        button.backgroundColor = .green

        return button
    }()

    // KVO
    private var observerContext = 0

    override init(frame: CGRect) {
        super.init(frame: frame)

        configure()
        addObserver(self, forKeyPath: #keyPath(subHeading.text), options: [.new, .old], context: &observerContext)
    }

    deinit {
        removeObserver(self, forKeyPath: #keyPath(subHeading.text), context: &observerContext)
    }

    @available(*, unavailable)
    required init(coder: NSCoder) {
        fatalError("Don't use this, use init(frame:)")
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard context == &observerContext else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
            return
        }

        guard let keyPath = keyPath, keyPath == "subHeading.text" else {
            return
        }

        guard let changeDict = change else {
            return print("No changes")
        }

        let newValue = changeDict[.newKey] as? Optional<String>
        if let nv = newValue {
            layoutWithSubHeading()
        } else {
            layoutForNoSubHeading()
        }
    }

    // Constraints
    var subHeadingToButton: NSLayoutConstraint!
    var headingToSubHeading: NSLayoutConstraint!
    var headingToButton: NSLayoutConstraint!

}

fileprivate extension PossibleView {
    func configure() {
        backgroundColor = .purple

        addSubview(header)
        addSubview(subHeading)
        addSubview(button)

        headingToSubHeading = subHeading.topAnchor.constraint(equalTo: header.bottomAnchor, constant: 8)
        subHeadingToButton = button.topAnchor.constraint(equalTo: subHeading.bottomAnchor, constant: 8)
        headingToButton = header.bottomAnchor.constraint(equalTo: button.topAnchor, constant: -4)


        NSLayoutConstraint.activate([
            header.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 8),
            header.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -8),
            header.topAnchor.constraint(equalTo: topAnchor),

            subHeading.leadingAnchor.constraint(equalTo: header.leadingAnchor),
            subHeading.trailingAnchor.constraint(equalTo: header.trailingAnchor),

            button.leadingAnchor.constraint(equalTo: subHeading.leadingAnchor),
            button.trailingAnchor.constraint(lessThanOrEqualTo: subHeading.trailingAnchor),

            ])
        layoutForNoSubHeading()
    }

    func layoutForNoSubHeading() {
        headingToSubHeading.isActive = false
        subHeadingToButton.isActive = false
        headingToButton.isActive = true

    }

    func layoutWithSubHeading() {
        headingToSubHeading.isActive = true
        subHeadingToButton.isActive = true
        headingToButton.isActive = false
    }

}

import PlaygroundSupport

let view = PossibleView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))

PlaygroundPage.current.liveView = view

view.header.text = "You should be layed out here"
view.subHeading.text = nil //setting this to nil, or some title will change the layout of the view
view.button.setTitle("This is a button", for: .normal)