Swift 从闭包内部添加 VC 不起作用

Question

So i'm trying to display a loading view when a user tap on a button. Below is the function

    /// Handles the shutter button tapped
    @objc func shutterTapped() {
        let parent = self.parent as! CameraActionViewController

        if parent.timerEngine.currentTime != 0 { parent.timerEngine.presentTimerDisplay() }
// This acts as a timer for the shutter to fire. (like a timer function for a camera)
        DispatchQueue.main.asyncAfter(deadline: .now() + Double(parent.timerEngine.currentTime)) {

            parent.cameraManager.capturePictureWithCompletion({ result in
                switch result {
                case .failure:
                    AlertHelper.shared.presentDefault(title: "Error", message: "Looks like there was an error capturing the image.", to: self)

                case .success(let content):
                    let loadingVC = LoadingViewController()
                    parent.parent!.addVC(loadingVC)
                    print("processing...")

                    if let image = content.asImage {
                        guard let editedImage = FilterEngine.shared.process(originalImage: image) else {
                            TapticHelper.shared.errorTaptic()
                            return
                        }

                        guard let data = editedImage.jpegData(compressionQuality: 1.0) else {
                            TapticHelper.shared.errorTaptic()
                            return
                        }

                        DataEngine.shared.save(imageData: data)
                        print("Success")
                        TapticHelper.shared.successTaptic()
                        loadingVC.removeVC()
                    }
                }
            })
        }
    }

When the camera successfully captures the image and start to edit with all the filters selected by the user, I want to display the loading view. Because this is a heavy task and will freeze the UI for a second.

let loadingVC = LoadingViewController()
parent.parent!.addVC(loadingVC)
...
loadingVC.removeVC()

The method to add is here:

    func addVC(_ child: UIViewController) {
        addChild(child)
        view.addSubview(child.view)
        child.didMove(toParent: self)
    }

    func removeVC() {
        guard parent != nil else { return }

        willMove(toParent: nil)
        view.removeFromSuperview()
        removeFromParent()
    }

But for some reason, it is not showing the view at all. I then found out that it is somehow executed after the DataEngine.shared.save(imageData: data) line (adding to VC). Therefore since it is adding and removing at the same time, it shows nothing.

Any help?

Answer 1

You need to Dispatch all UI related tasks to the main queue. Try:

DispatchQueue.main.async {
    parent.parent!.addVC(loadingVC)
}

And:

DispatchQueue.main.async {
    loadingVC.removeVC()
}

Or simply put everything in the main queue right after the background task is complete:

parent.cameraManager.capturePictureWithCompletion({ result in
    DispatchQueue.main.async {
        //...
    }
}