在AppDelegate中设置Tab Bar Item图片内边距无效，求排查

Hey there! Let's break down why your code isn't having the effect you want, and how to fix it.

The Problem With Your Current Code #

You're instantiating a new instance of RootTabBarController from the storyboard in AppDelegate, setting the image insets on its tab item—but this instance never gets displayed on the screen.

When your app launches with a main storyboard set, iOS automatically loads the initial view controller (your RootTabBarController) and assigns it to the window's rootViewController. The instance you create with storyboard.instantiateViewController(...) is a separate, unused object—so your insets are applied to something that never appears to the user.

Fix 1: Set Insets Directly in the Tab Bar Controller #

The cleanest approach is to let the RootTabBarController manage its own tab items. Add the inset code to its viewDidLoad method:

class RootTabBarController: UITabBarController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Access the first tab item and set its insets
        if let homeTabItem = tabBar.items?.first {
            homeTabItem.imageInsets = UIEdgeInsets(top: 12.0, left: 0.0, bottom: -12.0, right: 0.0)
        }
    }
}

This way, the insets are applied to the actual instance of the tab bar controller that's displayed to the user, right when it finishes loading its view.

Fix 2: Target the Correct Instance in AppDelegate #

If you really need to set this in AppDelegate, you need to modify the existing instance that iOS already loaded as the root view controller, not create a new one:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Get the already-loaded root tab bar controller
    guard let tabBarController = window?.rootViewController as? RootTabBarController else {
        return true
    }
    
    // Apply insets to the first tab item
    if let homeTabItem = tabBarController.tabBar.items?.first {
        homeTabItem.imageInsets = UIEdgeInsets(top: 12.0, left: 0.0, bottom: -12.0, right: 0.0)
    }
    
    return true
}

Why IB Works #

When you set the insets in Interface Builder, you're modifying the properties of the tab bar item inside the storyboard's instance of RootTabBarController. This is the same instance that iOS loads and displays at launch, so the changes take effect immediately.

内容的提问来源于stack exchange，提问作者Matt