Instantiating Initial View Controller Provides Nil

问题

For my tab bar controller, I want a transition for 4/5 tabs. The fifth tab i was able to successfully perform a different transition, but trying to add it to the other view controllers caused a crash.

Action Storyboard: This storyboard contains my tab bar controller, and my camera view controller. The transition for this view controller is the only one that works. When i try to transition different tabs that are not in the tab bar controllers storyboard, it crashes

This is the storyboard with the problem

For example, I want to provide a different transition for the view controller in this storyboard. However, i get the error: Unexpectedly found nil while unwrapping an optional value

This is my code:

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
    if let restoreID = viewController.restorationIdentifier {
        if restoreID == "NavigationCamera" {
        // This is the transition that works
            if let nav = tabBarController.viewControllers![tabBarController.selectedIndex] as? UINavigationController {
                print("Nav is allowed")
                let newVC = tabBarController.storyboard?.instantiateViewController(withIdentifier: "CameraView")
                let transition = CATransition()
                transition.duration = 0.25
                transition.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
                transition.type = kCATransitionPush
                transition.subtype = kCATransitionFromTop
                nav.view.layer.add(transition, forKey: nil)
                nav.pushViewController(newVC!, animated: false)
                return false
            }
        } else {
            if let otherNav = tabBarController.viewControllers![tabBarController.selectedIndex] as? UINavigationController {
                print("Other nav is allowed")
                let vcID = restoreID + "View"
                print(vcID)
                let myVC = otherNav.storyboard?.instantiateInitialViewController()
                let otherTransition = CATransition()
                otherTransition.duration = 0.25
                otherTransition.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
                otherTransition.type = kCATransitionPush
                otherTransition.subtype = kCATransitionFade
                otherNav.view.layer.add(otherTransition, forKey: nil)
                // This is where the error occurs and crashes
                otherNav.pushViewController(myVC!, animated: false)
                return false
            }
        }
    }
    return true
}

Once again the error i get is:

Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value

回答1:

let storyboard = UIStoryboard(name: <other storyboard>, bundle: nil)
let myVC = storyboard.instantiateViewController(withIdentifier: <your navigation controler>)

来源：https://stackoverflow.com/questions/47588214/instantiating-initial-view-controller-provides-nil

标签

ios

swift

xcode

storyboard