Can I hook when a weakly-referenced object (of arbitrary type) is freed?

Question

I\'m writing a container class in Swift, which works like as java.util.WeakHashMap in Java. My current implementation is here.

class WeakRefMap&         


        

Answer 1

Sad to say, didSet or willSet observer doesn't get called when weak var raw property value is deallocated.

So, you have to use objc_setAssociatedObject in this case:

// helper class to notify deallocation
class DeallocWatcher {
    let notify:()->Void
    init(_ notify:()->Void) { self.notify = notify }
    deinit { notify() }
}

class WeakRefMap {

    private var mapping = [Key: WeakBox]()

    subscript(key: Key) -> Value? {
        get { return mapping[key]?.raw }
        set {
            if let o = newValue {
                // Add helper to associated objects.
                // When `o` is deallocated, `watcher` is also deallocated.
                // So, `watcher.deinit()` will get called.
                let watcher = DeallocWatcher { [unowned self] in self.mapping[key] = nil }
                objc_setAssociatedObject(o, unsafeAddressOf(self), watcher, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
                mapping[key] = WeakBox(o)
            }
            else {
                mapping[key] = nil
            }
        }
    }

    var count: Int { return mapping.count }

    deinit {
        // cleanup
        for e in self.mapping.values {
            objc_setAssociatedObject(e.raw, unsafeAddressOf(self), nil, 0)
        }
    }
}

NOTE: Before Swift 1.2. this solution does not work for arbitrary Swift classes.