Reactive ECS-integrated signals

[JMS55]

Objective

• Provide a way for an entity to have some component C that automatically updates based on the value of some other piece of data of type S
• This "piece of data" is called a Signal https://docs.solidjs.com/concepts/signals

Solution

• Spawn an entity to hold the signal data S
• Provide a SubscribedComponentSetup component that given a mapping function from &S -> C computes an initial value c based on the current value of the signal, inserts c on the subscribed entity (the one with the setup component), and sets up an observer for monitoring changes to the signal data, and then calling the mapping function again to produce a new c and updating the subscribed entity's C.

Problems

• Automatic signal cleanup requires storing Arc<()> in Signal, which means Signal is no longer Copy, which is quite painful to pass into observer callbacks :/. We can remove this, but then unused signals never get cleaned up...

TODO

• Depends on first merging Mutate Observer #16183
• Derived signals
• Allow signals to add/remove components, not just set values
• Allow signals to control entity hierarchies, and not just components
• Add commands.spawn_signal() and app.spawn_signal()?
• Write tests
• Flesh out docs
• Add an example file

[ItsDoot]

I recommend doing what Asset handles do and have an Arc<T> where T queues an event to despawn the entity when the Arc data is dropped. SignalRefcount would probably want to hold a Weak<T> then. This means you don't have to iterate through all signals, just ones that actually need to be cleaned up.

Edit: Unless you actually need to check the reference count for other reasons, you probably don't need SignalRefcount at all with the above.

[JMS55]

Unless you actually need to check the reference count for other reasons, you probably don't need SignalRefcount at all with the above.

Not sure I get how, but I would love to get rid of it. Currently it's used for two things:

• If an entity (X) has a component subscribed to a signal, that's handled by spawning a new observer entity. Said observer entity gets a SignalRefcount component to ensure that the signal entity is not despawned while the observer entity is alive, i.e. while entity X has a subscription to the signal.
• If you have a Signal type (which is not a bevy component or resource), it internally holds a SignalRefcount so that the signal entity is not despawned while Signal is alive. You can pass this Signal into observer callbacks for button clicks or event handling systems or whatever in order to call signal.set(v) to update the signal's value.

I would love to get rid of the refcount so that Signal can be Copy again, but then I don't know how to automatically decide when to cleanup the signal entity.