在iOS 13中使用UITabBar.ItemPositioning

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建一个 UITabBarController
        let tabBarController = UITabBarController()
        
        // 设置 UITabBarItemPositioning
        if #available(iOS 13.0, *) {
            tabBarController.tabBar.itemPositioning = .fill
        }
        
        // 添加子控制器
        let firstViewController = UIViewController()
        firstViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .favorites, tag: 0)
        
        let secondViewController = UIViewController()
        secondViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .history, tag: 1)
        
        tabBarController.viewControllers = [firstViewController, secondViewController]
        
        // 设置为当前视图控制器
        self.addChild(tabBarController)
        self.view.addSubview(tabBarController.view)
        tabBarController.didMove(toParent: self)
    }
}