Validate transaction slots

src/inventory/ArmorInventory.php

@@ -24,9 +24,13 @@
 namespace pocketmine\inventory;
 
 use pocketmine\entity\Living;
+use pocketmine\inventory\transaction\validator\ArmorInventoryValidator;
 use pocketmine\item\Item;
+use pocketmine\utils\ObjectSet;
+
+class ArmorInventory extends SimpleInventory implements SlotSafeInventory{
+	use SlotSafeInventoryTrait;
 
-class ArmorInventory extends SimpleInventory{
 	public const SLOT_HEAD = 0;
 	public const SLOT_CHEST = 1;
 	public const SLOT_LEGS = 2;
@@ -73,4 +77,8 @@
 	public function setBoots(Item $boots) : void{
 		$this->setItem(self::SLOT_FEET, $boots);
 	}
+
+	protected static function initSlotValidators(ObjectSet $validators) : void{
+		$validators->add(new ArmorInventoryValidator());
+	}
 }

src/inventory/SlotSafeInventory.php

@@ -0,0 +1,45 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\inventory;
+
+use pocketmine\inventory\transaction\validator\InventoryTransactionValidator;
+use pocketmine\utils\ObjectSet;
+
+/**
+ * A "slot safe inventory" is an inventory that has a set of rules that determine whether an item can be placed in a
+ * particular slot. This ensures that the inventory's internal state is always valid and consistent.
+ */
+interface SlotSafeInventory{
+	/**
+	 * Return a set of validators that will be used to determine whether an item can be placed in a particular slot.
+	 * All validators need to return true for the transaction to be allowed.
+	 * If one of the validators returns false, the transaction will be cancelled.
+	 *
+	 * There is no guarantee that the validators will be called in any particular order.
+	 * All validators need to be stateless and not depend on the order in which they are called.
+	 *
+	 * @phpstan-return ObjectSet<InventoryTransactionValidator>
+	 */
+	public static function getSlotValidators() : ObjectSet;
+}

src/inventory/SlotSafeInventoryTrait.php

@@ -0,0 +1,45 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\inventory;
+
+use pocketmine\utils\ObjectSet;
+
+trait SlotSafeInventoryTrait{
+	private static ObjectSet $validators;
+
+	/**
+	 * Initialize the set of validators.
+	 * Put here all the internal validators of the inventory.
+	 */
+	abstract protected static function initSlotValidators(ObjectSet $validators) : void;
+
+	public static function getSlotValidators() : ObjectSet {
+		if(!isset(self::$validators)){
+			self::$validators = new ObjectSet();
+			self::initSlotValidators(self::$validators);
+		}
+
+		return self::$validators;
+	}
+}

src/inventory/transaction/action/SlotChangeAction.php

@@ -24,9 +24,11 @@
 namespace pocketmine\inventory\transaction\action;
 
 use pocketmine\inventory\Inventory;
+use pocketmine\inventory\SlotSafeInventory;
 use pocketmine\inventory\transaction\InventoryTransaction;
 use pocketmine\inventory\transaction\TransactionValidationException;
 use pocketmine\item\Item;
+use pocketmine\item\VanillaItems;
 use pocketmine\player\Player;
 
 /**
@@ -74,6 +76,17 @@ public function validate(Player $source) : void{
 		if($this->targetItem->getCount() > $this->inventory->getMaxStackSize()){
 			throw new TransactionValidationException("Target item exceeds inventory max stack size");
 		}
+		if($this->inventory instanceof SlotSafeInventory && !$this->targetItem->equals(VanillaItems::AIR())){
+			$result = false;
+			foreach($this->inventory::getSlotValidators() as $validator){
+				if(($result = $validator->validate($this->inventory, $this->targetItem, $this->inventorySlot))){
+					break;
+				}
+			}
+			if(!$result){
+				throw new TransactionValidationException("Target item is not accepted by the inventory");
+			}
+		}
 	}
 
 	/**

src/inventory/transaction/validator/ArmorInventoryValidator.php

@@ -0,0 +1,44 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\inventory\transaction\validator;
+
+use pocketmine\block\BlockTypeIds;
+use pocketmine\block\MobHead;
+use pocketmine\inventory\ArmorInventory;
+use pocketmine\inventory\Inventory;
+use pocketmine\item\Armor;
+use pocketmine\item\Item;
+use pocketmine\item\ItemBlock;
+
+class ArmorInventoryValidator implements InventoryTransactionValidator{
+	public function validate(Inventory $inventory, Item $item, int $slot) : bool{
+		return $inventory instanceof ArmorInventory && (
+				($item instanceof Armor && $item->getArmorSlot() === $slot) ||
+				($slot === ArmorInventory::SLOT_HEAD && $item instanceof ItemBlock && (
+						$item->getBlock()->getTypeId() === BlockTypeIds::CARVED_PUMPKIN ||
+						$item->getBlock() instanceof MobHead
+					))
+			);
+	}
+}

src/inventory/transaction/validator/InventoryTransactionValidator.php

@@ -0,0 +1,41 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\inventory\transaction\validator;
+
+use pocketmine\inventory\Inventory;
+use pocketmine\inventory\SlotSafeInventory;
+use pocketmine\item\Item;
+
+/**
+ * This interface is used to validate the transactions of a "safe" inventory. {@see SlotSafeInventory}
+ *
+ * In this way, each inventory can determine whether or not an item has the right to be placed in a particular slot.
+ */
+interface InventoryTransactionValidator{
+	/**
+	 * @return bool Return true, if the item CAN be placed in the given slot of the given inventory.
+	 * Otherwise, return false and the transaction will be cancelled.
+	 */
+	public function validate(Inventory $inventory, Item $item, int $slot) : bool;
+}