[weak self] vs [unowned self] — when to use which?

Middle PRO

The difference

Both avoid bumping self's strong count and so break the cycle "object stores closure, closure holds object". They differ in behavior once self is dead:

• [weak self] — self inside becomes Optional; after deallocation it reads as nil. Always safe; the cost is guard let self and a slightly more expensive read (side table).
• [unowned self] — self is non-optional; access after deallocation is a deterministic trap "Attempted to read an unowned reference but object was already deallocated". Cheaper (the count is in the object, no side table), but it is a promise to the compiler: "self is definitely alive".

final class ViewModel {
    var title = ""
    var onChange: (() -> Void)?

    func bind(label: UILabel) {
        onChange = { [weak self] in
            guard let self else { return }   // closure may outlive self? → weak
            label.text = self.title
        }
    }
}