public protocol Observable: class {
    associatedtype Observer: AnyObject
    var observers: [Observer] { get set }
    func addObserver(_ observer: Observer) -> Void
}

public extension Observable {
    public func addObserver(_ observer: Observer) -> Void {
        observers.append(observer)
    }
}

protocol TestObservableListener: class {
    func bla()
}

protocol TestObservable: Observable where Observer == TestObservableListener {
}

class TestObservableImpl: TestObservable {
    typealias Observer = TestObservableListener
    public var observers = [Observer]()
    func broadcast() {
        for observer in observers {
            //2
            observer.bla()
        }
    }
}

class TestObserverImpl: TestObservableListener {
    func bla() {
        print("bla")
    }
}

let observable = TestObservableImpl()
let observer = TestObserverImpl()
observable.addObserver(observer)
//1
observable.broadcast()