从子视图控制器调用函数不起作用

// 定义协议
protocol ParentViewControllerDelegate: AnyObject {
    func parentViewControllerDidSomething()
}

// 父VC
class ParentViewController: UIViewController, ParentViewControllerDelegate {
    override func viewDidLoad() {
        super.viewDidLoad()
        let childVC = ChildViewController()
        childVC.delegate = self
        addChild(childVC)
        view.addSubview(childVC.view)
        childVC.didMove(toParent: self)
    }

    func parentViewControllerDidSomething() {
        print("Parent VC did something")
    }
}

// 子VC
class ChildViewController: UIViewController {
    weak var delegate: ParentViewControllerDelegate?

    func someFunction() {
        delegate?.parentViewControllerDidSomething()
    }
}