New BiomeLoadingEvent that allows modders to edit biomes as they are being deserialized.

patches/minecraft/net/minecraft/util/registry/WorldSettingsImport.java.patch

@@ -0,0 +1,10 @@
+--- a/net/minecraft/util/registry/WorldSettingsImport.java
++++ b/net/minecraft/util/registry/WorldSettingsImport.java
+@@ -231,6 +231,7 @@
+ 
+          public <E> DataResult<Pair<E, OptionalInt>> func_241879_a(DynamicOps<JsonElement> p_241879_1_, RegistryKey<? extends Registry<E>> p_241879_2_, RegistryKey<E> p_241879_3_, Decoder<E> p_241879_4_) {
+             JsonElement jsonelement = this.field_244349_a.get(p_241879_3_);
++            if (jsonelement!= null) jsonelement.getAsJsonObject().addProperty("forge:registry_name", p_241879_3_.func_240901_a_().toString());
+             return jsonelement == null ? DataResult.error("Unknown element: " + p_241879_3_) : p_241879_4_.parse(p_241879_1_, jsonelement).setLifecycle(this.field_244351_c.get(p_241879_3_)).map((p_244353_2_) -> {
+                return Pair.of(p_244353_2_, OptionalInt.of(this.field_244350_b.getInt(p_241879_3_)));
+             });

patches/minecraft/net/minecraft/world/biome/Biome.java.patch

@@ -9,7 +9,18 @@
     public static final Logger field_150586_aC = LogManager.getLogger();
     public static final Codec<Biome> field_242418_b = RecordCodecBuilder.create((p_235064_0_) -> {
        return p_235064_0_.group(Biome.Climate.field_242459_a.forGetter((p_242446_0_) -> {
-@@ -200,7 +200,7 @@
+@@ -70,7 +70,9 @@
+          return p_242443_0_.field_242424_k;
+       }), MobSpawnInfo.field_242552_c.forGetter((p_242442_0_) -> {
+          return p_242442_0_.field_242425_l;
+-      })).apply(p_235064_0_, Biome::new);
++      }), ResourceLocation.field_240908_a_.fieldOf("forge:registry_name").forGetter(Biome::getRegistryName))
++      .apply(p_235064_0_, (climate, category, depth, scale, effects, gen, spawns, name) ->
++          net.minecraftforge.common.ForgeHooks.enhanceBiome(name, climate, category, depth, scale, effects, gen, spawns, p_235064_0_, Biome::new));
+    });
+    public static final Codec<Biome> field_242419_c = RecordCodecBuilder.create((p_242432_0_) -> {
+       return p_242432_0_.group(Biome.Climate.field_242459_a.forGetter((p_242441_0_) -> {
+@@ -200,7 +202,7 @@
        } else {
           if (p_201850_2_.func_177956_o() >= 0 && p_201850_2_.func_177956_o() < 256 && p_201850_1_.func_226658_a_(LightType.BLOCK, p_201850_2_) < 10) {
              BlockState blockstate = p_201850_1_.func_180495_p(p_201850_2_);

patches/minecraft/net/minecraft/world/biome/BiomeGenerationSettings.java.patch

@@ -0,0 +1,28 @@
+--- a/net/minecraft/world/biome/BiomeGenerationSettings.java
++++ b/net/minecraft/world/biome/BiomeGenerationSettings.java
+@@ -47,6 +47,7 @@
+    });
+    private final Supplier<ConfiguredSurfaceBuilder<?>> field_242482_d;
+    private final Map<GenerationStage.Carving, List<Supplier<ConfiguredCarver<?>>>> field_242483_e;
++   private final java.util.Set<GenerationStage.Carving> carversView;
+    private final List<List<Supplier<ConfiguredFeature<?, ?>>>> field_242484_f;
+    private final List<Supplier<StructureFeature<?, ?>>> field_242485_g;
+    private final List<ConfiguredFeature<?, ?>> field_242486_h;
+@@ -59,12 +60,17 @@
+       this.field_242486_h = p_i241935_3_.stream().flatMap(Collection::stream).map(Supplier::get).flatMap(ConfiguredFeature::func_242768_d).filter((p_242490_0_) -> {
+          return p_242490_0_.field_222737_a == Feature.field_227247_y_;
+       }).collect(ImmutableList.toImmutableList());
++      this.carversView = java.util.Collections.unmodifiableSet(field_242483_e.keySet());
+    }
+ 
+    public List<Supplier<ConfiguredCarver<?>>> func_242489_a(GenerationStage.Carving p_242489_1_) {
+       return this.field_242483_e.getOrDefault(p_242489_1_, ImmutableList.of());
+    }
+ 
++   public java.util.Set<GenerationStage.Carving> getCarvingStages() {
++       return this.carversView;
++   }
++
+    public boolean func_242493_a(Structure<?> p_242493_1_) {
+       return this.field_242485_g.stream().anyMatch((p_242494_1_) -> {
+          return (p_242494_1_.get()).field_236268_b_ == p_242493_1_;

