New Post: pub/sub: How to wait for all subscribers to complete

What you want is not possible without some other form of external locking based on the design of PubSubEvents (which I was directly part of). Events are supposed to be fire and forget from the publishers perspective. Because of the thread dispatching mechanisms of PubSubEvents that allows the subscriber to choose whether to handle the event on the publisher's thread (default), the UI thread, or a background thread, there is no way for the publisher to be sure if the subscriber has finished processing the event or not, nor in which order subscribers are notified.

If you want a more synchronous pub-sub like mechanism for something like you describe, I recommend using CompositeCommands. Those to have a pub-sub like semantic to them (you can register and unregister one or more loosely coupled handlers), but they have an imperative API to them - the "publisher" (command invoker) is explicitly saying "go do this now" and if multiple command handlers are registered, the Execute method will not return until all of the registered command handlers have been invoked and returned. There is no thread dispatching options with CompositeCommands.

There is still nothing stopping a handler from spinning up a separate thread in their handler for the command or handing it asynchronously with an async void Execute handler, but you at least get a guarantee of synchronous dispatch to the handler method start that you can't have (by design) with PubSubEvents.