Swap rootViewController with animation?

Im trying to swap to another root view controller with a tab bar; via app delegate, and I want to add transition animation. By default it would only show the view without any animation.

let tabBar = self.instantiateViewController(storyBoard: "Main", viewControllerID: "MainTabbar")
let appDelegate = UIApplication.shared.delegate as! AppDelegate
appDelegate.window = UIWindow(frame: UIScreen.main.bounds)
appDelegate.window?.rootViewController = tabBar
appDelegate.window?.makeKeyAndVisible()

That's how I swapped to another rootview controller.

You can use UIView.transition(with: view) to replace the rootViewController of a UIWindow:

guard let window = UIApplication.shared.keyWindow else {
    return
}

let storyboard = UIStoryboard(name: "Main", bundle: nil)
let vc = storyboard.instantiateViewController(withIdentifier: "MainTabbar")

// Set the new rootViewController of the window.
// Calling "UIView.transition" below will animate the swap.
window.rootViewController = vc

// A mask of options indicating how you want to perform the animations.
let options: UIView.AnimationOptions = .transitionCrossDissolve

// The duration of the transition animation, measured in seconds.
let duration: TimeInterval = 0.3

// Creates a transition animation.
// Though `animations` is optional, the documentation tells us that it must not be nil. ¯\_(ツ)_/¯
UIView.transition(with: window, duration: duration, options: options, animations: {}, completion:
{ completed in
    // maybe do something on completion here
})

Swift 4

Paste function into AppDelegate:

func setRootViewController(_ vc: UIViewController, animated: Bool = true) {
    guard animated, let window = self.window else {
        self.window?.rootViewController = vc
        self.window?.makeKeyAndVisible()
        return
    }
    
    window.rootViewController = vc
    window.makeKeyAndVisible()
    UIView.transition(with: window,
                      duration: 0.3,
                      options: .transitionCrossDissolve,
                      animations: nil,
                      completion: nil)
}

An alternative solution:

let stb = UIStoryboard(name: "YOUR_STORYBOARD_NAME", bundle: nil)
let rootVC = stb.instantiateViewController(withIdentifier: "YOUR_TABBAR_VIEWCONTROLLER_NAME")
let snapshot = (UIApplication.shared.keyWindow?.snapshotView(afterScreenUpdates: true))!
rootVC.view.addSubview(snapshot)

UIApplication.shared.keyWindow?.rootViewController = rootVC
UIView.transition(with: snapshot, 
                  duration: 0.4,
                  options: .transitionCrossDissolve,
                  animations: { 
                      snapshot.layer.opacity = 0
                  },
                  completion: { status in 
                      snapshot.removeFromSuperview()
                  })

> Im trying to swap to another root view controller ... and I want to add transition animation

I have an app that does this: it changes the root view controller with animation (it's called Albumen).

But my app actually doesn't actually change the root view controller. The root view controller is a custom container view controller that never changes. Its view is never seen and it has no functionality. Its only job is to be the place where the change happens: it swaps one child view controller for another — and thus the transition animation works.

In other words, you add one view controller to your view controller hierarchy, right at the top of the hierarchy, and the whole problem is solved neatly and correctly.

Try this:

UIView.transition(from: appdelegate.window.rootViewController!.view, to: tabbar.view, duration: 0.6, options: [.transitionCrossDissolve], completion: {
    _ in
    appdelegate.window.rootViewController = tabbar
})

Updated Swift 5.3 version:

    let foregroundedScenes = UIApplication.shared.connectedScenes.filter { $0.activationState == .foregroundActive }
    let window = foregroundedScenes.map { $0 as? UIWindowScene }.compactMap { $0 }.first?.windows.filter { $0.isKeyWindow }.first
    
    guard let uWindow = window else { return }

    uWindow.rootViewController = customTabBarController
    UIView.transition(with: uWindow, duration: 0.3, options: [.transitionCrossDissolve], animations: {}, completion: nil)
}

I have created a helper class for this based on d.felber's answer:

    import UIKit

    class ViewPresenter {
        
        public static func replaceRootView(for viewController: UIViewController,
                                   duration: TimeInterval = 0.3,
                                   options: UIView.AnimationOptions = .transitionCrossDissolve,
                                   completion: ((Bool) -> Void)? = nil) {
            guard let window = UIApplication.shared.keyWindow else {
                return
            }
            
            guard let rootViewController = window.rootViewController else {
                return
            }
            
            viewController.view.frame = rootViewController.view.frame
            viewController.view.layoutIfNeeded()
            
            UIView.transition(with: window, duration: duration, options: options, animations: {
                window.rootViewController = viewController
            }, completion: completion)
        }
    }

You can use it like this:

    let loginVC = SignInViewController(nibName: "SignInViewController", bundle: nil)
    ViewPresenter.replaceRootView(for: loginVC)

or

ViewPresenter.replaceRootView(for: loginVC, duration: 0.3, options: .transitionCrossDissolve) { 
(bool) in
       // do something
}