Clarification: can a service be registered/unregistered more than once?

[angusholder]

I'm trying to write something equivalent to val ViewModel.viewModelScope: CoroutineScope:

class FooViewModel : ScopedServices.Registered {
    private val serviceScope = CoroutineScope(SupervisorJob() + Dispatchers.Main.immediate)

    override fun onServiceRegistered() {
    }

    override fun onServiceUnregistered() {
        serviceScope.cancel()
    }
}

Am I correct in thinking that once unregistered, the service is destroyed and that instance won't ever be reused? So onServiceRegistered and onServiceUnregistered only ever get called once each?

Another question; with Jetpack ViewModel, I often have a loadInitialData() method that I call from Fragment.onViewCreated, because I'd rather not kick off database calls/network calls from the constructor, it feels hacky and is not nice for testing. onServiceRegistered looks ideal for kicking off initial data loading, would you recommend that?

[Zhuinden]

Am I correct in thinking that once unregistered, the service is destroyed and that instance won't ever be reused?

Back in Simple-Stack 1.x there was a "bug" (design oversight) caused by how re-registering the same instance would call onEnterScope() to run multiple times, 2,x exists specifically because this section was heavily reworked to ensure that 1 object is tracked only once per registration.

So even if the same instance is added in multiple scopes, it would only have onServiceRegistered called when 0 -> 1, and onServiceUnregistered() only called when 1 -> 0, but not later.

Therefore, unless you cache these poor objects in some singleton and "re-add the same instance" again, then these callbacks are ensured to be only called once. If the object is instantiated along with the scope as is done in the samples in bindServices(), then they are definitely only called once.

TL;DR:

Am I correct in thinking that once unregistered, the service is destroyed and that instance won't ever be reused? So onServiceRegistered and onServiceUnregistered only ever get called once each?

Yes

Another question; with Jetpack ViewModel, I often have a loadInitialData() method that I call from Fragment.onViewCreated, because I'd rather not kick off database calls/network calls from the constructor, it feels hacky and is not nice for testing. onServiceRegistered looks ideal for kicking off initial data loading, would you recommend that?

Yes

😉

---

btw, if the class is instantiated "from a scope" as it would happen from a Backstack, then onServiceRegistered and co. is invoked by Backstack normally. All unit tests in simple-stack are unit tests, so all scoping logic and "scoped service lifecycle management" is unit-testable.

[angusholder]

Okay, that's helpful thank you. Want me to make a PR to add that info onto onServiceRegistered/onServiceUnregistered?

[Zhuinden]

Hmm, there is already a comment like so:

    /**
     * When a service implements {@link Registered}, then it will receive a callback when the service has been registered to at least one scope.
     */
    public static interface Registered {
        /**
         * Called when the service has been registered to any scopes, and this is the first scope it was registered to.
         */
        void onServiceRegistered();

        /**
         * Called when the service is no longer registered in any scopes.
         */
        void onServiceUnregistered();
    }

Although if it's not actually lcear, then it can be useful. It also doesn't really mention how onServiceRegisterd runs after fromBundle.

[Zhuinden]

I will consider this solved.

[angusholder]

Sorry I didn't get back to you, yeah I think it's fine