在另一个类中访问RootViewController的数据成员

protocol RootViewControllerDelegate {
    func getData() -> Data
}

class RootViewController: UIViewController {
    var delegate: RootViewControllerDelegate?

    // 数据成员
    var data: Data?

    // 其他代码
}

class AnotherViewController: UIViewController, RootViewControllerDelegate {
    var rootViewController: RootViewController?

    override func viewDidLoad() {
        super.viewDidLoad()

        rootViewController?.delegate = self
    }

    // 实现协议方法
    func getData() -> Data {
        return rootViewController?.data ?? Data()
    }
}