patches/minecraft/net/minecraft/world/biome/MobSpawnInfo.java.patch

@@ -0,0 +1,38 @@
+--- a/net/minecraft/world/biome/MobSpawnInfo.java
++++ b/net/minecraft/world/biome/MobSpawnInfo.java
+@@ -42,23 +42,35 @@
+    private final Map<EntityClassification, List<MobSpawnInfo.Spawners>> field_242554_e;
+    private final Map<EntityType<?>, MobSpawnInfo.SpawnCosts> field_242555_f;
+    private final boolean field_242556_g;
++   private final java.util.Set<EntityClassification> typesView;
++   private final java.util.Set<EntityType<?>> costView;
+ 
+    private MobSpawnInfo(float p_i241946_1_, Map<EntityClassification, List<MobSpawnInfo.Spawners>> p_i241946_2_, Map<EntityType<?>, MobSpawnInfo.SpawnCosts> p_i241946_3_, boolean p_i241946_4_) {
+       this.field_242553_d = p_i241946_1_;
+       this.field_242554_e = p_i241946_2_;
+       this.field_242555_f = p_i241946_3_;
+       this.field_242556_g = p_i241946_4_;
++      this.typesView = java.util.Collections.unmodifiableSet(this.field_242554_e.keySet());
++      this.costView = java.util.Collections.unmodifiableSet(this.field_242555_f.keySet());
+    }
+ 
+    public List<MobSpawnInfo.Spawners> func_242559_a(EntityClassification p_242559_1_) {
+       return this.field_242554_e.getOrDefault(p_242559_1_, ImmutableList.of());
+    }
+ 
++   public java.util.Set<EntityClassification> getSpawnerTypes() {
++       return this.typesView;
++   }
++
+    @Nullable
+    public MobSpawnInfo.SpawnCosts func_242558_a(EntityType<?> p_242558_1_) {
+       return this.field_242555_f.get(p_242558_1_);
+    }
+ 
++   public java.util.Set<EntityType<?>> getEntityTypes() {
++       return this.costView;
++   }
++
+    public float func_242557_a() {
+       return this.field_242553_d;
+    }

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -39,6 +39,8 @@ import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonParseException;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import net.minecraft.advancements.Advancement;
@@ -104,9 +106,15 @@ import net.minecraft.world.IBlockReader;
 import net.minecraft.world.IWorld;
 import net.minecraft.world.IWorldReader;
 import net.minecraft.world.World;
+import net.minecraft.world.biome.Biome;
+import net.minecraft.world.biome.BiomeAmbience;
+import net.minecraft.world.biome.BiomeGenerationSettings;
+import net.minecraft.world.biome.MobSpawnInfo;
 import net.minecraftforge.common.loot.IGlobalLootModifier;
 import net.minecraftforge.common.loot.LootModifierManager;
 import net.minecraftforge.common.util.BlockSnapshot;
+import net.minecraftforge.common.world.BiomeGenerationSettingsBuilder;
+import net.minecraftforge.common.world.MobSpawnInfoBuilder;
 import net.minecraftforge.event.AnvilUpdateEvent;
 import net.minecraftforge.event.DifficultyChangeEvent;
 import net.minecraftforge.event.ForgeEventFactory;
@@ -130,6 +138,7 @@ import net.minecraftforge.event.entity.player.AttackEntityEvent;
 import net.minecraftforge.event.entity.player.CriticalHitEvent;
 import net.minecraftforge.event.entity.player.PlayerEvent;
 import net.minecraftforge.event.entity.player.PlayerInteractEvent;
+import net.minecraftforge.event.world.BiomeLoadingEvent;
 import net.minecraftforge.event.world.BlockEvent;
 import net.minecraftforge.event.world.NoteBlockEvent;
 import net.minecraftforge.eventbus.api.Event.Result;
@@ -839,6 +848,21 @@ public class ForgeHooks
         throw new RuntimeException("Mod fluids must override createAttributes.");
     }
 
