Expose IItemHandler on vanilla entities

src/main/java/net/minecraftforge/items/ItemStackHandler.java

@@ -20,6 +20,11 @@ public class ItemStackHandler implements IItemHandler, IItemHandlerModifiable, I
         stacks = new ItemStack[size];
     }
 
+    public ItemStackHandler(ItemStack[] stacks)
+    {
+        this.stacks = stacks;
+    }
+
     public void setSize(int size)
     {
         stacks = new ItemStack[size];

src/main/java/net/minecraftforge/items/wrapper/PlayerArmorInvWrapper.java

@@ -7,7 +7,7 @@ import net.minecraft.item.ItemStack;
 public class PlayerArmorInvWrapper extends InvWrapper
 {
     public final InventoryPlayer inventoryPlayer;
-    public final int offset;
+    private final int offset;
 
     public PlayerArmorInvWrapper(InventoryPlayer inv)
     {

src/main/java/net/minecraftforge/items/wrapper/PlayerInvWrapper.java

@@ -6,6 +6,6 @@ public class PlayerInvWrapper extends CombinedInvWrapper
 {
     public PlayerInvWrapper(InventoryPlayer inv)
     {
-        super(new PlayerMainInvWrapper(inv), new PlayerArmorInvWrapper(inv));
+        super(new PlayerMainInvWrapper(inv), new PlayerArmorInvWrapper(inv), new PlayerOffhandInvWrapper(inv));
     }
 }

src/main/java/net/minecraftforge/items/wrapper/PlayerMainInvWrapper.java

@@ -10,7 +10,6 @@ import net.minecraft.item.ItemStack;
  */
 public class PlayerMainInvWrapper extends InvWrapper
 {
-
     public final InventoryPlayer inventoryPlayer;
 
     public PlayerMainInvWrapper(InventoryPlayer inv)
@@ -29,7 +28,7 @@ public class PlayerMainInvWrapper extends InvWrapper
     @Override
     public void setStackInSlot(int slot, ItemStack stack)
     {
-        // prevent setting of armor inventory
+        // prevent setting of armor and offhand
         if (slot > getSlots())
         {
             return;
@@ -40,7 +39,7 @@ public class PlayerMainInvWrapper extends InvWrapper
     @Override
     public ItemStack getStackInSlot(int slot)
     {
-        // prevent getting of armor inventory
+        // prevent getting of armor and offhand
         if (slot > getSlots())
         {
             return null;
@@ -51,7 +50,7 @@ public class PlayerMainInvWrapper extends InvWrapper
     @Override
     public ItemStack insertItem(int slot, ItemStack stack, boolean simulate)
     {
-        // prevent inserting into armor inventory
+        // prevent inserting into armor and offhand
         if (slot > getSlots())
         {
             return stack;
@@ -79,7 +78,7 @@ public class PlayerMainInvWrapper extends InvWrapper
     @Override
     public ItemStack extractItem(int slot, int amount, boolean simulate)
     {
-        // prevent extraction from armor inventory
+        // prevent extraction from armor and offhand
         if (slot > getSlots())
         {
             return null;

src/main/java/net/minecraftforge/items/wrapper/PlayerOffhandInvWrapper.java

@@ -0,0 +1,49 @@
+package net.minecraftforge.items.wrapper;
+
+import net.minecraft.entity.player.InventoryPlayer;
+import net.minecraft.item.ItemStack;
+
+public class PlayerOffhandInvWrapper extends InvWrapper
+{
+    public final InventoryPlayer inventoryPlayer;
+    private final int offset;
+
+    public PlayerOffhandInvWrapper(InventoryPlayer inv)
+    {
+        super(inv);
+
+        inventoryPlayer = inv;
+        offset = inventoryPlayer.mainInventory.length + inventoryPlayer.armorInventory.length;
+    }
+
+    @Override
+    public int getSlots()
+    {
+        return inventoryPlayer.offHandInventory.length;
+    }
+
+    @Override
+    public ItemStack getStackInSlot(int slot)
+    {
+        return super.getStackInSlot(slot + offset);
+    }
+
+    @Override
+    public ItemStack insertItem(int slot, ItemStack stack, boolean simulate)
+    {
+        return super.insertItem(slot + offset, stack, simulate);
+    }
+
+    @Override
+    public void setStackInSlot(int slot, ItemStack stack)
+    {
+        super.setStackInSlot(slot + offset, stack);
+    }
+
+    @Override
+    public ItemStack extractItem(int slot, int amount, boolean simulate)
+    {
+        return super.extractItem(slot + offset, amount, simulate);
+    }
+
+}