iOS:如何检测外部键盘上的修改键状态更改？

import UIKit

class ViewController: UIViewController {
    
    override var canBecomeFirstResponder: Bool {
        return true
    }
    
    override var keyCommands: [UIKeyCommand]? {
        return [
            UIKeyCommand(input: "a", modifierFlags: .command, action: #selector(commandKeyPressed(_:))),
            UIKeyCommand(input: "b", modifierFlags: .control, action: #selector(controlKeyPressed(_:))),
            UIKeyCommand(input: "c", modifierFlags: .shift, action: #selector(shiftKeyPressed(_:)))
        ]
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        becomeFirstResponder()
    }
    
    @objc func commandKeyPressed(_ sender: UIKeyCommand) {
        // 处理按下 Command 键的逻辑
        print("Command key pressed")
    }
    
    @objc func controlKeyPressed(_ sender: UIKeyCommand) {
        // 处理按下 Control 键的逻辑
        print("Control key pressed")
    }
    
    @objc func shiftKeyPressed(_ sender: UIKeyCommand) {
        // 处理按下 Shift 键的逻辑
        print("Shift key pressed")
    }
}