如何在视图控制器被关闭后显示警报

Question

我有一个应用，用户在后台上传文件通常需要几秒钟。当他们点击 "Done" 按钮并且同时关闭视图控制器时，上传开始。我希望发生的是下载完成后出现警报。我以为我只是将下面的代码添加到上传功能，但它不起作用。我怎样才能出现一个警告框来确认上传成功？

@IBAction func tapDone(_ sender: Any) {
    self.dismiss(animated: true, completion: nil)
    if let image = newImage {
        submit2Parse(image: image)
    }
}

func submit2Parse (image: UIImage) {
    if let pfImage = image2PFFile(image: image) {
        // Insert PFFile into parse server
        let submittedImage = PFObject(className: "Images")
        submittedImage["imageFile"] = pfImage
        submittedImage["type"] = "submittedFromUserHome"
        submittedImage["bride"] = brideSwitch.isOn
        submittedImage["groom"] = groomSwitch.isOn
        submittedImage["user"] = userSwitch.isOn
        submittedImage["picturePeriod"] = pickerSelected
        submittedImage["uploadedByUserId"] = PFUser.current()?.objectId ?? ""            submittedImage["uploadedByUserName"] = PFUser.current()?.username ?? ""
        if !txtIsPlaceHolder { submittedImage["description"] = imageDescription.text }

        submittedImage.saveInBackground { (success, error) in
            if success {
                let message = "Save in bg worked"
                print(message)
                let alert = UIAlertController(title: title, message: message, preferredStyle: UIAlertController.Style.alert)
                alert.addAction(UIAlertAction(title: "Ok", style: UIAlertAction.Style.default, handler: {
                    (action) in
                    self.dismiss(animated: true, completion: nil)
                }))
                self.present(alert,animated: true, completion: nil)

            } else {
                print(error?.localizedDescription ?? "")
            }
        }
    }
}

代码给出了这个构建错误：

Implicit use of 'self' in closure; use 'self.' to make capture semantics explicit

Answer 1

在你的tapDone方法中，你需要使用completion关闭你的控制器，像这样：

self.dismiss(animated: true) {
    if let image = newImage {
        self.submit2Parse(image: image)
    }
}

这仅意味着 self.submit2Parse(image: image) 将仅在您关闭控制器后执行。此外，错误

Implicit use of 'self' in closure; use 'self.' to make capture semantics explicit

only 意味着您需要使用 self. 在闭包中显式调用变量或方法。所以你的 submit2Parse... 现在应该是这样的：

self.submit2Parse(image: image).

Answer 2

如果我对你的问题的理解正确，你想在用户点击 tapDone 按钮时关闭你的 SecondViewController。之后，一旦您的图片上传完成，您就会想要显示成功警报。

但是如果你 dismiss 它一旦点击按钮你将不会收到任何警报，因为你的 SecondViewController 不再在 window hierarchy 中，如果你尝试显示警报你将在控制台中收到如下错误：

Attempt to present on whose view is not in the window hierarchy!

要解决这个问题，您需要在您关闭第二个视图控制器后显示第一个视图控制器的警报，您可以使用 delegate.

来实现

考虑以下示例：

首先在你的 FirstViewController 之外声明一个 protocol:

protocol UplaodSuccessDelegate:class {
    func uploadSuccess(message: String)
}

然后确认您的代理人相同 class:

class ViewController: FirstViewController, UplaodSuccessDelegate {

那么在present的时候需要通过delegate SecondViewController:

let vc = self.storyboard?.instantiateViewController(withIdentifier: "SecondViewController") as! SecondViewController
vc.delegate = self. //Pass delegate
self.present(vc, animated: true, completion: nil)

并将委托方法添加到相同的 class:

func uploadSuccess(message: String) {

    let message = "Save in bg worked"
    print(message)
    let alert = UIAlertController(title: "title", message: message, preferredStyle: UIAlertController.Style.alert)
    alert.addAction(UIAlertAction(title: "Ok", style: UIAlertAction.Style.default, handler: {
        (action) in

    }))
    self.present(alert,animated: true, completion: nil)
}

现在在您的 SecondViewController 中您需要添加

weak var delegate: UplaodSuccessDelegate?

并在您的 tapDone 方法中将代码替换为：

self.dismiss(animated: true) {
    if let image = newImage {
        submit2Parse(image: image)
    }
}

上传完成后，您需要调用委托方法（上传完成后），例如：

self.delegate?.uploadSuccess(message: "your message")

这将从 FirstViewController.

调用您的委托方法

Answer 3

当你将下面的函数放到viewController中，在后台启动数据上传，然后在提交完成时触发的闭包中调用这个函数，它甚至能够成功创建一个警报认为最初的视图控制器被解雇了（如果是长时间上传，则被解雇了很长时间）。

// This is the function that performs my background upload
    func submit2Parse (image: UIImage) {
        if let pfImage = image2PFFile(image: image) {
            // Insert PFFile into parse server
            let submittedImage = PFObject(className: "Images")
            submittedImage["imageFile"] = pfImage
            submittedImage["imageCreateDt"] = newImageCreateDate
            submittedImage["type"] = "submittedFromUserHome"
            submittedImage["bride"] = brideSwitch.isOn
            submittedImage["groom"] = groomSwitch.isOn
            submittedImage["user"] = userSwitch.isOn
            submittedImage["picturePeriod"] = pickerSelected
            submittedImage["uploadedByUserId"] = PFUser.current()?.objectId ?? ""
            submittedImage["uploadedByUserName"] = PFUser.current()?.username ?? ""
            if !txtIsPlaceHolder { submittedImage["description"] = imageDescription.text }
            // Get the image timestamp, every photo has one
            // How do you get the thumbnail image too?

            submittedImage.saveInBackground { (success, error) in
                if success {
                    let message = "Save in bg worked"
                    print(message)
                    self.showAlertFromAppDelegates()
                } else {
                    print(error?.localizedDescription ?? "")
                }
            }
        }
    }

// This is the function that creates the alert
    func showAlertFromAppDelegates(){
        var topWindow: UIWindow? = UIWindow(frame: UIScreen.main.bounds)
        topWindow?.rootViewController = UIViewController()
        topWindow?.windowLevel = UIWindow.Level.alert + 1
        let alert: UIAlertController =  UIAlertController(title: "Upload Complete", message: "Your image was successfully submitted!", preferredStyle: .alert)
        alert.addAction(UIAlertAction.init(title: "OK", style: .default, handler: { (alertAction) in
            topWindow?.isHidden = true
            topWindow = nil
        }))
        topWindow?.makeKeyAndVisible()
        topWindow?.rootViewController?.present(alert, animated: true, completion:nil)
    }

如何在视图控制器被关闭后显示警报

How to present an alert after view controller has been dismissed

ios

swift

uialertcontroller