+    @FunctionalInterface
+    public interface BiomeCallbackFunction
+    {
+        Biome apply(final Biome.Climate climate, final Biome.Category category, final Float depth, final Float scale, final BiomeAmbience effects, final BiomeGenerationSettings gen, final MobSpawnInfo spawns);
+    }
+
+    public static Biome enhanceBiome(final ResourceLocation name, final Biome.Climate climate, final Biome.Category category, final Float depth, final Float scale, final BiomeAmbience effects, final BiomeGenerationSettings gen, final MobSpawnInfo spawns, final RecordCodecBuilder.Instance<Biome> codec, final BiomeCallbackFunction callback)
+    {
+        BiomeGenerationSettingsBuilder genBuilder = new BiomeGenerationSettingsBuilder(gen);
+        MobSpawnInfoBuilder spawnBuilder = new MobSpawnInfoBuilder(spawns);
+        BiomeLoadingEvent event = new BiomeLoadingEvent(name, climate, category, depth, scale, effects, genBuilder, spawnBuilder);
+        MinecraftForge.EVENT_BUS.post(event);
+        return callback.apply(event.getClimate(), event.getCategory(), event.getDepth(), event.getScale(), event.getEffects(), event.getGeneration().func_242508_a(), event.getSpawns().func_242577_b());
+    }
+
     private static class LootTableContext
     {
         public final ResourceLocation name;

src/main/java/net/minecraftforge/common/world/BiomeGenerationSettingsBuilder.java

@@ -0,0 +1,36 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.world;
+
+import net.minecraft.world.biome.BiomeGenerationSettings;
+
+import java.util.ArrayList;
+import java.util.Optional;
+
+public class BiomeGenerationSettingsBuilder extends BiomeGenerationSettings.Builder
+{
+    public BiomeGenerationSettingsBuilder(BiomeGenerationSettings orig)
+    {
+        field_242504_a = Optional.of(orig.func_242500_d());
+        orig.getCarvingStages().forEach(k -> field_242505_b.put(k, orig.func_242489_a(k)));
+        orig.func_242498_c().forEach(l -> field_242506_c.add(new ArrayList<>(l)));
+        field_242507_d.addAll(orig.func_242487_a());
+    }
+}

src/main/java/net/minecraftforge/common/world/MobSpawnInfoBuilder.java

@@ -0,0 +1,79 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.world;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+
+import javax.annotation.Nullable;
+
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.entity.EntityType;
+import net.minecraft.world.biome.MobSpawnInfo;
+
+public class MobSpawnInfoBuilder extends MobSpawnInfo.Builder
+{
+    private final Set<EntityClassification> typesView = Collections.unmodifiableSet(this.field_242567_a.keySet());
+    private final Set<EntityType<?>> costView = Collections.unmodifiableSet(this.field_242568_b.keySet());
+
+    public MobSpawnInfoBuilder(MobSpawnInfo orig)
+    {
+        orig.getSpawnerTypes().forEach(k -> {
+            field_242567_a.get(k).clear();
+            field_242567_a.get(k).addAll(new java.util.ArrayList<>(orig.func_242559_a(k)));
+        });
+        orig.getEntityTypes().forEach(k -> field_242568_b.put(k, orig.func_242558_a(k)));
+        field_242569_c = orig.func_242557_a();
+        field_242570_d = orig.func_242562_b();
+    }
+
+    public Set<EntityClassification> getSpawnerTypes()
+    {
+        return this.typesView;
+    }
+
+    public List<MobSpawnInfo.Spawners> getSpawner(EntityClassification type)
+    {
+        return this.field_242567_a.get(type);
+    }
+
+    public Set<EntityType<?>> getEntityTypes()
+    {
+        return this.costView;
+    }
+
+    @Nullable
+    public MobSpawnInfo.SpawnCosts getCost(EntityType<?> type)
+    {
+        return this.field_242568_b.get(type);
+    }
+
+    public float getProbability()
+    {
+        return this.field_242569_c;
+    }
+
+    public MobSpawnInfoBuilder disablePlayerSpawn()
+    {
+        this.field_242570_d = false;
+        return this;
+    }
+}

