Under what circumstances should a weak reference be used inside a closure?

Weak references inside closures in iOS are used to prevent strong reference cycles (retain cycles), which can lead to memory leaks. This is usually relevant when a closure captures an object (for example, self) and is retained by that object, creating a mutual strong reference.

Use weak to reference self inside a closure if the closure is stored in a property of an object or may live longer than the function call, to avoid holding the object in memory after it should be released.

Example in Swift:

class ViewController {
    var completion: (() -> Void)?

    func setup() {
        completion = { [weak self] in
            guard let self = self else { return }
            self.doSomething()
        }
    }

    func doSomething() {
        print("Action performed")
    }
}

Here, [weak self] prevents a strong cycle between ViewController and the completion closure.