How to achieve server/client sync for Entity data

[Will Bradley]

I am extending a vanilla entity, and for reasons beyond the scope of this post I chose to use reflection instead of subclassing my mob (EnderDragon, if it matters). At any rate, I have one event listener in my main mod class structured as follows:

@SubscribeEvent
public void setColorOnDragonSpawn(EntityJoinWorldEvent event) {
    if (!event.getWorld().isClientSide() && event.getEntity() instanceof EnderDragon dragon) {
        // ... more logic

        doThingOnServerSide();
        DragonModPacketHandler.INSTANCE.send( // sync with client
            PacketDistributor.ALL.noArg(),
            new DoSameThingButOnClientSidePacket(dragon, otherArgs)
        );

    }
}

For DoSameThingButOnClientSidePacket#handle, I have

public void handle() {
    Minecraft minecraft = Minecraft.getInstance();

    EnderDragon dragon = (EnderDragon) minecraft.level.getEntity(this.dragonID); // null because the client level hasn't loaded yet
    doStuffWithDragon(dragon); // throws NPE
}

What's the best way to tell DoSameThingButOnClientSidePacket#handle to run doStuffWithDragon only once the dragon is spawned in on the client? I've thought of two approaches:

1. A package-private static final ConcurrentMap<Integer, Foo> in my main mod class, where the keys are dragon IDs (see Entity#getId) and Foo is additional metadata I want to store (a name, or color, or something. Not relevant). DoSameThingButOnClientSidePacket#handle would write this.dragonID into the map and another EntityJoinWorldEvent listener in the mod class (configured to only run when event.getWorld().isClientSide()) would pop from it and execute at the appropriate time.
2. Can I dynamically register and unregister events? Then I could have a helper @SubscribeEvent in DoSameThingButOnClientSidePacket that is pushed onto the event bus for each instance of the packet, executes on the client side (see option #1), and then removes itself from the event bus.
3. A built-in approach?

[diesieben07]

• Why not use EntityJoinWorldEvent on the client, too, instead of sending a packet?
• EntityJoinWorldEvent is entirely the wrong place to send clientbound entity packets and sending the packet to all players is not helpful at all (some players might not be near the entity and won't have it loaded, some might not even be in the same dimension). You need to use PlayerEvent.StartTracking, which is fired whenever an entity is started to be tracked by a new player (i.e. that player's client now has the entity loaded).

[Will Bradley]

Thanks!

[Will Bradley]

Ok @diesieben07 here's the issue. My entity has a random component, and if I have it join on the client and server, two different random numbers will be generated. This is a problem— how can I send a packet from the server to all clients that would load that entity, telling each client to spawn it with the proper random number?

[diesieben07]

Quote

You need to use PlayerEvent.StartTracking

 

[Will Bradley]

Here's my structure:

I have two event handlers for this stuff, one to add data to the dragon on the server side (called on EntityJoinWorldEvent with type casting and checks for Level::isClientSide(), and one to send message to the client using PlayerEvent.StartTracking, which looks like

@SubscribeEvent
public void setColorOnDragonSpawnClient(@NotNull PlayerEvent.StartTracking event) {
    if (event.getTarget() instanceof EnderDragon dragon
            && event.getPlayer() instanceof ServerPlayer player) {
        DragonModPacketHandler.INSTANCE.send( // sync with client
            PacketDistributor.ALL.noArg(),
            new ClientBoundDragonColorPacket(dragon, player.getId())
        );
    }
}

ClientBoundDragonColorPacket looks like

import ...

public class ClientBoundDragonColorPacket {
    private final Logger LOGGER = LogManager.getLogger();
    private final @Nullable
    String colorName;
    private final @Nonnull
    int playerID;
    private final int dragonID;
    private final Listeners listeners = new Listeners();

    public ClientBoundDragonColorPacket(@NotNull EnderDragon dragon, int playerID) {
        this.colorName = dragon.getCapability(MulticoloredDragonMod.COLOR_CAPABILITY).resolve().get()
            .getColorName();
        this.dragonID = dragon.getId();
        this.playerID = playerID;
    }

    public ClientBoundDragonColorPacket(@NotNull FriendlyByteBuf buffer) {
        boolean colorNameIsNull = buffer.readBoolean();
        if (colorNameIsNull)
            this.colorName = null;
        else
            this.colorName = new String(buffer.readByteArray());

        this.dragonID = buffer.readVarInt();
        this.playerID = buffer.readVarInt();
    }

    public void write(@NotNull FriendlyByteBuf buffer) {
        buffer.writeBoolean(this.colorName == null);
        if (this.colorName != null)
            buffer.writeByteArray(this.colorName.getBytes());

        buffer.writeVarInt(this.dragonID);
        buffer.writeVarInt(this.playerID);
    }

    public class Listeners {
        @SubscribeEvent
        public void changeDragonColorOnceLoadedOnClient(@NotNull EntityJoinWorldEvent event) {
            ClientBoundDragonColorPacket self = ClientBoundDragonColorPacket.this;
            if (event.getWorld().isClientSide()
                    && event.getEntity().getId() == self.dragonID) {

