Adjust when the EntityTrackingEvents are fired.

fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/impl/attachment/sync/AttachmentSync.java

@@ -29,6 +29,7 @@
 import net.minecraft.util.Identifier;
 
 import net.fabricmc.api.ModInitializer;
+import net.fabricmc.fabric.api.networking.v1.EntityTrackingEvents;
 import net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry;
 import net.fabricmc.fabric.api.networking.v1.ServerConfigurationConnectionEvents;
 import net.fabricmc.fabric.api.networking.v1.ServerConfigurationNetworking;
@@ -113,7 +114,14 @@ public void onInitialize() {
 			}
 		});
 
-		// entity tracking handled in EntityTrackerEntryMixin instead, see comment
+		EntityTrackingEvents.START_TRACKING.register((trackedEntity, player) -> {
+			List<AttachmentChange> changes = new ArrayList<>();
+			((AttachmentTargetImpl) trackedEntity).fabric_computeInitialSyncChanges(player, changes::add);
+
+			if (!changes.isEmpty()) {
+				AttachmentChange.partitionAndSendPackets(changes, player);
+			}
+		});
 	}
 
 	private record AttachmentSyncTask() implements ServerPlayerConfigurationTask {

fabric-data-attachment-api-v1/src/main/resources/fabric-data-attachment-api-v1.mixins.json

@@ -11,7 +11,6 @@
     "ClientConnectionMixin",
     "CustomPayloadS2CPacketAccessor",
     "EntityMixin",
-    "EntityTrackerEntryMixin",
     "SerializedChunkMixin",
     "ServerWorldMixin",
     "VarIntsAccessor",

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/api/networking/v1/EntityTrackingEvents.java

@@ -27,9 +27,9 @@
  */
 public final class EntityTrackingEvents {
 	/**
-	 * An event that is called before player starts tracking an entity.
+	 * An event that is called after a player has started tracking an entity.
 	 * Typically, this occurs when an entity enters a client's view distance.
-	 * This event is called before the player's client is sent the entity's {@linkplain Entity#createSpawnPacket spawn packet}.
+	 * This event is called after the entity's {@linkplain Entity#createSpawnPacket spawn packet} is sent to the player.
 	 */
 	public static final Event<StartTracking> START_TRACKING = EventFactory.createArrayBacked(StartTracking.class, callbacks -> (trackedEntity, player) -> {
 		for (StartTracking callback : callbacks) {
@@ -38,9 +38,8 @@ public final class EntityTrackingEvents {
 	});
 
 	/**
-	 * An event that is called after a player has stopped tracking an entity.
-	 * The client at this point was sent a packet to {@link net.minecraft.network.packet.s2c.play.EntitiesDestroyS2CPacket destroy} the entity on the client.
-	 * The entity still exists on the server.
+	 * An event that is called before a player stops tracking an entity.
+	 * The entity still exists on the server and on the player's client.
 	 */
 	public static final Event<StopTracking> STOP_TRACKING = EventFactory.createArrayBacked(StopTracking.class, callbacks -> (trackedEntity, player) -> {
 		for (StopTracking callback : callbacks) {
@@ -51,7 +50,7 @@ public final class EntityTrackingEvents {
 	@FunctionalInterface
 	public interface StartTracking {
 		/**
-		 * Called before an entity starts getting tracked by a player.
+		 * Called after a player has started tracking an entity.
 		 *
 		 * @param trackedEntity the entity that will be tracked
 		 * @param player the player that will track the entity
@@ -62,10 +61,10 @@ public interface StartTracking {
 	@FunctionalInterface
 	public interface StopTracking {
 		/**
-		 * Called after an entity stops getting tracked by a player.
+		 * Called before an entity stops getting tracked by a player.
 		 *
-		 * @param trackedEntity the entity that is no longer being tracked
-		 * @param player the player that is no longer tracking the entity
+		 * @param trackedEntity the entity that is about to stop being tracked
+		 * @param player the player that is about to stop tracking the entity
 		 */
 		void onStopTracking(Entity trackedEntity, ServerPlayerEntity player);
 	}

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/mixin/networking/EntityTrackerEntryMixin.java

@@ -35,12 +35,12 @@ abstract class EntityTrackerEntryMixin {
 	@Final
 	private Entity entity;
 
-	@Inject(method = "startTracking", at = @At("HEAD"))
+	@Inject(method = "startTracking", at = @At("TAIL"))
 	private void onStartTracking(ServerPlayerEntity player, CallbackInfo ci) {
 		EntityTrackingEvents.START_TRACKING.invoker().onStartTracking(this.entity, player);
 	}
 
-	@Inject(method = "stopTracking", at = @At("TAIL"))
+	@Inject(method = "stopTracking", at = @At("HEAD"))
 	private void onStopTracking(ServerPlayerEntity player, CallbackInfo ci) {
 		EntityTrackingEvents.STOP_TRACKING.invoker().onStopTracking(this.entity, player);
 	}