Simple class for fading animation between UIViewControllers in UINavigationController

TMFadeAnimator.swift

//
//  NavigationVC.swift
//  WorkUp
//
//  Created by Simon on 4/23/17.
//  Copyright © 2017 Simon. All rights reserved.
//

import UIKit

//Usage example
// class NavigationVC: UINavigationController, UINavigationControllerDelegate { 
//     func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationControllerOperation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
//         return TMFadeAnimator()
//     }
// }

//The VC fading animator
class TMFadeAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.5
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        
        let toViewController   = transitionContext.viewController(forKey: .to)
        let fromViewController   = transitionContext.viewController(forKey: .from)
        
        toViewController?.view.frame = transitionContext.containerView.frame
        
        toViewController?.beginAppearanceTransition(true, animated: true)
        fromViewController?.beginAppearanceTransition(false, animated: true)
        
        transitionContext.containerView.addSubview(toViewController!.view)
        
        
        toViewController!.view.alpha = 0.0
        
        UIView.animate(withDuration: self.transitionDuration(using: transitionContext), animations: {
            toViewController!.view.alpha = 1.0
            fromViewController!.view.alpha = 0.0
        }) { (finished) in
            toViewController?.endAppearanceTransition()
            fromViewController?.endAppearanceTransition()
            transitionContext.completeTransition(finished)
        }
    }
}