Retain cycle with Swift closure context

You should better include the core part of your code as text. Many readers are not allowed to download online resources.

Generally, if you observe unexpectedly retained instances between multiple view controllers, the retain cycle would very likely be problematic.

Do I need to be worried about this kind of retain cycle?

Yes.

Will the retain cycle be broken eventually?

No.

For the sake of OOPer and others, here’s a snippet of your code, slightly edited to simplify things:

class BlueViewModel: NSObject {
    var updateUI: (() -> Void)!
    var titleText: String {
        return "..."
    }
}

class BlueViewController: UIViewController {

    var viewModel: BlueViewModel!

    init(viewModel: BlueViewModel) {
        self.viewModel = viewModel
        super.init(nibName: "BlueViewController", bundle: nil)
        viewModel.updateUI = { [weak self] in
            self?.titleLabel.text = viewModel.titleText
        }
    }

    … more stuff …
}

You’ve done the weak self dance in

BlueViewController

, so that there’s nothing to stop that getting deallocated. However, you’ve still get a retain cycle between your closure (lines 15 through 17) and the

viewModel

object itself. Specifically, line 16 references the

viewModel

local variable from line 12, not the

viewModel

property from line 10.

There’s a bunch of ways you can fix this. Probably the easiest is to change line 16 to this:

self?.titleLabel.text = self?.viewModel.titleText

At that point the closure is only capturing

self

and thus doesn’t retain the view model directly.

However, I think it would be better to rethink your strategy a bit because the current structure just leaves potential footguns lying around your project. One approach I often use is to have the closure pass a ‘self’ value to the callback, which rules out a whole class of problems like this. For example:

class BlueViewModel: NSObject {
    var updateUI: ((BlueViewModel) -> Void)!
    …
}

Alas this won’t work in your real project because

BlueViewModel

is a subclass. These days I avoid that by avoiding inheritance, using structs for my model ‘objects’, but that’s a pretty big change of direction.

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