Add ModifyCustomSpawnersEvent (#1272)

patches/net/minecraft/server/level/ServerLevel.java.patch

@@ -9,12 +9,23 @@
      private final StructureManager structureManager;
      private final StructureCheck structureCheck;
      private final boolean tickTime;
-@@ -298,6 +_,8 @@
+@@ -232,7 +_,6 @@
+         );
+         this.tickTime = p_215009_;
+         this.server = p_214999_;
+-        this.customSpawners = p_215008_;
+         this.serverLevelData = p_215002_;
+         ChunkGenerator chunkgenerator = p_215004_.generator();
+         boolean flag = p_214999_.forceSynchronousWrites();
+@@ -298,6 +_,11 @@
          this.randomSequences = Objects.requireNonNullElseGet(
              p_288977_, () -> this.getDataStorage().computeIfAbsent(RandomSequences.factory(i), "random_sequences")
          );
 +
 +        net.neoforged.neoforge.attachment.LevelAttachmentsSavedData.init(this);
++        // Neo: Move the initialization of customSpawners to the end of costructor
++        // Providing a fully initialized ServerLevel instance for the ServerLevelEvent.CustomSpawners
++        this.customSpawners = net.neoforged.neoforge.event.EventHooks.getCustomSpawners(this, p_215008_);
      }
 
      @Deprecated

src/main/java/net/neoforged/neoforge/event/EventHooks.java

@@ -10,6 +10,7 @@
 import com.mojang.datafixers.util.Either;
 import com.mojang.serialization.DynamicOps;
 import java.io.File;
+import java.util.ArrayList;
 import java.util.EnumSet;
 import java.util.List;
 import java.util.Optional;
@@ -81,6 +82,7 @@
 import net.minecraft.world.level.BaseSpawner;
 import net.minecraft.world.level.BlockGetter;
 import net.minecraft.world.level.ChunkPos;
+import net.minecraft.world.level.CustomSpawner;
 import net.minecraft.world.level.Explosion;
 import net.minecraft.world.level.GameRules;
 import net.minecraft.world.level.Level;
@@ -169,6 +171,7 @@
 import net.neoforged.neoforge.event.level.ExplosionEvent;
 import net.neoforged.neoforge.event.level.ExplosionKnockbackEvent;
 import net.neoforged.neoforge.event.level.LevelEvent;
+import net.neoforged.neoforge.event.level.ModifyCustomSpawnersEvent;
 import net.neoforged.neoforge.event.level.PistonEvent;
 import net.neoforged.neoforge.event.level.SleepFinishedTimeEvent;
 import net.neoforged.neoforge.event.level.block.CreateFluidSourceEvent;
@@ -1120,4 +1123,17 @@ public static MobSplitEvent onMobSplit(Mob parent, List<Mob> children) {
         NeoForge.EVENT_BUS.post(event);
         return event;
     }
+
+    /**
+     * Fires the {@link ModifyCustomSpawnersEvent}. Returns the custom spawners list.
+     * 
+     * @param serverLevel    The server level.
+     * @param customSpawners The original custom spawners.
+     * @return The new custom spawners list.
+     */
+    public static List<CustomSpawner> getCustomSpawners(ServerLevel serverLevel, List<CustomSpawner> customSpawners) {
+        ModifyCustomSpawnersEvent event = new ModifyCustomSpawnersEvent(serverLevel, new ArrayList<>(customSpawners));
+        NeoForge.EVENT_BUS.post(event);
+        return event.getCustomSpawners();
+    }
 }

src/main/java/net/neoforged/neoforge/event/level/ModifyCustomSpawnersEvent.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) NeoForged and contributors
+ * SPDX-License-Identifier: LGPL-2.1-only
+ */
+
+package net.neoforged.neoforge.event.level;
+
+import java.util.List;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.CustomSpawner;
+import net.neoforged.bus.api.Event;
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * This event is fired on the {@linkplain net.neoforged.fml.LogicalSide#SERVER logical server} when a {@link ServerLevel} is building its {@linkplain CustomSpawner custom spawners}.
+ * <p>
+ * Subscribe to this event to add/remove {@linkplain CustomSpawner custom spawners} for the level.
+ * <p>
+ * This event is not {@link net.neoforged.bus.api.ICancellableEvent cancellable} and does not have a result.
+ */
+public class ModifyCustomSpawnersEvent extends Event {
+    private final ServerLevel serverLevel;
+    private final List<CustomSpawner> customSpawners;
+
+    @ApiStatus.Internal
+    public ModifyCustomSpawnersEvent(ServerLevel serverLevel, List<CustomSpawner> customSpawners) {
+        this.serverLevel = serverLevel;
+        this.customSpawners = customSpawners;
+    }
+
+    /**
+     * {@return the server level this event is affecting}
+     */
+    public ServerLevel getLevel() {
+        return this.serverLevel;
+    }
+
+    /**
+     * @return the modifiable custom spawner list.
+     */
+    public List<CustomSpawner> getCustomSpawners() {
+        return this.customSpawners;
+    }
+
+    /**
+     * Adds a custom spawner to the list.
+     */
+    public void addCustomSpawner(CustomSpawner customSpawner) {
+        this.customSpawners.add(customSpawner);
+    }
+}