Swift implementation of C#-style Events concept (eg a manager with multiple weakly-referenced delegates)

WeakDelegates.swift

import Foundation

/// Since there can be multiple delegates, they can only be used to signifying events (output), not for
/// supplying data for the manager (input).
@objc protocol MyManagerDelegate {
    func manager(manager: MyManager, isNowLoading: Bool)
}

@objc class MyManager {

    /// Multiple delegates.
    let delegates = NSHashTable.weakObjectsHashTable()
    
    // Add a delegate.
    func addDelegate(delegate: MyManagerDelegate) {
        delegates.addObject(delegate)
    }
    
    /// Example of how you'd call the multiple delegates.
    private func tellDelegatesIsNowLoading(loading: Bool) {
        for object in delegates.allObjects {
            let delegate = object as MyManagerDelegate
            delegate.manager(self, isNowLoading: loading)
        }
    }
}

Granted, KVO is often a good alternative to this. Unfortunately, KVO isn't a first-class member of Swift, and combined with its antique API, I anticipate it being replaced by something else soon.