在将图像保存到文档目录后更新 bool 变量在 Swift 中没有按预期工作

Question

我正在处理我的应用程序的用户个人资料页面。我有一个默认设置为 false 的全局 bool 变量 (updateProfile)。当用户对他们的个人资料信息进行任何更改时，例如 change/remove 他们的个人资料图片，数据库会更新，图像会被下载并保存在文档目录中。这是下载后保存图像的代码：

struct Downloads {

    // Create a static variable to start the download after tapping on the done button in the editUserProfile page
    static var updateProfile: Bool = false

    static func downloadUserProfilePic() {

        Database.database().reference().child("Users").child(userID!).child("Profile Picture URL").observeSingleEvent(of: .value) { (snapshot) in
        guard let profilePictureString = snapshot.value as? String else { return }
        guard let profilePicURL = URL(string: profilePictureString) else { return }

        let session = URLSession(configuration: .default)

        let downloadPicTask = session.dataTask(with: profilePicURL) {
                (data, response, error) in

            if let e = error {
                print("error downloading with error: \(e)")

            } else {
                if let res = response as? HTTPURLResponse {
                    Downloads.imageCached = true // The image has been downloaded
                    print("Downloaded with \(res.statusCode)")

                    if let imageData = data {
                        let image = UIImage(data: imageData)

                        // Now save the image in the documents directory
                        // Get the url of the documents directory
                        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
                        // Name your image with the user ID to make it unique
                        let imageName = userID! + "profilePic.jpg"
                        // Create the destination file URL to save the image
                        let imageURL = documentsDirectory.appendingPathComponent(imageName)
                        print(imageURL)
                        let data = image?.jpegData(compressionQuality: 0.5)

                        do {
                            // Save the downloaded image to the documents directory

                            // Write the image data to disk
                            try data!.write(to: imageURL)
                            print("Image Saved")
                            updateProfile = true

                        } catch {
                            print("Error saving file \(error)")
                        }


                    } else {
                        print("Couldnt get image")
                    }

                } else {
                    print("Couldnt't get response")
                }
            }
        }
        downloadPicTask.resume()        

    }
}

SomeOtherViewController

// When the user taps on the 'done' button
@objc func doneButtonTapped() {

    uploadUserSelectedPicture()

}

func uploadUserSelectedPicture() {

    // Download the profile picture and save it
    Downloads.downloadUserProfilePic()

    if Downloads.updateProfile == true {
        // Go to the user profile page
        let userProfilePage = UserProfilePage()
        self.present(userProfilePage, animated: true)
    }

}

如您所见，我在将图像保存到文档目录并且 updateProfile 全局变量更改为 true 后立即打印 "Image saved"。在 SomeOtherViewController 上，仅当 updateProfile 变量为 true 时（这意味着图像应保存到文档目录）才会显示页面（当点击完成按钮时）。

但唯一的问题是，在保存图像之前变量设置为 true，我怎么知道的？我知道这一点是因为页面是在执行 print 语句 print("Image Saved") 之前呈现的。为什么会这样？有什么办法可以解决这个问题吗？

Answer 1

实际上您的代码应该永远不会显示页面。但是，由于您在调用 downloadUserProfilePic 之前忘记将 updateProfile 显式设置为 false，因此显示的页面没有第二次调用时的图像。

然而，observe 和 dataTask 都是异步工作的。您必须添加一个完成处理程序。

简单形式：

~~static var updateProfile: Bool = false~~

static func downloadUserProfilePic(completion: @escaping () -> Void) {

...

    do {
       // Save the downloaded image to the documents directory

       // Write the image data to disk
       try data!.write(to: imageURL)
       print("Image Saved")          
       completion()
    }

...

func uploadUserSelectedPicture() {

    // Download the profile picture and save it
    Downloads.downloadUserProfilePic { [weak self] in
        // Go to the user profile page
        DispatchQueue.main.async {
           let userProfilePage = UserProfilePage()
           self?.present(userProfilePage, animated: true)
        }
    }

}

在将图像保存到文档目录后更新 bool 变量在 Swift 中没有按预期工作

Updating a bool variable after saving image to documents directory doesn’t work as expected in Swift

ios

documentsdirectory

swift