ServerEntityEvents.ALLOW_FRESH_LOAD and AFTER_FRESH_LOAD event

fabric-lifecycle-events-v1/src/main/java/net/fabricmc/fabric/api/event/lifecycle/v1/ServerEntityEvents.java

@@ -40,6 +40,56 @@
 		}
 	});
 
+	/**
+	 * Called before an Entity is added to a ServerLevel.
+	 *
+	 * <p>If return value is {@code false} entity will not be added to a server.</p>
+	 *
+	 * <p>Should be used when you want to add another entity instead of added or to block adding specific entities.</p>
+	 *
+	 * {@snippet :
+	 * ServerEntityEvents.ALLOW_FRESH_LOAD.register((entity, level) -> {
+	 * 	// Spawn with 25% chance zombie instead of creeper
+	 * 	if (entity instanceof Creeper && level.getRandom().nextFloat() <= 0.25f) {
+	 * 	 var zombie = EntityType.ZOMBIE.create(level, null, entity.blockPosition(), EntitySpawnReason.EVENT, true, false);
+	 * 	 if (zombie != null) return !level.addFreshEntity(zombie);
+	 * 	}
+	 * 	return true;
+	 * });
+	 * }
+	 */
+	public static final Event<ServerEntityEvents.AllowFreshLoad> ALLOW_FRESH_LOAD = EventFactory.createArrayBacked(ServerEntityEvents.AllowFreshLoad.class, callbacks -> (entity, level) -> {
+		boolean bl = true;
+
+		for (AllowFreshLoad callback : callbacks) {
+			bl = bl && callback.onFreshLoad(entity, level);
+		}
+
+		return bl;
+	});
+
+	/**
+	 * Called when an Entity is added to a ServerLevel.
+	 *
+	 * <p>When this event is called, the entity is already in the level.</p>
+	 *
+	 * <p>Should be used when you need to do something after entity summon, naturally spawn or any other add reason.</p>
+	 * <p>If you need to do something after entity every entity load (not the first one) use ENTITY_LOAD event.</p>
+	 *
+	 * {@snippet :
+	 * ServerEntityEvents.AFTER_FRESH_LOAD.register((entity, level) -> {
+	 * 	if (entity instanceof Creeper) {
+	 * 	 level.players().forEach(player -> player.sendSystemMessage(Component.literal("Creeper was added")));
+	 * 	}
+	 * });
+	 * }
+	 */
+	public static final Event<ServerEntityEvents.AfterFreshLoad> AFTER_FRESH_LOAD = EventFactory.createArrayBacked(ServerEntityEvents.AfterFreshLoad.class, callbacks -> (entity, level) -> {
+		for (AfterFreshLoad callback : callbacks) {
+			callback.afterFreshLoad(entity, level);
+		}
+	});
+
 	/**
 	 * Called when an Entity is unloaded from a ServerLevel.
 	 *
@@ -68,6 +118,16 @@
 		void onLoad(Entity entity, ServerLevel level);
 	}
 
+	@FunctionalInterface
+	public interface AllowFreshLoad {
+		boolean onFreshLoad(Entity entity, ServerLevel level);
+	}
+
+	@FunctionalInterface
+	public interface AfterFreshLoad {
+		void afterFreshLoad(Entity entity, ServerLevel level);
+	}
+
 	@FunctionalInterface
 	public interface Unload {
 		void onUnload(Entity entity, ServerLevel level);

fabric-lifecycle-events-v1/src/main/java/net/fabricmc/fabric/mixin/event/lifecycle/ServerLevelMixin.java

@@ -18,14 +18,18 @@
 
 import java.util.function.BooleanSupplier;
 
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import com.llamalad7.mixinextras.injector.wrapoperation.WrapOperation;
 import org.objectweb.asm.Opcodes;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 
 import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.entity.Entity;
 
+import net.fabricmc.fabric.api.event.lifecycle.v1.ServerEntityEvents;
 import net.fabricmc.fabric.api.event.lifecycle.v1.ServerTickEvents;
 
 @Mixin(ServerLevel.class)
@@ -40,4 +44,18 @@ private void startLevelTick(BooleanSupplier shouldKeepTicking, CallbackInfo ci)
 	private void endLevelTick(BooleanSupplier shouldKeepTicking, CallbackInfo ci) {
 		ServerTickEvents.END_LEVEL_TICK.invoker().onEndTick((ServerLevel) (Object) this);
 	}
+
+	@WrapOperation(method = "addFreshEntity", at = @At(value = "INVOKE", target = "Lnet/minecraft/server/level/ServerLevel;addEntity(Lnet/minecraft/world/entity/Entity;)Z"))
+	private boolean afterEntityFreshLoad(ServerLevel instance, Entity entity, Operation<Boolean> original) {
+		ServerLevel serverLevel = (ServerLevel) (Object) this;
+		if (!ServerEntityEvents.ALLOW_FRESH_LOAD.invoker().onFreshLoad(entity, serverLevel)) return false;
+
+		boolean result = original.call(instance, entity);
+
+		if (result) {
+			ServerEntityEvents.AFTER_FRESH_LOAD.invoker().afterFreshLoad(entity, serverLevel);
+		}
+
+		return result;
+	}
 }

fabric-lifecycle-events-v1/src/testmod/java/net/fabricmc/fabric/test/event/lifecycle/ServerEntityLifecycleTests.java

@@ -50,6 +50,20 @@ public void onInitialize() {
 			}
 		});
 
+		ServerEntityEvents.ALLOW_FRESH_LOAD.register((entity, level) -> {
+			if (PRINT_SERVER_ENTITY_MESSAGES) {
+				logger.info("[SERVER] ALLOW FRESH LOAD {} IN {}", entity, level);
+			}
+
+			return true;
+		});
+
+		ServerEntityEvents.AFTER_FRESH_LOAD.register((entity, level) -> {
+			if (PRINT_SERVER_ENTITY_MESSAGES) {
+				logger.info("[SERVER] FRESH LOADED {} IN {}", entity, level);
+			}
+		});
+
 		ServerEntityEvents.ENTITY_UNLOAD.register((entity, level) -> {
 			this.serverEntities.remove(entity);