Add hook for custom damage logic (#964)

* Add hook for custom damage logic

* Address comments

* Change to use new FabricItemSettings

* Make CustomDamageHandler a functional interface

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/api/item/v1/CustomDamageHandler.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.api.item.v1;
+
+import java.util.function.Consumer;
+
+import net.minecraft.entity.LivingEntity;
+import net.minecraft.item.ItemStack;
+
+/**
+ * Allows an item to run custom logic when {@link ItemStack#damage(int, LivingEntity, Consumer)} is called.
+ * This is useful for items that, for example, may drain durability from some other source before damaging
+ * the stack itself.
+ *
+ * <p>Custom damage handlers can be set with {@link FabricItemSettings#customDamage}.
+ */
+@FunctionalInterface
+public interface CustomDamageHandler {
+	/**
+	 * Called to apply damage to the given stack.
+	 * This can be used to e.g. drain from a battery before actually damaging the item.
+	 * @param amount The amount of damage originally requested
+	 * @param breakCallback Callback when the stack reaches zero damage. See {@link ItemStack#damage(int, LivingEntity, Consumer)} and its callsites for more information.
+	 * @return The amount of damage to pass to vanilla's logic
+	 */
+	int damage(ItemStack stack, int amount, LivingEntity entity, Consumer<LivingEntity> breakCallback);
+}

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/api/item/v1/FabricItemSettings.java

@@ -19,6 +19,7 @@ package net.fabricmc.fabric.api.item.v1;
 import net.minecraft.item.FoodComponent;
 import net.minecraft.item.Item;
 import net.minecraft.item.ItemGroup;
+import net.minecraft.item.ItemStack;
 import net.minecraft.util.Rarity;
 
 import net.fabricmc.fabric.impl.item.FabricItemInternals;
@@ -42,6 +43,16 @@ public class FabricItemSettings extends Item.Settings {
 		return this;
 	}
 
+	/**
+	 * Sets the custom damage handler of the item.
+	 * Note that this is only called on an ItemStack if {@link ItemStack#isDamageable()} returns true.
+	 * @see CustomDamageHandler
+	 */
+	public FabricItemSettings customDamage(CustomDamageHandler handler) {
+		FabricItemInternals.computeExtraData(this).customDamage(handler);
+		return this;
+	}
+
 	// Overrides of vanilla methods
 
 	@Override

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/impl/item/FabricItemInternals.java

@@ -20,6 +20,7 @@ import java.util.WeakHashMap;
 
 import net.minecraft.item.Item;
 
+import net.fabricmc.fabric.api.item.v1.CustomDamageHandler;
 import net.fabricmc.fabric.api.item.v1.EquipmentSlotProvider;
 
 public final class FabricItemInternals {
@@ -37,14 +38,20 @@ public final class FabricItemInternals {
 
 		if (data != null) {
 			((ItemExtensions) item).fabric_setEquipmentSlotProvider(data.equipmentSlotProvider);
+			((ItemExtensions) item).fabric_setCustomDamageHandler(data.customDamageHandler);
 		}
 	}
 
 	public static final class ExtraData {
 		private /* @Nullable */ EquipmentSlotProvider equipmentSlotProvider;
+		private /* @Nullable */ CustomDamageHandler customDamageHandler;
 
 		public void equipmentSlot(EquipmentSlotProvider equipmentSlotProvider) {
 			this.equipmentSlotProvider = equipmentSlotProvider;
 		}
+
+		public void customDamage(CustomDamageHandler handler) {
+			this.customDamageHandler = handler;
+		}
 	}
 }

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/impl/item/ItemExtensions.java

@@ -16,9 +16,12 @@
 
 package net.fabricmc.fabric.impl.item;
 
+import net.fabricmc.fabric.api.item.v1.CustomDamageHandler;
 import net.fabricmc.fabric.api.item.v1.EquipmentSlotProvider;
 
 public interface ItemExtensions {
-	EquipmentSlotProvider fabric_getEquipmentSlotProvider();
+	/* @Nullable */ EquipmentSlotProvider fabric_getEquipmentSlotProvider();
 	void fabric_setEquipmentSlotProvider(EquipmentSlotProvider equipmentSlotProvider);
+	/* @Nullable */ CustomDamageHandler fabric_getCustomDamageHandler();
+	void fabric_setCustomDamageHandler(CustomDamageHandler handler);
 }

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/mixin/item/ItemMixin.java

@@ -24,6 +24,7 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 
 import net.minecraft.item.Item;
 
+import net.fabricmc.fabric.api.item.v1.CustomDamageHandler;
 import net.fabricmc.fabric.api.item.v1.EquipmentSlotProvider;
 import net.fabricmc.fabric.impl.item.FabricItemInternals;
 import net.fabricmc.fabric.impl.item.ItemExtensions;
@@ -33,6 +34,9 @@ abstract class ItemMixin implements ItemExtensions {
 	@Unique
 	private EquipmentSlotProvider equipmentSlotProvider;
 
