Question

我提出了一个模态视图控制器，用户可以在其中输入一些信息，然后使用此功能保存该信息...

func handleSave() {

    guard let newProductUrl = NSURL(string: urlTextField.text!) else {
        print("error getting text from product url field")
        return
    }
    guard let newProductName = self.nameTextField.text else {
        print("error getting text from product name field")
        return
    }
    guard let newProductImage = self.logoTextField.text else {
        print("error getting text from product logo field")
        return
    }

    DispatchQueue.main.async {
        self.productController?.save(name: newProductName, url: newProductUrl as URL, image: newProductImage)
    }


    // Present reloaded view controller with new product added
    let ac = UINavigationController()
    let pController = ProductController()
    productController = pController
    ac.viewControllers = [pController]
    present(ac, animated: true, completion: nil)

}

...我在viewWillAppear ProductController（显示模态视图控制器的控制器，现在正试图回复）中收到错误

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    guard let appDelegate =
        UIApplication.shared.delegate as? AppDelegate else {
            return
    }

    let managedContext =
        appDelegate.persistentContainer.viewContext

    let companyToDisplay = self.navigationItem.title!

    let fetchRequest =
        NSFetchRequest<NSManagedObject>(entityName: "Product")

    fetchRequest.predicate = NSPredicate(format:"company.name == %@",companyToDisplay)

    do {
        products = try managedContext.fetch(fetchRequest)
        print(products)
    } catch let error as NSError {
        print("Could not fetch. \(error), \(error.userInfo)")
    }
}

错误是：在let companyToDisplay = self.navigationItem.title!行上打开一个可选项时意外发现nil。如何指定它所寻找（和缺失）的self.navigationItem.title是发送模态视图的控制器的self.navigationItem.title？

感谢您提供任何帮助，我一直试图将此问题排除数日并且无法解决问题。

编辑：这就是我在AddProductController

中展示模态视图ProductController的方式

func presentModalView() {

    let nc = UINavigationController()
    let addProductController = AddProductController()
    nc.viewControllers = [addProductController]

    self.modalTransitionStyle = UIModalTransitionStyle.coverVertical
    self.modalPresentationStyle = .currentContext
    self.present(nc, animated: true, completion: nil)
}

编辑：将代码放入调度块：

    DispatchQueue.main.async {
        self.productController?.save(name: newProductName, url: newProductUrl, image: newProductImage)

        let pController = ProductController()
        self.productController = pController
        self.navigationController?.pushViewController(pController, animated: true)
    }

Answer 1

这是你的问题，你使用present（ac，animated：true，completion：nil）来获取这个视图，所以这个演示文稿是模态的，没有navigationController。您必须使用UINavigationController.pushViewController来呈现视图，这样您将获得navigationController。

编辑：

        let nc = UINavigationController()
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let addProductController = storyboard.instantiateViewController(withIdentifier: "addProductVC")
    nc.viewControllers = [addProductController]

    self.modalTransitionStyle = UIModalTransitionStyle.coverVertical
    self.modalPresentationStyle = .currentContext
    self.present(nc, animated: true, completion: nil)

不要忘记在故事板中设置addProductController的标识符，将故事板和VC标识符更改为您的标识符。

编辑2：

            let nc = UINavigationController()
    let addProductController = ProductController()
    addProductController.navigationItem.title = "Have a good one"
    nc.viewControllers = [addProductController]

    self.modalTransitionStyle = UIModalTransitionStyle.coverVertical
    self.modalPresentationStyle = .currentContext
    self.present(nc, animated: true, completion: nil)

Answer 2

问题在于此调用中的阻止：

DispatchQueue.main.async {
    self.productController?.save(name: newProductName, url: newProductUrl as URL, image: newProductImage)
}

实际执行后，handleSave()方法返回。看起来你期望它相对于该方法中的其他代码顺序执行。

DispathQueue.main.async将一个块添加到将来某个时间点执行的代码队列中 - 它不会立即执行。

要解决此问题，您需要将代码放在调度块中，以执行下一步需要执行的操作。这与你在这里的相似：

// Present reloaded view controller with new product added let ac = UINavigationController() let pController = ProductController() productController = pController ac.viewControllers = [pController] present(ac, animated: true, completion: nil)

但是你可能想要清理你创建和解雇视图控制器的方式/位置 - 你在这里看到它会不必要地堆积一堆视图控制器。

重新显示视图控制器时出错

2 个答案: