implement item.OffHand to offhand items

server/item/arrow.go

@@ -16,3 +16,8 @@ func (a Arrow) EncodeItem() (name string, meta int16) {
 	}
 	return "minecraft:arrow", 0
 }
+
+// OffHand ...
+func (Arrow) OffHand() bool {
+	return true
+}

server/item/firework.go

@@ -82,6 +82,11 @@ func (f Firework) RandomisedDuration() time.Duration {
 	return f.Duration + time.Duration(rand.IntN(int(time.Millisecond*600)))
 }
 
+// OffHand ...
+func (Firework) OffHand() bool {
+	return true
+}
+
 // EncodeItem ...
 func (Firework) EncodeItem() (name string, meta int16) {
 	return "minecraft:firework_rocket", 0

server/item/inventory/armour.go

@@ -21,7 +21,7 @@ type Armour struct {
 // The function passed is called when a slot is changed. It may be nil to not call anything.
 func NewArmour(f func(slot int, before, after item.Stack)) *Armour {
 	inv := New(4, f)
-	inv.canAdd = canAddArmour
+	inv.validator = canAddArmour
 	return &Armour{inv: inv}
 }
 

server/item/inventory/inventory.go

@@ -20,13 +20,16 @@ type Inventory struct {
 	h     Handler
 	slots []item.Stack
 
-	f      SlotFunc
-	canAdd func(s item.Stack, slot int) bool
+	f         SlotFunc
+	validator SlotValidator
 }
 
 // SlotFunc is a function called for each item changed in an Inventory.
 type SlotFunc func(slot int, before, after item.Stack)
 
+// SlotValidator is a function that limits changes in the Inventory slot.
+type SlotValidator func(s item.Stack, slot int) bool
+
 // ErrSlotOutOfRange is returned by any methods on inventory when a slot is passed which is not within the
 // range of valid values for the inventory.
 var ErrSlotOutOfRange = errors.New("slot is out of range: must be in range 0 <= slot < inventory.Size()")
@@ -42,7 +45,7 @@ func New(size int, f SlotFunc) *Inventory {
 	if f == nil {
 		f = func(slot int, before, after item.Stack) {}
 	}
-	return &Inventory{h: NopHandler{}, slots: make([]item.Stack, size), f: f, canAdd: func(s item.Stack, slot int) bool { return true }}
+	return &Inventory{h: NopHandler{}, slots: make([]item.Stack, size), f: f, validator: func(s item.Stack, slot int) bool { return true }}
 }
 
 // Clone copies an Inventory and returns it, calling the SlotFunc passed for any
@@ -51,7 +54,7 @@ func (inv *Inventory) Clone(f SlotFunc) *Inventory {
 	if f == nil {
 		f = func(slot int, before, after item.Stack) {}
 	}
-	return &Inventory{h: NopHandler{}, slots: inv.Slots(), f: f, canAdd: func(s item.Stack, slot int) bool { return true }}
+	return &Inventory{h: NopHandler{}, slots: inv.Slots(), f: f, validator: func(s item.Stack, slot int) bool { return true }}
 }
 
 // SlotFunc changes the function called when a slot in the inventory is changed.
@@ -61,6 +64,13 @@ func (inv *Inventory) SlotFunc(f SlotFunc) {
 	inv.f = f
 }
 
+// SlotValidator changes the function that limits item placement in the inventory slot.
+func (inv *Inventory) SlotValidator(f SlotValidator) {
+	inv.mu.Lock()
+	defer inv.mu.Unlock()
+	inv.validator = f
+}
+
 // Item attempts to obtain an item from a specific slot in the inventory. If an item was present in that slot,
 // the item is returned and the error is nil. If no item was present in the slot, a Stack with air as its item
 // and a count of 0 is returned. Stack.Empty() may be called to check if this is the case.
@@ -356,7 +366,7 @@ func (inv *Inventory) Handler() Handler {
 // setItem sets an item to a specific slot and overwrites the existing item. It calls the function which is
 // called for every item change and does so without locking the inventory.
 func (inv *Inventory) setItem(slot int, it item.Stack) func() {
-	if !inv.canAdd(it, slot) {
+	if !inv.validator(it, slot) {
 		return func() {}
 	}
 	if it.Count() > it.MaxCount() {

server/item/nautilus_shell.go

@@ -7,3 +7,8 @@ type NautilusShell struct{}
 func (NautilusShell) EncodeItem() (name string, meta int16) {
 	return "minecraft:nautilus_shell", 0
 }
+
+// OffHand ...
+func (NautilusShell) OffHand() bool {
+	return true
+}

server/item/totem.go

@@ -12,3 +12,8 @@ func (Totem) MaxCount() int {
 func (Totem) EncodeItem() (name string, meta int16) {
 	return "minecraft:totem_of_undying", 0
 }
+
+// OffHand ...
+func (Totem) OffHand() bool {
+	return true
+}

server/player/type.go

@@ -20,6 +20,14 @@ func (t ptype) Open(tx *world.Tx, handle *world.EntityHandle, data *world.Entity
 		playerData: pd,
 	}
 
+	pd.offHand.SlotValidator(func(s item.Stack, _ int) bool {
+		if s.Empty() {
+			return true
+		}
+		_, allowedInOffhand := s.Item().(item.OffHand)
+		return allowedInOffhand
+	})
+
 	if pd.s != nil {
 		pd.s.HandleInventories(tx, p, pd.inv, pd.offHand, pd.enderChest, pd.ui, pd.armour, pd.heldSlot)
 	} else {