                EnderDragon dragon = (EnderDragon) event.getEntity();
                dragon.getCapability(MulticoloredDragonMod.COLOR_CAPABILITY).resolve().get()
                    .setColorName(self.colorName);

                MinecraftForge.EVENT_BUS.unregister(self.listeners);
            }
        }
    }

    public void handle() { // 495
        MinecraftForge.EVENT_BUS.register(this.listeners);
    }
}

 

[diesieben07]

Quote

PacketDistributor.ALL.noArg()

No... you have to send it to the player now tracking the entity.

Quote

private final @Nonnull int playerID;

Why are you sending this? Also, an int cannot be null.

Quote

    public class Listeners {
        @SubscribeEvent
        public void changeDragonColorOnceLoadedOnClient(@NotNull EntityJoinWorldEvent event) {
            ClientBoundDragonColorPacket self = ClientBoundDragonColorPacket.this;
            if (event.getWorld().isClientSide()
                    && event.getEntity().getId() == self.dragonID) {

                EnderDragon dragon = (EnderDragon) event.getEntity();
                dragon.getCapability(MulticoloredDragonMod.COLOR_CAPABILITY).resolve().get()
                    .setColorName(self.colorName);

                MinecraftForge.EVENT_BUS.unregister(self.listeners);
            }
        }
    }

    public void handle() { // 495
        MinecraftForge.EVENT_BUS.register(this.listeners);
    }

None of this makes any sense.

[Will Bradley]

Oops, I should've been more vigilant with the PacketDistributor. My bad; that's fixed now.

As for the rest, it compiles and seems to work— instead of being rude, would you mind telling me how dynamically registering/unregistering callbacks is a problem, and alternatives?

[Will Bradley]

For example, how about this?

public class ClientBoundDragonColorPacket {
    private final Logger LOGGER = LogManager.getLogger();
    private final @Nullable
    String colorName;

    private final int dragonID;

    public ClientBoundDragonColorPacket(@NotNull EnderDragon dragon) {
        this.colorName = dragon.getCapability(MulticoloredDragonMod.COLOR_CAPABILITY).resolve().get()
            .getColorName();
        this.dragonID = dragon.getId();
    }

    public ClientBoundDragonColorPacket(@NotNull FriendlyByteBuf buffer) {
        boolean colorNameIsNull = buffer.readBoolean();
        if (colorNameIsNull)
            this.colorName = null;
        else
            this.colorName = new String(buffer.readByteArray());

        this.dragonID = buffer.readVarInt();
    }

    public void write(@NotNull FriendlyByteBuf buffer) {
        buffer.writeBoolean(this.colorName == null);
        if (this.colorName != null)
            buffer.writeByteArray(this.colorName.getBytes());

        buffer.writeVarInt(this.dragonID);
    }

    public void handle() {
        Minecraft minecraft = Minecraft.getInstance();
        EnderDragon dragon = (EnderDragon) minecraft.level.getEntity(this.dragonID);
        dragon.getCapability(MulticoloredDragonMod.COLOR_CAPABILITY).resolve().get()
            .setColorName(this.colorName);
    }
}

@SubscribeEvent
    public void setColorOnDragonSpawnClient(@NotNull PlayerEvent.StartTracking event) {
        if (event.getTarget() instanceof EnderDragon dragon
                && event.getPlayer() instanceof ServerPlayer player) {
            DragonModPacketHandler.INSTANCE.send( // sync with client
                PacketDistributor.PLAYER.with(() -> player),
                new ClientBoundDragonColorPacket(dragon)
            );
        }
    }

 

[diesieben07]

That's fine, except that you are not serializing the strings correctly. String#getBytes uses the default charset, which might be different between server and client, leading to garbled data. You should use FriendlyByteBuf#writeUtf.

Your packet should also do some sanity checks, in case the server sends a wrong entity ID. Currently this will hard-crash the client.

[Will Bradley]

Ok, got it, thanks!