Added InventoryAction->onPreExecute(), fixed PlayerDropItemEvent deleting items

src/pocketmine/inventory/transaction/SimpleInventoryTransaction.php

@@ -222,6 +222,12 @@ class SimpleInventoryTransaction implements InventoryTransaction{
 		return $this->matchItems($needItems, $haveItems) and count($this->actions) > 0 and count($haveItems) === 0 and count($needItems) === 0;
 	}
 
+	protected function handleFailed(){
+		foreach($this->actions as $action){
+			$action->onExecuteFail($this->source);
+		}
+	}
+
 	/**
 	 * @return bool
 	 */
@@ -232,7 +238,15 @@ class SimpleInventoryTransaction implements InventoryTransaction{
 
 		Server::getInstance()->getPluginManager()->callEvent($ev = new InventoryTransactionEvent($this));
 		if($ev->isCancelled()){
-			return false;
+			$this->handleFailed();
+			return true;
+		}
+
+		foreach($this->actions as $action){
+			if(!$action->onPreExecute($this->source)){
+				$this->handleFailed();
+				return true;
+			}
 		}
 
 		foreach($this->actions as $action){

src/pocketmine/inventory/transaction/action/DropItemAction.php

@@ -23,6 +23,7 @@ declare(strict_types=1);
 
 namespace pocketmine\inventory\transaction\action;
 
+use pocketmine\event\player\PlayerDropItemEvent;
 use pocketmine\Player;
 
 /**
@@ -41,6 +42,15 @@ class DropItemAction extends InventoryAction{
 		return $this->sourceItem->isNull();
 	}
 
+	public function onPreExecute(Player $source) : bool{
+		$source->getServer()->getPluginManager()->callEvent($ev = new PlayerDropItemEvent($source, $this->targetItem));
+		if($ev->isCancelled()){
+			return false;
+		}
+
+		return true;
+	}
+
 	/**
 	 * Drops the target item in front of the player.
 	 *

src/pocketmine/inventory/transaction/action/InventoryAction.php

@@ -74,6 +74,18 @@ abstract class InventoryAction{
 	 */
 	abstract public function isValid(Player $source) : bool;
 
+	/**
+	 * Called by inventory transactions before any actions are processed. If this returns false, the transaction will
+	 * be cancelled.
+	 *
+	 * @param Player $source
+	 *
+	 * @return bool
+	 */
+	public function onPreExecute(Player $source) : bool{
+		return true;
+	}
+
 	/**
 	 * Performs actions needed to complete the inventory-action server-side. Returns if it was successful. Will return
 	 * false if plugins cancelled events. This will only be called if the transaction which it is part of is considered