public Subscription(PeriodicObservable <T> observable, IObserver <T> observer)
 {
     this.observable = observable;
     this.observer   = observer;
 }
 public DisposablePeriodicObservable(TimeSpan period, Func <T> factory, Action onDispose)
 {
     periodicObservable = new PeriodicObservable <T>(period, factory);
     this.onDispose     = onDispose;
 }