Fix unsubscribe not called when: disposing an observe on a child node or unmounting a synced useObservable

[msantang78]

This PR is a proposed fix for #621 and #624

It solves:

1. Unsubscribe is not called when disposing an observe on a child node #621

const store = observable(
  synced({
    initial: { value: 1 },
    subscribe: ({ update }) => {
      console.log("subscribed");
      return () => {
        console.log("unsubscribed");
      };
    },
  })
);

const u = observe(() => {
  store.value.get();
});

u();

Changes

• It ensures that listeners-cleared is triggered for the parents node when there is no more listeners
• It also triggers the listener-added for parents (it was not triggered again after the first un-subscription)

At the moment, listener-added is only triggered for synced nodes due to the test “should only trigger middleware for the registered node”. It’s unclear whether this is the intended behavior or if I should also trigger for all parent nodes.

2. Unsubscribe not called when unmounting a synced useObservable #624

const obs$ = useObservable(
    synced({
        initial: 0,
        subscribe: () => {
            numSubscribes++;
            return () => {
                numUnsubscribes++;
                console.log('unsubscribed'); // <--- never executed
            };
        },
    }),
);

Changes

• Added tests for useObservable
• Ensure listeners-cleared is dispatched when unmounted

[msantang78]

I also found that:

• The synced observables are not unsubscribed on unmount
• The synced observables are not reset when the useObservable dependencies changes

Besides the missing functionality this should be causing memory leaks.
I'm will try to add some extra changes to this PR to fix those cases too

[msantang78]

@jmeistrich sorry to tag you, but this one seems important, It leads to memory leaks and not been able to use subscriptions. Can you please check it out when you have a moment 🙏