DeinitObserver.swift

final class DeinitObserver {
    private let key: String
    private weak var target: AnyObject?
    private let deinitHandler: () -> Void

    deinit {
        deinitHandler()
    }

    init(for target: AnyObject, deinitHandler: @escaping () -> Void) {
        self.target = target
        self.key = UUID().uuidString
        self.deinitHandler = deinitHandler

        objc_setAssociatedObject(target, key, self, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }

    func cancel() {
        guard let target = target else { return }
        objc_setAssociatedObject(target, key, nil, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }
}