Protocol 'ObserverProtocol' can only be used as a generic constraint because it has Self or associated type requirements

gistfile1.txt

protocol ObserverProtocol: Equatable {
    func notified()
}

class Observable: NSObject {
    var observers: [ObserverProtocol] = []
    override init() {
        super.init()
    }
    func addObserver(observer: ObserverProtocol) {
        observers.append(observer)
    }
    func removeObserver(observer: ObserverProtocol) {
        if let index = find(observers, observer) {
            observers.removeAtIndex(index)
        }
    }
}