By makig your own service that uses Obervables indeed, like you said.
Building a BehaviorSubject service that emits a string.
@rapropos has a great post for this: Ionic 4 Tab to page then back to Tab did not trigger ionViewWillEnter [Solved]
Even though, the better way would be not to do that and use router events or a more strictly typed emitter for whatever async stuff you want to capture or share as state within your app…
The string Event emitter is a gateway to event-soup, if you actually did not think of the above before yourself
Tom