+	@Unique
+	private CustomDamageHandler customDamageHandler;
+
 	@Inject(method = "<init>", at = @At("RETURN"))
 	private void onConstruct(Item.Settings settings, CallbackInfo info) {
 		FabricItemInternals.onBuild(settings, (Item) (Object) this);
@@ -47,4 +51,14 @@ abstract class ItemMixin implements ItemExtensions {
 	public void fabric_setEquipmentSlotProvider(EquipmentSlotProvider equipmentSlotProvider) {
 		this.equipmentSlotProvider = equipmentSlotProvider;
 	}
+
+	@Override
+	public CustomDamageHandler fabric_getCustomDamageHandler() {
+		return customDamageHandler;
+	}
+
+	@Override
+	public void fabric_setCustomDamageHandler(CustomDamageHandler handler) {
+		this.customDamageHandler = handler;
+	}
 }

fabric-item-api-v1/src/main/java/net/fabricmc/fabric/mixin/item/ItemStackMixin.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.mixin.item;
+
+import java.util.function.Consumer;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.ModifyArg;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+import net.minecraft.entity.LivingEntity;
+import net.minecraft.item.Item;
+import net.minecraft.item.ItemStack;
+
+import net.fabricmc.fabric.api.item.v1.CustomDamageHandler;
+import net.fabricmc.fabric.impl.item.ItemExtensions;
+
+@Mixin(ItemStack.class)
+public abstract class ItemStackMixin {
+	@Shadow public abstract Item getItem();
+
+	@Unique
+	private LivingEntity fabric_damagingEntity;
+
+	@Unique
+	private Consumer<LivingEntity> fabric_breakCallback;
+
+	@Inject(method = "damage(ILnet/minecraft/entity/LivingEntity;Ljava/util/function/Consumer;)V", at = @At("HEAD"))
+	private void saveDamager(int amount, LivingEntity entity, Consumer<LivingEntity> breakCallback, CallbackInfo ci) {
+		this.fabric_damagingEntity = entity;
+		this.fabric_breakCallback = breakCallback;
+	}
+
+	@ModifyArg(method = "damage(ILnet/minecraft/entity/LivingEntity;Ljava/util/function/Consumer;)V", at = @At(value = "INVOKE", target = "Lnet/minecraft/item/ItemStack;damage(ILjava/util/Random;Lnet/minecraft/server/network/ServerPlayerEntity;)Z"), index = 0)
+	private int hookDamage(int amount) {
+		CustomDamageHandler handler = ((ItemExtensions) getItem()).fabric_getCustomDamageHandler();
+
+		if (handler != null) {
+			return handler.damage((ItemStack) (Object) this, amount, fabric_damagingEntity, fabric_breakCallback);
+		}
+
+		return amount;
+	}
+
+	@Inject(method = "damage(ILnet/minecraft/entity/LivingEntity;Ljava/util/function/Consumer;)V", at = @At("RETURN"))
+	private <T extends LivingEntity> void clearDamager(int amount, T entity, Consumer<T> breakCallback, CallbackInfo ci) {
+		this.fabric_damagingEntity = null;
+		this.fabric_breakCallback = null;
+	}
+}

fabric-item-api-v1/src/main/resources/fabric-item-api-v1.mixins.json

@@ -3,6 +3,7 @@
   "package": "net.fabricmc.fabric.mixin.item",
   "compatibilityLevel": "JAVA_8",
   "mixins": [
+    "ItemStackMixin",
     "ItemMixin",
     "MobEntityMixin"
   ],

fabric-item-api-v1/src/testmod/java/net/fabricmc/fabric/test/item/CustomDamageTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.test.item;
+
+import net.minecraft.item.ItemStack;
+import net.minecraft.item.PickaxeItem;
+import net.minecraft.item.ToolMaterials;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.text.Text;
+import net.minecraft.util.Identifier;
+import net.minecraft.util.registry.Registry;
+
+import net.fabricmc.api.ModInitializer;
+import net.fabricmc.fabric.api.item.v1.CustomDamageHandler;
+import net.fabricmc.fabric.api.item.v1.FabricItemSettings;
+
+public class CustomDamageTest implements ModInitializer {
+	@Override
+	public void onInitialize() {
+		Registry.register(Registry.ITEM, new Identifier("fabric-item-api-v1-testmod", "weird_pickaxe"), new WeirdPick());
+	}
+
+	public static final CustomDamageHandler WEIRD_DAMAGE_HANDLER = (stack, amount, entity, breakCallback) -> {
+		// If sneaking, apply all damage to vanilla. Otherwise, increment a tag on the stack by one and don't apply any damage
+		if (entity.isSneaking()) {
+			return amount;
+		} else {
+			CompoundTag tag = stack.getOrCreateTag();
+			tag.putInt("weird", tag.getInt("weird") + 1);
+			return 0;
+		}
+	};
+
+	public static class WeirdPick extends PickaxeItem {
+		protected WeirdPick() {
+			super(ToolMaterials.GOLD, 1, -2.8F, new FabricItemSettings().customDamage(WEIRD_DAMAGE_HANDLER));
+		}
+
+		@Override
+		public Text getName(ItemStack stack) {
+			int v = stack.getOrCreateTag().getInt("weird");
+			return super.getName(stack).shallowCopy().append(" (Weird Value: " + v + ")");
+		}
+	}
+}

fabric-item-api-v1/src/testmod/resources/assets/fabric-item-api-v1-testmod/models/item/weird_pickaxe.json

@@ -0,0 +1,6 @@
+{
+  "parent": "minecraft:item/handheld",
+  "textures": {
+    "layer0": "minecraft:item/golden_pickaxe"
+  }
+}

fabric-item-api-v1/src/testmod/resources/fabric.mod.json

@@ -14,6 +14,9 @@
     ],
     "client": [
       "net.fabricmc.fabric.test.item.client.TooltipTests"
+    ],
+    "main": [
+      "net.fabricmc.fabric.test.item.CustomDamageTest"
     ]
   }
 }