Add RenderArmEvent to make overriding just the arm rendering not require copying nearly as much vanilla code (#8254)

src/main/java/net/minecraftforge/client/ForgeHooksClient.java

@@ -31,6 +31,7 @@ import net.minecraft.client.model.geom.builders.LayerDefinition;
 import net.minecraft.client.multiplayer.ClientLevel;
 import net.minecraft.client.multiplayer.MultiPlayerGameMode;
 import net.minecraft.client.multiplayer.ServerData;
+import net.minecraft.client.player.AbstractClientPlayer;
 import net.minecraft.client.player.LocalPlayer;
 import net.minecraft.locale.Language;
 import net.minecraft.network.Connection;
@@ -38,6 +39,7 @@ import net.minecraft.network.chat.FormattedText;
 import net.minecraft.network.chat.TextComponent;
 import net.minecraft.network.protocol.status.ServerStatus;
 import net.minecraft.world.effect.MobEffectInstance;
+import net.minecraft.world.entity.HumanoidArm;
 import net.minecraft.world.inventory.tooltip.TooltipComponent;
 import net.minecraft.world.level.block.state.BlockState;
 import com.mojang.blaze3d.platform.Window;
@@ -238,6 +240,11 @@ public class ForgeHooksClient
         return MinecraftForge.EVENT_BUS.post(new RenderHandEvent(hand, mat, buffers, light, partialTicks, interpPitch, swingProgress, equipProgress, stack));
     }
 
+    public static boolean renderSpecificFirstPersonArm(PoseStack poseStack, MultiBufferSource multiBufferSource, int packedLight, AbstractClientPlayer player, HumanoidArm arm)
+    {
+        return MinecraftForge.EVENT_BUS.post(new RenderArmEvent(poseStack, multiBufferSource, packedLight, player, arm));
+    }
+
     public static void onTextureStitchedPre(TextureAtlas map, Set<ResourceLocation> resourceLocations)
     {
         StartupMessageManager.mcLoaderConsumer().ifPresent(c->c.accept("Atlas Stitching : "+map.location().toString()));

src/main/java/net/minecraftforge/client/event/RenderArmEvent.java

@@ -0,0 +1,86 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * 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.client.event;
+
+import com.mojang.blaze3d.vertex.PoseStack;
+import net.minecraft.client.player.AbstractClientPlayer;
+import net.minecraft.client.renderer.MultiBufferSource;
+import net.minecraft.world.entity.HumanoidArm;
+import net.minecraftforge.eventbus.api.Cancelable;
+import net.minecraftforge.eventbus.api.Event;
+
+/**
+ * This is a more targeted version of {@link RenderHandEvent} event that is fired specifically when
+ * a player's arm is being rendered in first person, and should be used instead if the desired
+ * outcome is just to replace the rendering of the arm, such as to make armor render on it or
+ * instead of it.
+ *
+ * This event is fired on the {@link net.minecraftforge.common.MinecraftForge#EVENT_BUS}
+ * Canceling the event causes the arm to not render.
+ */
+@Cancelable
+public class RenderArmEvent extends Event
+{
+    private final PoseStack poseStack;
+    private final MultiBufferSource multiBufferSource;
+    private final int packedLight;
+    private final AbstractClientPlayer player;
+    private final HumanoidArm arm;
+
+    public RenderArmEvent(PoseStack poseStack, MultiBufferSource multiBufferSource, int packedLight, AbstractClientPlayer player, HumanoidArm arm)
+    {
+        this.poseStack = poseStack;
+        this.multiBufferSource = multiBufferSource;
+        this.packedLight = packedLight;
+        this.player = player;
+        this.arm = arm;
+    }
+
+    /**
+     * @return The arm being rendered.
+     */
+    public HumanoidArm getArm()
+    {
+        return arm;
+    }
+
+    public PoseStack getPoseStack()
+    {
+        return poseStack;
+    }
+
+    public MultiBufferSource getMultiBufferSource()
+    {
+        return multiBufferSource;
+    }
+
+    public int getPackedLight()
+    {
+        return packedLight;
+    }
+
+    /**
+     * @return the client player that is having their arm rendered. In general this will be the same as {@link net.minecraft.client.Minecraft#player}.
+     */
+    public AbstractClientPlayer getPlayer()
+    {
+        return player;
+    }
+}