propertyWrapper WeakReference

WeakReference.swift

@propertyWrapper
struct WeakReference<T> {
    private weak var storage: AnyObject? = nil
    private var value: T?{
        get { return storage.map { $0 as! T } }
        set {
            storage = newValue.map {
                let asObject = $0 as AnyObject
                assert(asObject === $0 as AnyObject)
                return asObject
            }
        }
    }

    init(value: T? = nil) {
        self.value = value
    }

    var wrappedValue: T? {
        get { value }
        set { value = newValue }
    }
}