An experiment in method swizzling and Combine to expose traitCollectionDidChange as a Publisher

1_UIViewController+TraitCollectionChangePublisher.swift

import Combine
import ObjectiveC
import UIKit

typealias TraitCollectionChange = (UITraitCollection, previous: UITraitCollection?)

private typealias TraitChangeSubject = PassthroughSubject<TraitCollectionChange, Never>
private var traitSubjectKey = "_voodoo_trait_subject"
private var swizzlingPerformed = false

extension UIViewController {
    func traitCollectionChangePublisher() -> AnyPublisher<TraitCollectionChange, Never> {
        objc_sync_enter(self)
        defer { objc_sync_exit(self) }
        objc_sync_enter(UIViewController.self)
        defer { objc_sync_exit(UIViewController.self) }

        if let subject = objc_getAssociatedObject(self, &traitSubjectKey) as? TraitChangeSubject {
            return subject.eraseToAnyPublisher()
        }

        let subject = TraitChangeSubject()

        objc_setAssociatedObject(
            self,
            &traitSubjectKey,
            subject,
            .OBJC_ASSOCIATION_RETAIN_NONATOMIC)

        if !swizzlingPerformed {
            let originalSelector = #selector(UIViewController.traitCollectionDidChange)
            let swizzledSelector = #selector(UIViewController._swizzled_traitCollectionDidChange)

            let originalMethod = class_getInstanceMethod(UIViewController.self, originalSelector)
            let swizzledMethod = class_getInstanceMethod(UIViewController.self, swizzledSelector)

            guard let m1 = originalMethod, let m2 = swizzledMethod else {
                return Empty().eraseToAnyPublisher()
            }

            method_exchangeImplementations(m1, m2)
            swizzlingPerformed = true
        }

        return subject.eraseToAnyPublisher()
    }

    @objc private dynamic func _swizzled_traitCollectionDidChange(_ previous: UITraitCollection?) {
        _swizzled_traitCollectionDidChange(previous)

        guard let subject = objc_getAssociatedObject(self, &traitSubjectKey) as? TraitChangeSubject else {
            return
        }

        subject.send((traitCollection, previous: previous))
    }
}

2_ExampleViewController.swift

import Combine
import UIKit

class ExampleViewController: UIViewController {
    private var cancellables = Set<AnyCancellable>()

    override func viewDidLoad() {
        super.viewDidLoad()
        let label = UILabel()
        label.translatesAutoresizingMaskIntoConstraints = false
        label.font = UIFont.preferredFont(forTextStyle: .title1)
        label.text = "Hello World!"
        label.textColor = .label
        view.addSubview(label)
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])

        let pushButton = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(pushNewViewController))
        navigationItem.rightBarButtonItem = pushButton

        traitCollectionChangePublisher()
            .print()
            .sink { _ in }
            .store(in: &cancellables)
    }

    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        print("THIS IS FINE")
    }

    @objc private func pushNewViewController() {
        let vc = ViewController()
        show(vc, sender: self)
    }
}