src/main/java/net/minecraftforge/event/world/BiomeLoadingEvent.java

@@ -0,0 +1,131 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.event.world;
+
+import net.minecraft.util.ResourceLocation;
+import net.minecraft.world.biome.Biome;
+import net.minecraft.world.biome.BiomeAmbience;
+import net.minecraftforge.common.world.BiomeGenerationSettingsBuilder;
+import net.minecraftforge.common.world.MobSpawnInfoBuilder;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraftforge.eventbus.api.EventPriority;
+
+/**
+ * This event fires when a Biome is created from json or when a registered biome is re-created for worldgen.
+ * It allows mods to edit a biome (like add a mob spawn) before it gets used for worldgen.
+ *
+ * In order to maintain the most compatibility possible with other mods' modifications to a biome,
+ * the event should be assigned a {@link net.minecraftforge.eventbus.api.EventPriority} as follows:
+ *
+ * - Additions to any list/map contained in a biome : {@link EventPriority#HIGH}
+ * - Removals to any list/map contained in a biome : {@link EventPriority#NORMAL}
+ * - Any other modification : {@link EventPriority#LOW}
+ *
+ * Be aware that another mod could have done an operation beforehand, so an expected value out of a vanilla biome might not
+ * always be the same, depending on other mods.
+ */
+public class BiomeLoadingEvent extends Event
+{
+    private final ResourceLocation name;
+    private Biome.Climate climate;
+    private Biome.Category category;
+    private float depth;
+    private float scale;
+    private BiomeAmbience effects;
+    private final BiomeGenerationSettingsBuilder gen;
+    private final MobSpawnInfoBuilder spawns;
+
+    public BiomeLoadingEvent(final ResourceLocation name, final Biome.Climate climate, final Biome.Category category, final float depth, final float scale, final BiomeAmbience effects, final BiomeGenerationSettingsBuilder gen, final MobSpawnInfoBuilder spawns)
+    {
+        this.name = name;
+        this.climate = climate;
+        this.category = category;
+        this.depth = depth;
+        this.scale = scale;
+        this.effects = effects;
+        this.gen = gen;
+        this.spawns = spawns;
+    }
+
+    public ResourceLocation getName()
+    {
+        return name;
+    }
+
+    public Biome.Climate getClimate()
+    {
+        return climate;
+    }
+
+    public void setClimate(final Biome.Climate value)
+    {
+        this.climate = value;
+    }
+
+    public Biome.Category getCategory()
+    {
+        return category;
+    }
+
+    public void setCategory(final Biome.Category value)
+    {
+        this.category = value;
+    }
+
+    public float getDepth()
+    {
+        return depth;
+    }
+
+    public void setDepth(final float value)
+    {
+        this.depth = value;
+    }
+
+    public float getScale()
+    {
+        return scale;
+    }
+
+    public void setScale(final float value)
+    {
+        this.scale = value;
+    }
+
+    public BiomeAmbience getEffects()
+    {
+        return effects;
+    }
+
+    public void setEffects(final BiomeAmbience value)
+    {
+        this.effects = value;
+    }
+
+    public BiomeGenerationSettingsBuilder getGeneration()
+    {
+        return gen;
+    }
+
+    public MobSpawnInfoBuilder getSpawns()
+    {
+        return spawns;
+    }
+}

src/main/resources/META-INF/accesstransformer.cfg

@@ -298,6 +298,15 @@ public net.minecraft.world.World field_73003_n #prevRainingStrength
 public net.minecraft.world.World field_73004_o #rainingStrength
 public net.minecraft.world.World field_73017_q #thunderingStrength
 public net.minecraft.world.World field_73018_p #prevThunderingStrength
+public net.minecraft.world.biome.Biome$Climate
+protected net.minecraft.world.biome.BiomeGenerationSettings$Builder field_242504_a
+protected net.minecraft.world.biome.BiomeGenerationSettings$Builder field_242505_b
+protected net.minecraft.world.biome.BiomeGenerationSettings$Builder field_242506_c
+protected net.minecraft.world.biome.BiomeGenerationSettings$Builder field_242507_d
+protected net.minecraft.world.biome.MobSpawnInfo$Builder field_242567_a
+protected net.minecraft.world.biome.MobSpawnInfo$Builder field_242568_b
+protected net.minecraft.world.biome.MobSpawnInfo$Builder field_242569_c
+protected net.minecraft.world.biome.MobSpawnInfo$Builder field_242570_d
 public net.minecraft.world.chunk.ChunkStatus <init>(Ljava/lang/String;Lnet/minecraft/world/chunk/ChunkStatus;ILjava/util/EnumSet;Lnet/minecraft/world/chunk/ChunkStatus$Type;Lnet/minecraft/world/chunk/ChunkStatus$IGenerationWorker;Lnet/minecraft/world/chunk/ChunkStatus$ILoadingWorker;)V
 private-f net.minecraft.world.gen.DebugChunkGenerator field_177462_b #GRID_WIDTH
 private-f net.minecraft.world.gen.DebugChunkGenerator field_177464_a #ALL_VALID_STATES

src/test/java/net/minecraftforge/debug/block/ScaffoldingTest.java

@@ -1,3 +1,22 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
 package net.minecraftforge.debug.block;
 
 import net.minecraft.block.AbstractBlock;