Watcher Callbacks Should Use JSManagedValue to Avoid GC Issues

[alkene5]

Currently, various watchers in the HSApplicationModule (e.g., _addWatcher) store their JavaScript callbacks directly as JSValue. This can lead to unexpected behavior or crashes if the JavaScript garbage collector collects the callback, since the watcher holds a strong reference to a JSValue that might no longer be valid.

To improve stability and prevent potential memory issues, these watchers should store callbacks using JSManagedValue. JSManagedValue allows the JavaScriptCore garbage collector to manage the lifecycle of the JS objects safely while still keeping a weak reference from Swift, ensuring that callbacks remain valid for the lifetime of the watcher without creating retain cycles.

This change would make the watcher system more robust and safer in scenarios where callbacks are added and removed dynamically.

[cmsj]

Hmm, I've been reading the docs for JSManagedValue and I'm not sure I really understand what the right choice here is.

As far as I can tell, we want a strong reference to the supplied function because it could easily be a lambda that has no other references on the JS side?

[alkene5]

Yes, that’s possible. The callback might just be an inline lambda with no other references on the JS side, for example:

hs.task.new(() => {
  // ...
})

So we can’t assume it’s retained elsewhere.