关闭表视图后，单元格没有被释放，被闭包的上下文引用

Question

I'm creating a custom table view cell, which allows the user to add, take, or view uploaded photos.

I discovered that this cell stays in memory forever even after the table view's dismissal, creating weird memory graph . I want the cell to be dismissed properly , but I have a hard time understanding what is going on.

The graph shows that my cell is being strongly referenced by a addPhotoTapAction.context .

addPhotoTapAction: ((ItemInfoCell) -> Void)? is the cell's class variable, used to store the closure handling user input. The closure is defined in the view controller:

let infocell = tableView.dequeueReusableCell(withIdentifier: K.infocellID) as! ItemInfoCell
    if item?.imageUrl == nil {
        self.imageManager.actionController?.actions[2].isEnabled = false
    } else {
        self.imageManager.actionController?.actions[2].isEnabled = true
    }
    infocell.addPhotoTapAction = { [unowned self] _ in
        infocell.addPhotoButton.isEnabled = false
        self.imageManager.pickImage(self) { [weak self] image in
            self?.imageToSave = image
            infocell.itemPhoto.image = self?.imageToSave
            infocell.addPhotoButton.tintColor = UIColor(ciColor: .clear)
            infocell.addPhotoButton.isEnabled = true
            self?.imageManager.actionController?.actions[2].isEnabled = true
        }

The pickImage method is shown below. It's used to present action controller with image picker options (take photo or choose from lib):

func pickImage(_ viewController: UIViewController, _ callback: @escaping ((UIImage) -> ())) {
    picker.delegate = self
    picker.mediaTypes = ["public.image"]
    picker.allowsEditing = true
    pickImageCallback = callback
    self.viewController = viewController
    actionController!.popoverPresentationController?.sourceView = viewController.view
    viewController.present(actionController!, animated: true, completion: nil)
}

...and the callback is stored to be used in picker's didFinishPickingMediaWithInfo call:

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any]) {
    picker.dismiss(animated: true, completion: nil)
    if let image = info[.editedImage] as? UIImage {
        let squareImage = makeSquare(image)
        pickImageCallback?(squareImage)
    } else if let image = info[.originalImage] as? UIImage {
        let squareImage = makeSquare(image)
        pickImageCallback?(squareImage)
    }
    viewController = nil
}

I've tried to manually set variable with closure to nil , then switched from [weak self] to [unowned self] to the combination of both. No luck.

I think that either the pickImage(self) , or using the class' properties within the closure are creating a strong reference even when using [weak/unowned] capture lists, but I'm still not sure and not being able to fix it.

Update: ItemInfoCell class' code

class ItemInfoCell: UITableViewCell {

@IBOutlet weak var itemPhoto: UIImageView!
@IBOutlet weak var itemLabel: UILabel!
@IBOutlet weak var addPhotoButton: UIButton!

var addPhotoTapAction: ((ItemInfoCell) -> Void)?

override func awakeFromNib() {
    super.awakeFromNib()
}

override func setSelected(_ selected: Bool, animated: Bool) {
    super.setSelected(selected, animated: animated)
}

@IBAction func takePhoto(_ sender: Any) {
    if let addPhoto = self.addPhotoTapAction {
        addPhoto(self)
    }
}

}

Answer 1

The problem is that you access the infocell inside it's callback. If you use variable inside own callback, you should mark it as a weak by adding it to the capture list.

In your code it should be like this:

   infocell.addPhotoTapAction = { [unowned self, weak infocell] _ in
      ...
    }

Answer 2

In your code, there is a strong reference from infocell to closure and inside closure you are referring to infocell ie both have a strong reference to each other.

infocell.addPhotoTapAction = { [unowned self] _ in
    infocell.addPhotoButton.isEnabled = false
     ...
}

This leads to a retain cycle and doesn't allow the cell to be deallocated.

Since you are already using capture list for self, you can easily solve this issue by adding infocell to the capture list, like so:

infocell.addPhotoTapAction = { [weak self, weak infocell] _ in
    guard let self = self else { return }
    infocell.addPhotoButton.isEnabled = false
    ...
}

Generally speaking, it is suggested to use unowned only when you are completely sure that the reference will not become nil before execution of the closure, because using unowned is like forcefully unwrapping an optional value. If it is nil your app will crash. So weak self is generally the safer way to go.