Fix subscribers memory leak

[speigg]

Memory Leak Fix

Problem

There was a memory leak in the hookstate library where states don't properly unsubscribe on store switchovers, introduced in PR #409. The issue occurs when incrementally switching to new global stores.

Solution

We fixed the memory leak by:

1. Adding an internal cleanupOrphanedSubscribers method to the Store class that cleans up subscribers that are no longer mounted:

// Internal method to clean up orphaned subscribers
// This is the key fix for the memory leak
cleanupOrphanedSubscribers() {
    if (this._subscribers.size > 1) {
        // Create a new set to avoid modification during iteration
        const subscribers = new Set(this._subscribers);
        subscribers.forEach(subscriber => {
            // Always keep the root state subscriber
            if (subscriber === this._stateMethods) {
                return;
            }

            // Only remove subscribers that are unmounted
            if (subscriber instanceof StateMethodsImpl &&
                !subscriber.isMounted) {
                this.unsubscribe(subscriber);
            }
        });
    }
}

2. Modifying the useHookstate function to mark the old state as unmounted BEFORE creating a new one:

if (value.store !== parentMethods.store || !('source' in value)) {
    // Get a reference to the old store before we create a new one
    const oldStore = value.store;
    const oldState = value.state;

    // Mark the old state as unmounted BEFORE creating a new one
    // This is the key fix for the memory leak
    oldState.onUnmount();

    // Create a new value with the new store
    value = initializer();

    // Now properly clean up the old store
    oldStore.unsubscribe(oldState);

    // Force cleanup of orphaned subscribers
    // This is needed because the store keeps a reference to the state
    // even after the state is unmounted
    oldStore.cleanupOrphanedSubscribers();
}

3. Calling cleanupOrphanedSubscribers in the unmount handler to ensure all orphaned subscribers are properly cleaned up when the component unmounts:

return () => {
    // This is the key fix for the memory leak
    // We need to properly clean up when the component unmounts
    value.state.onUnmount()
    value.store.unsubscribe(value.state);

    // Force cleanup of the store's subscribers
    // This is needed because the store keeps a reference to the state
    // even after the state is unmounted
    value.store.cleanupOrphanedSubscribers();

    // Deactivate the store which will clean up all subscribers
    value.store.deactivate() // this will destroy the extensions
}

Key Improvements

2. The cleanup is selective - it only removes subscribers that are unmounted, rather than all subscribers. This ensures that legitimate subscribers remain active.

3. The cleanup happens automatically as part of the normal component lifecycle.

[avkonst]

Hi, why do you need to _ prefix method when it can be simply non public method enforced by typescript?

[speigg]

Oh, good point, I'll unprefix :)