How can I control UI Tab Bar Appearance from my user variables

My IOS tab bar app has both bright and dark display modes. To make this work well, I want to set the tab bar appearance accordingly to display mode. I have the following code in place to do this.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {


        displayMode=UserDefaults.standard.integer(forKey: "displayMode")

        if displayMode==0 // bright display. want bright tab bar
        {   UITabBar.appearance().backgroundColor=UIColor.white
            UITabBar.appearance().barTintColor=UIColor.white
            UITabBar.appearance().unselectedItemTintColor = UIColor.darkGray
            UITabBar.appearance().tintColor = UIColor.blue
            UITabBar.appearance().isTranslucent = false
        }

        if displayMode==1 // dark display. want dark tab bar
        {   UITabBar.appearance().backgroundColor=UIColor.black
            UITabBar.appearance().barTintColor=UIColor.black
            UITabBar.appearance().unselectedItemTintColor = UIColor.lightGray
            UITabBar.appearance().tintColor = UIColor.white
            UITabBar.appearance().isTranslucent = false
        }

        return true
    }

This works but poorly. It can only change the tab bar colors when the app restarts. I would like to make it more immediate. The choice of places to put the controls appears limited to the App Delegate. I would like to control the tab bar colors from my main program area instead.

Solution

Appearance proxy is not limited to AppDelegate. You can apply it anywhere anytime but note the following:

iOS applies appearance changes when a view enters a window, it doesn’t change the appearance of a view that’s already in a window. To change the appearance of a view that’s currently in a window, remove the view from the view hierarchy and then put it back.

Ref: https://developer.apple.com/documentation/uikit/uiappearance

Solution:

Present and dismiss a dummy UIViewController. Thereby refreshing the visible viewController of the window.

Example:

func randomTabBarAppearance() {
    UITabBar.appearance().backgroundColor = UIColor.init(red: CGFloat.random(in: 0...1),
                                                         green: CGFloat.random(in: 0...1),
                                                         blue: CGFloat.random(in: 0...1),
                                                         alpha: 1)

    present(UIViewController(), animated: false, completion: nil)
    dismiss(animated: false, completion: nil)
}