foo.swift

import Foundation

protocol Observer {
    associatedtype T
}

class ObservableSparseArray<Element> : NSObject {
    typealias T = Element?

    let elements : [T] = []
    let weaklyHeldObservers = NSHashTable<AnyObject>.weakObjects()

    //MARK: Observable
    func attach<O>(observer: O) where O : Observer & AnyObject, T == O.T {
        weaklyHeldObservers.add(observer)
    }
}