依据：How to change the Color of text in UITabBarItem in iOS 5
看起来解决方案可能将消息发送到外观代理，而不是一个项目：

（在iOS 7.0以上版本中已弃用）

[[UITabBarItem appearance] setTitleTextAttributes:@{UITextAttributeFont: [UIFont fontWithName:@"AmericanTypewriter" size:20.0f]} forState:UIControlStateNormal];

适用于iOS 7.0以上用途：

[[UITabBarItem appearance] setTitleTextAttributes:@{NSFontAttributeName: [UIFont fontWithName:@"AmericanTypewriter" size:20.0f]} forState:UIControlStateNormal];

快捷的方式，为懒惰：

UITabBarItem.appearance().setTitleTextAttributes([NSFontAttributeName: UIFont.systemFontOfSize(10)], forState: .normal)
UITabBarItem.appearance().setTitleTextAttributes([NSFontAttributeName: UIFont.systemFontOfSize(10)], forState: .selected)

Swift 4.1和自定义字体

UITabBarItem.appearance().setTitleTextAttributes([NSAttributedString.Key.font: UIFont(name: "Montserrat-Medium", size: 11)], for: .normal)
UITabBarItem.appearance().setTitleTextAttributes([NSAttributedString.Key.font: UIFont(name: "Montserrat-Medium", size: 11)], for: .selected)

雨燕3

UITabBarItem.appearance().setTitleTextAttributes([NSFontAttributeName: UIFont(name: "OpenSans", size: 10)!], for: .normal)

雨燕4

UITabBarItem.appearance().setTitleTextAttributes([NSAttributedString.Key.font: UIFont.tabbar], for: .normal)

如果我在viewDidLoad（）中添加了代码，那么当选项卡栏被选中时，我永远无法更改字体。
这是一个伟大的文章，解释了如何做到这一点与更多的细节：HolySwift Article
简而言之，您需要在标签栏控制器中添加以下代码：

override var selectedIndex: Int { 
    didSet {
        guard let selectedViewController = viewControllers?[selectedIndex] else {
            return
        }
        selectedViewController.tabBarItem.setTitleTextAttributes([.font: UIFont.boldSystemFont(ofSize: 13)], for: .normal) 
    }
}

还有这个

override var selectedViewController: UIViewController? { 
    didSet {

        guard let viewControllers = viewControllers else {
            return
        }

        for viewController in viewControllers {
            if viewController == selectedViewController {
                viewController.tabBarItem.setTitleTextAttributes([.font: UIFont.boldSystemFont(ofSize: 13)], for: .normal)
            } else {
                viewController.tabBarItem.setTitleTextAttributes([.font: UIFont.systemFont(ofSize: 12)], for: .normal)
            }
        }
    }
}

PS：这也可以用自定义字体。

雨燕5

假设您有一个继承UITabBarController的类，如下所示：

final class YourTabBarController: UITabBarController { .. }

在此YourTabBarController中定义2种类型的属性：

let normalTabBarAttributes: [NSAttributedString.Key: Any] = [
    .font: NotSelectedFont,
    .foregroundColor: NotSelectedColor
]
let selectedTabBarAttributes: [NSAttributedString.Key: Any] = [
    .font: SelectedFont,
    .foregroundColor: SelectedColor
]

您有一个名为YourViewController的ViewController，它包含TabBar并继承UIViewController，如下所示：

final class YourViewController: UIViewController {...}

iOS 15.0或更高版本

在YourViewController中定义方法：

private func setTabBarAttributes() {
    guard let yourTabBarController = tabBarController as? YourTabBarController else {
        return
    }
    let appearance = UITabBarAppearance()
    appearance.stackedLayoutAppearance.normal.titleTextAttributes = yourTabBarController.normalTabBarAttributes
    appearance.stackedLayoutAppearance.selected.titleTextAttributes = yourTabBarController.selectedTabBarAttributes
    yourTabBarController.tabBar.standardAppearance = appearance
    yourTabBarController.tabBar.scrollEdgeAppearance = appearance
}

从YourViewController的override func viewDidLoad()方法调用该方法：

setTabBarAttributes()

iOS 15.0之前的版本

在YourTabBarController中，您可以定义一个方法：

private func updateTabBarAttributes() {
    guard let viewControllers = viewControllers else { return }
    for viewController in viewControllers {
        if viewController == selectedViewController {
            viewController.tabBarItem.setTitleTextAttributes(selectedTabBarAttributes, for: .normal)
        } else {
            viewController.tabBarItem.setTitleTextAttributes(normalTabBarAttributes, for: .normal)
        }
    }
}

现在使用如下方法：

override var selectedIndex: Int {
    didSet {
        updateTabBarAttributes()
    }
}

override var selectedViewController: UIViewController? {
    didSet {
        updateTabBarAttributes()
    }
}