Storing weak reference sometimes causes leak

Question

I have a class A that stores a optional weak variable to other object, that is a subclass of A. Sometimes when I store something in this variable I get a leak - this happens rarely, but it does. I'm assigning this variable in forEach loop, but I also found similar leak in other place of application once when using weak var, so I don't think that loop has anything to do with this. Responsible library is libswiftCore.dylib and responsible frame is swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::formWeakReference(). Did someone else have had similar issue? Is there some way I can fix this?

Answer 1

Try setting the variable instead of being weak set it to be unowned. This is a Swift bug sort of as there is no warning for the developer that he is capturing a strong reference of the nested closure, however, setting it to unowned should do as a workaround for now.

EDIT1: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001197.html

Mentioned as an improvement from here here:

class ViewControllerBase: UIViewController {
let timer:DispatchSourceTimer = DispatchSource.makeTimerSource(flags: [], queue:  DispatchQueue(label: "q.q"))

deinit {
    NSLog("deinit of \(NSStringFromClass(type(of: self)))")
}

override func viewDidLoad() {
    super.viewDidLoad()

    timer.scheduleRepeating(deadline: .now(), interval: .seconds(1))

    timer.setEventHandler {
        UIView.animate(withDuration: 0.2) { [weak self] in
            self?.view.backgroundColor = UIColor.green
        }
    }
}

This leaks memory in a subtle way...there is a retain loop even though self is only used in a "weak" manner. This is because the nested closure captures a strong reference for use in the closure that follows.

EDIT 2: I might be wrong but the OP is using changes.forEach closure and then another closure with changes.added.forEach I could be wrong but this might be the cause of the whole issue perhaps declaring it there [weak self] (cluster, change) in ... could possibly remove the issue. It is swifts built in closure but still a closure, that could technically cause them to be nested.

Try changing your code to:

changes.forEach{[weak self] (cluster, change) in

see if that helps

or

changes.forEach{[unowned self] (cluster, change) in

Also any chance that you could paste your code instead of doing a screenshot as it is easier to recreate your code instead of retyping it.