iOS在更改用户界面时,我应该从主线程调用什么?

我有一个提醒演示的扩展。什么时候? showAlert() 不是从主线程调用的,我得到了“更改用户界面而不是从主线程”的行为。有时警报根本就没有出现,有时有很大的延迟。没有撞车事故。但是如果我把函数的所有代码都放在 DispatchQueue.main.async 关闭-一切正常。为什么?我不应该只从主线程调用实际更改用户界面的代码吗?

@objc extension UIAlertController {

static func showAlert(title: String?, message: String? = nil, closeActionTitle: String? = "OK", preferredStyle: UIAlertControllerStyle = .alert, actions: [UIAlertAction]? = nil) {
        let alertController = UIAlertController(title: title,
                                                message: message,
                                                preferredStyle: preferredStyle)
        var allActions = [UIAlertAction]()
        if let closeTitle = closeActionTitle {
            allActions.append(UIAlertAction(title: closeTitle, style: .cancel))
        }
        allActions.append(contentsOf: actions ?? [])
        allActions.forEach { alertController.addAction($0) }

        let vc = ClearViewController()
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = vc
        window.backgroundColor = AppTheme.color.clear
        window.windowLevel = UIWindowLevelAlert

    DispatchQueue.main.async {
        window.makeKeyAndVisible()
        vc.present(alertController, animated: true)
    }
}

}