feat: Allow sending spawnData when spawning entities using the MultiplayerService

[NotBjoggisAtAll]

An attempt at resolving my own question on the discussion page.

Tested with a local project where I first encountered the need for this.

[AlmasB]

Thanks for this. The public API seems fine. However, implementation-wise, SpawnData can carry non-serializable data, so I'm just a bit cautious it may break compatibility with existing projects and/or cause confusion regarding the contractual obligation of SpawnData.

Is there a way we can achieve the same without making SpawnData serializable? Perhaps some internal or public data structure (which is serializable) that can carry the spawn data, NetworkSpawnData extends Serializable, which contractually forces developers to keep only Serializable data in it, possibly a serialization friendly version of PropertyMap.

Another aspect we need to try and figure out is what to do when SpawnData does carry non-serializable data. Using NetworkSpawnData from above may solve this. I'm curious to know what your thoughts are on this.

[NotBjoggisAtAll]

I agree with your concerns about changing SpawnData serializable,
I can make a NetworkSpawnData that is serializable instead. I think it should be public so that others can make subclasses to provide more data to send from the server to the client.

I think it should be up the the users to make sure all the data is serializable if they choose to create a subclass of NetworkSpawnData

Does this sound okay? I can make the changes and then we can see if its enough

[NotBjoggisAtAll]

Where do you want the NetworkSpawnData class to live? Under entity like with SpawnData or together with the MultiplayerService under the multiplayer package?

[AlmasB]

Probably alongside MultiplayerService, we can change later as needed. Above all sounds good.

[AlmasB]

Not sure if helpful, but perhaps you can make use of Bundle to keep the data inside NetworkSpawnData. Bundle is fully serializable and is what's used when saving to the file system

[NotBjoggisAtAll]

I wonder if NetworkSpawnData should extend SpawnData since the GameWorld.spawn() method expects a object of type SpawnData. Otherwise I would need to find a way to convert between the two SpawnData classes

[AlmasB]

I'd go with a constructor for now NetworkSpawnData(SpawnData)

[NotBjoggisAtAll]

I tried to do something like this but my demo fails since SpawnData is not serializable.

open class NetworkSpawnData(val spawnData: SpawnData) : Serializable {
}
-----------------

    fun spawn(connection: Connection<Bundle>, entity: Entity, entityName: String, spawnData: SpawnData) {
        if (!entity.hasComponent(NetworkComponent::class.java)) {
            log.warning("Attempted to network-spawn entity $entityName, but it does not have NetworkComponent")
            return
        }

        val networkComponent = entity.getComponent(NetworkComponent::class.java)

        val event = EntitySpawnEvent(networkComponent.id, entityName, NetworkSpawnData(spawnData))

        // TODO: if not available
        val data = replicatedEntitiesMap[connection]!!
        data.entities += entity

        fire(connection, event)
    }

----------------------
Demo

     // This does not work.
      SpawnDataspawnData = new SpawnData(x, y);
      Entity entity = spawn("test", spawnData);
      getMultiplayerService().spawn(connection, entity, "test", spawnData);

      //This work because I send my custom spawn data class which implements Serializable
      CardSpawnData cardSpawnData = new CardSpawnData(x, y, card.suit(), card.rank());
      Entity entity = spawn("card", cardSpawnData);
      getMultiplayerService().spawn(connection, entity, "card", cardSpawnData);
   
      

I could try to use a Bundle somehow, but I'm not too sure on how to use it. I could also set data inside SpawnData x,y,z,data manually on the NetworkSpawnData, but then I'm not sure to get custom variables/fields that exists on subclasses of SpawnData

[AlmasB]

NetworkSpawnData(val spawnData: SpawnData) -- the val there makes spawnData part of the class. Perhaps get rid of val and parse / convert all you need from spawnData's data object into NetworkSpawnData

[AlmasB]

I was thinking something like this.

NetworkSpawnData(spawnData: SpawnData) {

    val bundle = Bundle("NetworkSpawnData")

    init {
        spawnData.data.forEach((key, value) -> bundle.put(key, value));
    }
}

Not sure what we should do if value is not serializable. I guess for now you could check if it is, then put(), otherwise ignore.

Once we get NetworkSpawnData through the network to the other end, we need to reconstruct local SpawnData, which we should be able to do via the bundle object and its data field. Let me know if this makes sense.

Incidentally, I don't know/remember why SpawnData is an open class -- its internal Map should be able to carry anything, meaning we can potentially remove the open modifier.

[NotBjoggisAtAll]

I implemented something like you said, but I also included the position x,y,z because it didn't look like they were a part of SpawnData::data.

I left the SpawnData open, you can decide to remove the open modifier at a later stage. I do not know what will break or not if I remove it in this feature.

Does the new changes look like something you had in mind?

[AlmasB]

Many thanks, yes that's exactly what I had in mind. I'll take a look and merge later today.

I'll have a think about how to write a simple test for it, but in the meantime, can you confirm this implementation works for your use case?

[NotBjoggisAtAll]

Yes, I had to do some adjustments since I extended SpawnData expecting that to work. But I was able to retrieve the data in my spawn method from the spawnData.data variable instead of my custom class

[AlmasB]

Looks good, thank you