UINavigationBar styling between UINavigationController transitions

NavStyleBug.swift

//  NavStyleBug
//
//  Created by Saniul Ahmed on 22/01/2015.
//  Copyright (c) 2015 Saniul Ahmed. All rights reserved.
//

import UIKit

//MARK: Environment

@objc
protocol Styleable {
    var barStyle: UIBarStyle { get }
}

class ViewControllerA: UIViewController, Styleable {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = "Default"
    }
    
    var barStyle: UIBarStyle {
        return .Default
    }
}

class ViewControllerB: UIViewController, Styleable {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.title = "Black"
    }
    
    var barStyle: UIBarStyle {
        return .Black
    }
}

class NavController: UINavigationController {
    var stylingDelegate: StylingNavigationDelegate?
    
    override func viewDidLoad() {
        self.stylingDelegate = StylingNavigationDelegate(navigationController: self)
        self.delegate = self.stylingDelegate
    }
}

@objc
class StylingNavigationDelegate: NSObject, UINavigationControllerDelegate {
    var navigationController: UINavigationController
    
    var popTransitionHelper: InteractivePopTransitionHelper
    
    init(navigationController: UINavigationController) {
        self.navigationController = navigationController
        self.popTransitionHelper = InteractivePopTransitionHelper(navigationController: navigationController)
        
        super.init()
        
        self.popTransitionHelper.began = self.interactivePopTransitionBegan
    }
    
    func updateBarStyleWith(barStyle: UIBarStyle) {
        self.navigationController.navigationBar.barStyle = barStyle
    }
    
    //If we only do this, we will have a problem when cancelling the interactive pop transition
    func navigationController(navigationController: UINavigationController, willShowViewController viewController: UIViewController, animated: Bool) {
        if let styleable = navigationController.topViewController as? Styleable {
            self.updateBarStyleWith(styleable.barStyle)
        }
    }
    
    //This restores the bar style to what it was at the beginning of the gesture.
    func interactivePopTransitionBegan() {
        let preTransitionStyle = self.navigationController.navigationBar.barStyle
        self.popTransitionHelper.finished = { cancelled in
            if cancelled {
                self.updateBarStyleWith(preTransitionStyle)
            }
        }
    }
    
}

//MARK: Interactive Pop Transition Helper

@objc
public final class InteractivePopTransitionHelper {
    public let navigationController: UINavigationController
    
    public var began: (() -> Void)?
    public var finished: (Bool -> Void)?
    
    public init(navigationController: UINavigationController) {
        self.navigationController = navigationController
        
        navigationController.interactivePopGestureRecognizer.addTarget(self, action: Selector("interactivePopGestureRecognizerUpdate:"))
    }
    
    private final func interactivePopGestureRecognizerUpdate(gesture: UIGestureRecognizer) {
        if gesture.state != .Began {
            return;
        }
        
        if let coordinator = self.navigationController.transitionCoordinator() {
            self.began?()
            coordinator.notifyWhenInteractionEndsUsingBlock { context -> Void in
                self.finished?(context.isCancelled())
                () //argh, swift :(
            }
        }
    }
    
}