Add BakedModel unwrapping API (#2686)

* Add BakedModel unwrapping API

* Allow WrapperBakedModel to not have an inner model

(cherry picked from commit 2063beff56)

fabric-renderer-api-v1/src/client/java/net/fabricmc/fabric/api/renderer/v1/model/ForwardingBakedModel.java

@@ -37,7 +37,7 @@ import net.fabricmc.fabric.api.renderer.v1.render.RenderContext;
  * Base class for specialized model implementations that need to wrap other baked models.
  * Avoids boilerplate code for pass-through methods.
  */
-public abstract class ForwardingBakedModel implements BakedModel, FabricBakedModel {
+public abstract class ForwardingBakedModel implements BakedModel, FabricBakedModel, WrapperBakedModel {
 	/** implementations must set this somehow. */
 	protected BakedModel wrapped;
 
@@ -95,4 +95,9 @@ public abstract class ForwardingBakedModel implements BakedModel, FabricBakedMod
 	public ModelOverrideList getOverrides() {
 		return wrapped.getOverrides();
 	}
+
+	@Override
+	public BakedModel getWrappedModel() {
+		return wrapped;
+	}
 }

fabric-renderer-api-v1/src/client/java/net/fabricmc/fabric/api/renderer/v1/model/WrapperBakedModel.java

@@ -0,0 +1,58 @@
+/*
+ * 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.renderer.v1.model;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.minecraft.client.render.model.BakedModel;
+
+/**
+ * An interface to be implemented by models that wrap and replace another model, such as {@link ForwardingBakedModel}.
+ * This allows mods to access the wrapped model without having to know the exact type of the wrapper model.
+ *
+ * <p>If you need to access data stored in one of your {@link BakedModel} subclasses,
+ * and you would normally access the model by its identifier and then cast it:
+ * call {@link #unwrap(BakedModel)} on the model before, in case another mod is wrapping your model to alter its rendering.
+ */
+public interface WrapperBakedModel {
+	/**
+	 * Return the wrapped model, if there is one at the moment, or {@code null} otherwise.
+	 *
+	 * <p>If there are multiple layers of wrapping, this method does not necessarily return the innermost model.
+	 */
+	@Nullable
+	BakedModel getWrappedModel();
+
+	/**
+	 * Fully unwrap a model, i.e. return the innermost model.
+	 */
+	static BakedModel unwrap(BakedModel model) {
+		while (model instanceof WrapperBakedModel wrapper) {
+			BakedModel wrapped = wrapper.getWrappedModel();
+
+			if (wrapped == null) {
+				return model;
+			} else if (wrapped == model) {
+				throw new IllegalArgumentException("Model " + model + " is wrapping itself!");
+			} else {
+				model = wrapped;
+			}
+		}
+
+		return model;
+	}
+}