使用异步 PHPicker 选择图像后执行操作

Question

Problem

Want to perform operation BitwiseAND(UIImage1, UIImage2) this is function of OpenCV framework . UIImage1 is already presented in MainVC:

@IBOutlet weak var presentedImage: UIImageView!

So, UIImage2 must be picked additionally, whenever user wants it. Worth to mention:

@IBOutlet weak var presentedImage: UIImageView!

is also picked with the same PHPicker, as UIImage2

Current idea of solving problem...

Created 2 flags:

• First one to check, if current picked image with PHPicker is for

@IBOutlet weak var presentedImage: UIImageView!

or for "Bitwise's purpose".

This is the flag:

private var isBitwisePick: Bool = false

• Second "flag" determines type of Bitwise to perform ( AND XOR NOR OR )

 private var bitwiseOperationType: OperationTypes.ControllerTypes.BitwiseTypes?

 enum ControllerTypes{
 /*...*/
    enum BitwiseTypes{ case AND,NOR,XOR,OR }
/*.../
}

Attempt of handling user's image picking

/* This is switch-case of callback from other viewcontroller.
In our case only this one matters
*/
case .BITWISE_VC(let bitwise):
               
            self.isBitwisePick = true //Pick image for "Bitwise purpose"
            self.bitwiseOperationType = bitwise // determines type of Bitwise 
 
            // config and present PHPicker
            var config = PHPickerConfiguration()
            config.filter = .images
            config.selectionLimit = 1
            let picker = PHPickerViewController(configuration: config)
            picker.delegate = self
            self.present(picker, animated: true)

PHPickerViewControllerDelegate

extension MainViewController:  PHPickerViewControllerDelegate{
    func picker(_ picker: PHPickerViewController, didFinishPicking results: [PHPickerResult]) {
        dismiss(animated: true)
        print("test before async \(self.isBitwisePick)")
        if let itemProvider = results.first?.itemProvider, itemProvider.canLoadObject(ofClass: UIImage.self){
            let previousImage = self.presentedImage.image
            itemProvider.loadObject(ofClass: UIImage.self){ [weak self] image, error in
                DispatchQueue.main.async { [self] in
                    guard let self = self, let image = image as? UIImage, self.presentedImage.image == previousImage else {
                        return
                    }
                    print("Im in async \(self.isBitwisePick)")
                    if(self.isBitwisePick){
                        switch self.bitwiseOperationType{
                            case .AND:
                                self.presentedImage.image = OpenCVWrapper.bitwiseAND(self.presentedImage.image, with: image)
                                break
                            default:
                                print("the same for XOR OR NOR")
                            break
                        }
                    }
                    else{ self.presentedImage.image = image }
                }
            }
            self.isBitwisePick = false
        }
    }
}

With the line

print(" test before async (self.isBitwisePick)")

I can see, that flag changed and its value is true

However... The line:

print("Im in async (self.isBitwisePick)") prints value: false

I've just started with swift, read some posts and docs about solutions like DispatchQueue or DispatchGroup... But i couldn't fix it out. That's why I'm asking for your help and tips. Thank You for your time:)

Answer 1

Your data structure should be something like

private var firstImage: UIImage?
private var secondImage: UIImage?

then you can override setters or apply some other mechanism like

private var firstImage: UIImage? { didSet { reevaluateImagesSet() } }
private var secondImage: UIImage? { didSet { reevaluateImagesSet() } }

private func reevaluateImagesSet() {
    imageView1.image = firstImage
    imageView2.image = secondImage

    if let firstImage = firstImage, let secondImage = secondImage {
        applyCombinedImage(firstImage, secondImage)
    }
}

private func applyCombinedImage(_ firstImage: UIImage, _ secondImage: UIImage) {
    // do the bitwise operation of yours in here
}

Now the only problem is how to define which image you are setting. You could use a boolean flag or create a custom class which makes things like so:

   @IBAction private func selectFirstImagePressed() {
        let picker = MyPicker(controller: self)
        self.currentPicker = picker
        picker.selectImage { image in
            self.firstImage = image
        }
   }

   @IBAction private func selectSecondImagePressed() {
        let picker = MyPicker(controller: self)
        self.currentPicker = picker
        picker.selectImage { image in
            self.secondImage = image
        }
   }

This class MyPicker then corresponds to PHPickerViewControllerDelegate and implements the logic to only return an image in a provided closure. I hope you will have no problems implementing it. It is mostly extracting the code that you already have.

Answer 2

The problem is the line

 self.isBitwisePick = false

In your didFinishPicking() function. That line will execute before the completion handler of your itemProvider.loadObject(ofClass:) function is called. Then when you check isBitwisePick in your completion handler, it will be false. You should move that line inside your completion handler.