I agree that, all things considered, an Event will be the most appropriate way to handle this.

In my experience, “murder is always messy.”   :-D   More to the point, “it's never exactly the same way twice.” So, it's very difficult to test.

You might have one thread in the application that does nothing but wait for the closing whistle. It starts-up the app, waits for the whistle, quickly notifies the workers, waits briefly for them to respond, and then closes down the app as neatly as it can manage.