gistfile1.swift

//
//  ViewController.swift
//  test
//
//  Created by Andrew Carter on 6/7/17.
//  Copyright © 2017 Andrew Carter. All rights reserved.
//

import UIKit

class PushOperation: Operation {
    
    private let viewController: UIViewController
    private let view: UIView
    private var didAnimateInViewController = false
    private var isAnimatingViewController = false
    
    init(viewController: UIViewController, for view: UIView) {
        self.viewController = viewController
        self.view = view
    }
    
    override var isFinished: Bool {
        return didAnimateInViewController
    }
    
    override var isAsynchronous: Bool {
        return true
    }
    
    override var isExecuting: Bool {
        return isAnimatingViewController
    }
    
    override func start() {
        willChangeValue(forKey: "isExecuting")
        isAnimatingViewController = true
        didChangeValue(forKey: "isExecuting")
        
        DispatchQueue.main.async {
            
            UIView.animate(withDuration: 2.0, animations: {
                // Add you view controller anmiation code here, I'm just doing this 
                // for the sake of something happening
                self.view.addSubview(self.viewController.view)
                self.viewController.view.transform = CGAffineTransform(scaleX: 0.5, y: 0.5)
                
                print("Animating in \(self.viewController.title ?? "viewController")")
                
            }, completion: { _ in
                
                self.willChangeValue(forKey: "isFinished")
                self.didAnimateInViewController = true
                self.didChangeValue(forKey: "isFinished")
                
                print("Finished animating in \(self.viewController.title ?? "viewController")")
            })
        }
    }
    
}

class ViewController: UIViewController {
    
    let operationQueue: OperationQueue = {
        var operationQueue = OperationQueue()
        operationQueue.maxConcurrentOperationCount = 1
        return operationQueue
    }()
    
    func push(controller: UIViewController) {
        operationQueue.addOperation(PushOperation(viewController: controller, for: view))
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        let viewControllerOne = UIViewController()
        viewControllerOne.view.backgroundColor = UIColor.blue
        viewControllerOne.title = "One"
        
        let viewControllerTwo = UIViewController()
        viewControllerTwo.view.backgroundColor = UIColor.red
        viewControllerTwo.title = "Two"
        
        let viewControllerThree = UIViewController()
        viewControllerThree.view.backgroundColor = UIColor.green
        viewControllerThree.title = "Three"
        
        push(controller: viewControllerOne)
        push(controller: viewControllerTwo)
        push(controller: viewControllerThree)
    }

}