Lets a view controller embed a child view controller.

Embedding.swift

import UIKit

protocol Embedding: class {
    var embeddedViewController: UIViewController? { get set }
}

enum EmbeddingAnimation {
    case none
    case flipFromLeft
    case crossDissolve
    
    func animationOption() -> UIViewAnimationOptions {
        switch self {
        case .none:
            return []
        case .flipFromLeft:
            return .transitionFlipFromLeft
        case .crossDissolve:
            return .transitionCrossDissolve
        }
    }
    
    func duration() -> TimeInterval {
        switch self {
        case .none:
            return 0
        case .flipFromLeft:
            return 0.75
        case .crossDissolve:
            return 0.3
        }
    }
}

extension Embedding {
    func embedViewController(_ viewController: UIViewController, animation: EmbeddingAnimation) {
        func embed() {
            let s = self as! UIViewController

            if let viewControllerForRemoval = self.embeddedViewController {
                viewControllerForRemoval.willMove(toParentViewController: nil)
                viewControllerForRemoval.view.removeFromSuperview()
                viewControllerForRemoval.removeFromParentViewController()
            }
            
            self.embeddedViewController = viewController
            s.addChildViewController(viewController)
            viewController.view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
            s.view.addSubview(viewController.view)
            viewController.didMove(toParentViewController: s)
        }
        
        switch animation {
        case .none:
            embed()
        default:
            let s = self as! UIViewController
            UIView.transition(with: s.view, duration: animation.duration(), options: animation.animationOption(), animations: embed, completion: nil)
        }
    }
}