UIViewController containment bug demo

ViewController.swift

//
//  ViewController.swift
//  ContainerTimingBugDemo
//
//  Created by Jason Pepas on 11/12/15.
//  Copyright © 2015 Jason Pepas. All rights reserved.
//

/*

Instructions:

1. Create a new "Single View Application" Xcode project.
2. Replace the contents of ViewController.swift with this gist.
3. Uncomment the below test cases one-by-one.

*/

import UIKit

extension UIViewController
{
    func containChildViewController(childVC:UIViewController, inView containerView:UIView)
    {
        addChildViewController(childVC)
        containerView.addSubview(childVC.view)
        childVC.didMoveToParentViewController(self)
        containerView.bringSubviewToFront(childVC.view)
        
        childVC.view.translatesAutoresizingMaskIntoConstraints = false
    }
    
    func containChildViewController(child:UIViewController)
    {
        self.containChildViewController(child, inView:view)
    }
}

class ViewController: UIViewController
{
    override func viewDidAppear(animated: Bool)
    {
        super.viewDidAppear(animated)
        
//        self.presentViewController(VCTestCase1(), animated: true, completion: nil)
//        self.presentViewController(VCTestCase2(), animated: true, completion: nil)
//        self.presentViewController(VCTestCase3(), animated: true, completion: nil)
        self.presentViewController(VCTestCase4(), animated: true, completion: nil)
//        self.presentViewController(VCTestCase5(), animated: true, completion: nil)
//        self.presentViewController(VCTestCase6(), animated: true, completion: nil)
    }
}

class VerboseVC: UIViewController
{
    override func viewDidLoad()
    {
        debugPrint("\(self.dynamicType): \(__FUNCTION__)")
        super.viewDidLoad()
    }
    
    override func viewWillAppear(animated: Bool)
    {
        debugPrint("\(self.dynamicType): \(__FUNCTION__)")
        super.viewWillAppear(animated)
    }
    
    override func viewDidAppear(animated: Bool)
    {
        debugPrint("\(self.dynamicType): \(__FUNCTION__)")
        super.viewDidAppear(animated)
    }
    
    override func viewWillDisappear(animated: Bool)
    {
        debugPrint("\(self.dynamicType): \(__FUNCTION__)")
        super.viewWillDisappear(animated)
    }
    
    override func viewDidDisappear(animated: Bool)
    {
        debugPrint("\(self.dynamicType): \(__FUNCTION__)")
        super.viewDidDisappear(animated)
    }
}

class ChildVC: VerboseVC {}

class VCTestCase1: VerboseVC
{
    override func viewDidLoad()
    {
        super.viewDidLoad()
        
        // CORRECT: SubViewController will correctly receive viewWillAppear and viewDidAppear
        containChildViewController(ChildVC())
    }
}

class VCTestCase2: VerboseVC
{
    override func viewWillAppear(animated: Bool)
    {
        // CORRECT: SubViewController will correctly receive viewWillAppear and viewDidAppear
        self.containChildViewController(ChildVC())
        
        super.viewWillAppear(animated)
    }
}

class VCTestCase3: VerboseVC
{
    override func viewWillAppear(animated: Bool)
    {
        super.viewWillAppear(animated)
        
        // CORRECT: SubViewController will correctly receive viewWillAppear and viewDidAppear
        self.containChildViewController(ChildVC())
    }
}

class VCTestCase4: VerboseVC
{
    override func viewWillAppear(animated: Bool)
    {
        super.viewWillAppear(animated)
        
        dispatch_async(dispatch_get_main_queue()) { () -> Void in
            /*
            
            ERROR: SubViewController receives viewDidLoad, then viewDidAppear.  viewWillAppear is not called.
            
            Specifically, you should see this in the console:
            
            "VCTestCase4: viewDidLoad()"
            "VCTestCase4: viewWillAppear"
            "ChildVC: viewDidLoad()"
            "VCTestCase4: viewDidAppear"
            "ChildVC: viewDidAppear"
            
            */
            self.containChildViewController(ChildVC())
        }
    }
}

class VCTestCase5: VerboseVC
{
    override func viewDidAppear(animated: Bool)
    {
        // CORRECT: SubViewController will correctly receive viewWillAppear and viewDidAppear
        self.containChildViewController(ChildVC())
        
        super.viewDidAppear(animated)
    }
}

class VCTestCase6: VerboseVC
{
    override func viewDidAppear(animated: Bool)
    {
        super.viewDidAppear(animated)
        
        // CORRECT: SubViewController will correctly receive viewWillAppear and viewDidAppear
        self.containChildViewController(ChildVC())
    }
}