当我模拟后台抓取时，我收到一个警告，说完成处理程序从未被调用过

Question

为了设置background fetch，我遵循了所有步骤，但我怀疑在AppDelegate中编写函数performFetchWithCompletionHandler时犯了错误。

以下是我在模拟background fetch时立即收到的警告

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

下面是我的代码：

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")
      }
    }
  }
}

如何测试background-fetch是否正常工作？

Answer 1

如果不输入第一个If语句，则永远不会调用完成处理程序。此外，当您循环遍历视图控制器时，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，您可能应该在调用完成处理程序之后放置一个return。

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)
}