取决于你想做多少...
对于复杂的、功能齐全的"富文本编辑"，我建议搜索已经构建的解决方案--有很多开源的解决方案。如果你不能找到一个完全满足你需要的解决方案，我相信你可以找到一个足够接近的解决方案，你可以浏览代码并编辑它以满足你的需要。
对于一个相当简单的实现，您可以设置文本字段的.typingAttributes-参见Apple的docs。
这里有一个非常简单的例子：

class ViewController: UIViewController {
    
    let textField = UITextField()
    
    let normalFont: UIFont = .systemFont(ofSize: 17, weight: .regular)
    let boldFont: UIFont = .systemFont(ofSize: 17, weight: .bold)
    
    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
        
        textField.borderStyle = .roundedRect
        textField.font = normalFont
        
        let v = UILabel()
        v.text = "Bold on/off"
        
        let sw = UISwitch()
        
        let ctrlStack = UIStackView(arrangedSubviews: [v, sw])
        ctrlStack.spacing = 8
        
        ctrlStack.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(ctrlStack)
        
        textField.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(textField)
        
        let g = view.safeAreaLayoutGuide
        
        NSLayoutConstraint.activate([
            
            ctrlStack.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            ctrlStack.centerXAnchor.constraint(equalTo: g.centerXAnchor),
            
            textField.topAnchor.constraint(equalTo: ctrlStack.bottomAnchor, constant: 20.0),
            textField.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
            textField.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),

        ])
        
        sw.addTarget(self, action: #selector(swChanged(_:)), for: .valueChanged)
    }
    
    @objc func swChanged(_ sender: UISwitch) {
        textField.typingAttributes?[NSAttributedString.Key.font] = sender.isOn ? boldFont : normalFont
    }
}

看起来像这样：

轻触开关以启用粗体"On"，然后再键入一些内容：

关闭粗体并输入更多内容：

当你试图“取消粗体”你的文本，你设置默认字体属性字符串作为字体从你的文本字段

UIFont.boldSystemFont(ofSize: textField.font!.pointSize) as Any

以前设置为粗体
因此，将代码更改为

var isBoldEnabled = false
let defaultFont = UIFont.systemFont(ofSize: 16)

override func viewDidLoad() {
    super.viewDidLoad()

    // Set the initial font for the text field
    textField.font = defaultFont
}

@IBAction func boldButtonTapped(_ sender: Any) {
    // Toggle the isBoldEnabled variable
    isBoldEnabled = !isBoldEnabled

    // Create a new attributed string with the existing text and the current font
    let attributedString = NSMutableAttributedString(string: textField.text ?? "")
    let attributes: [NSAttributedString.Key: Any] = [NSAttributedString.Key.font: defaultFont as Any]
    attributedString.addAttributes(attributes, range: NSRange(location: 0, length: attributedString.length))
    ...
}