Add tag builder method for the new optional entries (#1173)

* Add tag builder method for the new optional entries

* Address comments

* Rename addOptional to addOptionalObject

* Flesh out doc

* Move mixin to target proper class

fabric-tag-extensions-v0/src/main/java/net/fabricmc/fabric/api/tag/FabricDataGeneratorTagBuilder.java

@@ -0,0 +1,44 @@
+/*
+ * 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.tag;
+
+import net.minecraft.tag.Tag;
+import net.minecraft.util.Identifier;
+
+/**
+ * Interface implemented by {@link net.minecraft.data.server.AbstractTagProvider.ObjectBuilder} instances when
+ * Fabric API is present. Useful for data generators.
+ */
+public interface FabricDataGeneratorTagBuilder<T> {
+	/**
+	 * Add an optional entry of type {@code <T>} to the tag.
+	 * The object identified by {@code id} is not required to be present on load,
+	 * which is useful for integration with other mods.
+	 * @param id The ID of the object to add
+	 * @see net.minecraft.data.server.AbstractTagProvider.ObjectBuilder#add(T) for the non-optional version of this method.
+	 */
+	void addOptionalObject(Identifier id);
+
+	/**
+	 * Add an optional tag entry to the tag.
+	 * The tag identified by {@code id} is not required to be present on load,
+	 * which is useful for integration with other mods.
+	 * @param id The ID of the tag to add
+	 * @see net.minecraft.data.server.AbstractTagProvider.ObjectBuilder#addTag(Tag.Identified)  for the non-optional version of this method.
+	 */
+	void addOptionalTag(Identifier id);
+}

fabric-tag-extensions-v0/src/main/java/net/fabricmc/fabric/mixin/tag/extension/MixinObjectBuilder.java

@@ -0,0 +1,46 @@
+/*
+ * 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.tag.extension;
+
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+
+import net.minecraft.data.server.AbstractTagProvider;
+import net.minecraft.tag.Tag;
+import net.minecraft.util.Identifier;
+
+import net.fabricmc.fabric.api.tag.FabricDataGeneratorTagBuilder;
+
+@Mixin(AbstractTagProvider.ObjectBuilder.class)
+public class MixinObjectBuilder<T> implements FabricDataGeneratorTagBuilder<T> {
+	@Shadow
+	@Final private Tag.Builder field_23960;
+
+	@Shadow
+	@Final private String field_23962;
+
+	@Override
+	public void addOptionalObject(Identifier id) {
+		field_23960.add(new Tag.OptionalObjectEntry(id), field_23962);
+	}
+
+	@Override
+	public void addOptionalTag(Identifier id) {
+		field_23960.add(new Tag.OptionalTagEntry(id), field_23962);
+	}
+}

fabric-tag-extensions-v0/src/main/resources/fabric-tag-extensions-v0.mixins.json

@@ -4,6 +4,7 @@
   "compatibilityLevel": "JAVA_8",
   "mixins": [
     "AccessorFluidTags",
+    "MixinObjectBuilder",
     "MixinTagImpl",
     "MixinTagBuilder"
   ],