A swizzling method which introduces a `statusBarStyle` to control the display of the status bar on a view controller.

UIViewController+StatusBar.swift

//
//  UIViewController+StatusBar.swift
//  StatusBarTesApp
//
//  Created by Jonathan Cardasis (C) on 7/10/18.
//  Copyright © 2018 Jonathan Cardasis. All rights reserved.
//

import UIKit

extension UIViewController {
    
    /// A convenience variable for setting the status bar style of the current controller.
    var statusBarStyle: UIStatusBarStyle {
        get {
            return statusBarController?.controllerStatusBarStyle ?? preferredStatusBarStyle
        }
        set {
            if statusBarController == nil {
                self.statusBarController = StatusBarController()
            }
            statusBarController?.controllerStatusBarStyle = newValue
        }
    }
}

// MARK: - Pass StatusBarStyle to Host Controllers

extension UITabBarController {
    open override var childViewControllerForStatusBarStyle: UIViewController? {
        return selectedViewController
    }
}

extension UINavigationController {
    open override var childViewControllerForStatusBarStyle: UIViewController? {
        return visibleViewController
    }
}

// MARK: - Private Implementation

fileprivate extension UIViewController {
    
    private struct AssociatedKey {
        static var statusBar = "uiviewcontroller_statusBarController"
    }
    
    private var statusBarController: StatusBarController? {
        get {
            return objc_getAssociatedObject(self, &AssociatedKey.statusBar) as? StatusBarController
        }
        set(newValue) {
            self.statusBarController?.detach()
            newValue?.attach(to: self)
            
            objc_setAssociatedObject(self, &AssociatedKey.statusBar, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
    
    // MARK: - Swizzling
    
    @objc var swizzledChildViewControllerForStatusBarStyle: UIViewController? {
        return statusBarController
    }
    
    fileprivate static func setupChildViewControllerForStatusBarStyleSwizzling() {
        let original = #selector(getter: UIViewController.childViewControllerForStatusBarStyle)
        let swizzled = #selector(getter: UIViewController.swizzledChildViewControllerForStatusBarStyle)
        
        let originalMethod = class_getInstanceMethod(UIViewController.self, original)
        let swizzledMethod = class_getInstanceMethod(UIViewController.self, swizzled)
        
        method_exchangeImplementations(originalMethod!, swizzledMethod!)
    }
    
    fileprivate static var isSwizzlingPerformed: Bool = false
    fileprivate static func swizzleIfNeeded() {
        guard !isSwizzlingPerformed else { return }
        UIViewController.setupChildViewControllerForStatusBarStyleSwizzling()
        isSwizzlingPerformed = true
    }
}

/// An invisible viewcontroller which becomes a child VC and controls
/// the display of the device's status bar.
fileprivate class StatusBarController: UIViewController {
    
    var controllerStatusBarStyle: UIStatusBarStyle = .default {
        didSet {
            guard oldValue != controllerStatusBarStyle else { return }
            self.parent?.setNeedsStatusBarAppearanceUpdate()
        }
    }
    
    init() {
        UIViewController.swizzleIfNeeded()
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.isHidden = true
        view.frame = .zero
    }
    
    override var preferredStatusBarStyle: UIStatusBarStyle {
        return controllerStatusBarStyle
    }
    
    func attach(to viewController: UIViewController) {
        viewController.addChildViewController(self)
        viewController.view.addSubview(view)
        didMove(toParentViewController: viewController)
    }
    
    func detach() {
        willMove(toParentViewController: nil)
        view.removeFromSuperview()
        removeFromParentViewController()
    }
}