我收到一条警告，说当我模拟后台提取时从未调用完成处理程序

Question

我按照所有步骤进行设置，background fetch但我怀疑performFetchWithCompletionHandler在 AppDelegate 中编写函数时犯了一个错误。

这是我在模拟后立即收到的警告 background fetch

Warning: Application delegate received call to -  application:
performFetchWithCompletionHandler:but the completion handler was never called.

这是我的代码：

func application(application: UIApplication, performFetchWithCompletionHandler completionHandler: (UIBackgroundFetchResult) -> Void) {
    if let tabBarController = window?.rootViewController as? UITabBarController,
            viewControllers = tabBarController.viewControllers as [UIViewController]! {
      for viewController in viewControllers {
        if let notificationViewController = viewController as? NotificationsViewController {
         firstViewController.reloadData()
         completionHandler(.NewData)
         print("background fetch done")
      }
    }
  }
}

我如何测试它background-fetch是否有效？

Answer 1

如果您不输入第一个 if 语句，则永远不会调用完成处理程序。此外，当您循环访问视图控制器时，您可能找不到您正在寻找的视图控制器，这意味着永远不会调用完成。最后，您可能应该return在调用完成处理程序后放置一个。

func application(
    application: UIApplication,
    performFetchWithCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void
) {
    guard let tabBarController = window?.rootViewController as? UITabBarController,
        let viewControllers = tabBarController.viewControllers else {
        completionHandler(.failed)
        return
    }

    guard let notificationsViewController = viewControllers.first(where: { $0 is NotificationsViewController }) as? NotificationsViewController else {
        completionHandler(.failed)
        return
    }

    notificationViewController.reloadData()
    completionHandler(.newData)
}