/// <summary>
/// Subscribes our inbox to another process state publish stream.
/// When a process state updates at the end of its message loop it announces it arrives in our inbox.
/// You should use this for notification only. Never modify the state object belonging to a process.
/// Best practice is to make the state type immutable.
/// </summary>
/// <remarks>
/// The process can publish any number of types, any published messages not of type T will be ignored.
/// This should be used from within a process' message loop only
/// </remarks>
/// <returns>IObservable T</returns>
public static Unit subscribeState <T>(ProcessId pid) =>
InMessageLoop
? ActorContext.SelfProcess.Actor.AddSubscription(
pid,
ActorContext.ObserveState <T>(pid).Subscribe(x => tell(Self, x, pid)))
: raiseUseInMsgLoopOnlyException <Unit>(nameof(subscribeState));
/// <summary>
/// Get an IObservable for a process's state stream. When a process state updates at the end of its
/// message loop it announces it on the stream returned from this method. You should use this for
/// notification only. Never modify the state object belonging to a process. Best practice is to make
/// the state type immutable.
/// </summary>
/// <remarks>
/// The process can publish any number of types, any published messages not of type T will be ignored.
///
/// Because this call is asychronous it could allow access to the message loop, therefore
/// you can't call it from within a process message loop.
/// </remarks>
/// <returns>IObservable T</returns>
public static IObservable <T> observeState <T>(ProcessId pid) =>
InMessageLoop
? raiseDontUseInMessageLoopException <IObservable <T> >(nameof(observeState))
: ActorContext.ObserveState <T>(pid);
