DelegateForwarder.swift

import Foundation

private class Weakbox {
    weak var object: NSObjectProtocol?
    init(_ object: NSObjectProtocol) {
        self.object = object
    }
}

class DelegateForwarder: NSObject {
    private var delegates = [Selector: Weakbox]()

    func setDelegate(delegate: NSObjectProtocol?, for selectors: [Selector]) {
        if let delegate = delegate {
            selectors.forEach { selector in
                self.delegates[selector] = Weakbox(delegate)
            }
        } else {
            selectors.forEach { selector in
                self.delegates.removeValue(forKey: selector)
            }
        }
    }

    override func responds(to aSelector: Selector!) -> Bool {
        return delegates[aSelector]?.object?.responds(to: aSelector) ?? super.responds(to: aSelector)
    }

    override func forwardingTarget(for aSelector: Selector!) -> Any? {
        return delegates[aSelector]?.object ?? super.forwardingTarget(for: aSelector)
    }
}