Separate held item index change listener logic from PlayerInventory

src/entity/Human.php

@@ -264,7 +264,12 @@ class Human extends Living implements ProjectileSource, InventoryHolder{
 			}
 		}
 
-		$this->inventory->setHeldItemIndex($nbt->getInt("SelectedInventorySlot", 0), false);
+		$this->inventory->setHeldItemIndex($nbt->getInt("SelectedInventorySlot", 0));
+		$this->inventory->getHeldItemIndexChangeListeners()->add(function(int $oldIndex) : void{
+			foreach($this->getViewers() as $viewer){
+				$viewer->getNetworkSession()->onMobEquipmentChange($this);
+			}
+		});
 
 		$this->hungerManager->setFood((float) $nbt->getInt("foodLevel", (int) $this->hungerManager->getFood()));
 		$this->hungerManager->setExhaustion($nbt->getFloat("foodExhaustionLevel", $this->hungerManager->getExhaustion()));
@@ -457,6 +462,7 @@ class Human extends Living implements ProjectileSource, InventoryHolder{
 
 	protected function onDispose() : void{
 		$this->inventory->removeAllViewers();
+		$this->inventory->getHeldItemIndexChangeListeners()->clear();
 		$this->enderChestInventory->removeAllViewers();
 		parent::onDispose();
 	}

src/inventory/PlayerInventory.php

@@ -23,6 +23,7 @@ declare(strict_types=1);
 
 namespace pocketmine\inventory;
 
+use Ds\Set;
 use pocketmine\entity\Human;
 use pocketmine\item\Item;
 use pocketmine\player\Player;
@@ -35,8 +36,15 @@ class PlayerInventory extends BaseInventory{
 	/** @var int */
 	protected $itemInHandIndex = 0;
 
+	/**
+	 * @var \Closure[]|Set
+	 * @phpstan-var Set<\Closure(int $oldIndex) : void>
+	 */
+	protected $heldItemIndexChangeListeners;
+
 	public function __construct(Human $player){
 		$this->holder = $player;
+		$this->heldItemIndexChangeListeners = new Set();
 		parent::__construct(36);
 	}
 
@@ -73,25 +81,27 @@ class PlayerInventory extends BaseInventory{
 	/**
 	 * Sets which hotbar slot the player is currently loading.
 	 *
-	 * @param int  $hotbarSlot 0-8 index of the hotbar slot to hold
-	 * @param bool $send Whether to send updates back to the inventory holder. This should usually be true for plugin calls.
-	 *                    It should only be false to prevent feedback loops of equipment packets between client and server.
+	 * @param int $hotbarSlot 0-8 index of the hotbar slot to hold
 	 *
 	 * @throws \InvalidArgumentException if the hotbar slot is out of range
 	 */
-	public function setHeldItemIndex(int $hotbarSlot, bool $send = true) : void{
+	public function setHeldItemIndex(int $hotbarSlot) : void{
 		$this->throwIfNotHotbarSlot($hotbarSlot);
 
+		$oldIndex = $this->itemInHandIndex;
 		$this->itemInHandIndex = $hotbarSlot;
 
-		if($this->holder instanceof Player and $send){
-			$this->holder->getNetworkSession()->getInvManager()->syncSelectedHotbarSlot();
-		}
-		foreach($this->holder->getViewers() as $viewer){
-			$viewer->getNetworkSession()->onMobEquipmentChange($this->holder);
+		foreach($this->heldItemIndexChangeListeners as $callback){
+			$callback($oldIndex);
 		}
 	}
 
+	/**
+	 * @return \Closure[]|Set
+	 * @phpstan-return Set<\Closure(int $oldIndex) : void>
+	 */
+	public function getHeldItemIndexChangeListeners() : Set{ return $this->heldItemIndexChangeListeners; }
+
 	/**
 	 * Returns the currently-held item.
 	 */

src/network/mcpe/InventoryManager.php

@@ -76,6 +76,8 @@ class InventoryManager{
 	 * @phpstan-var array<int, array<int, Item>>
 	 */
 	private $initiatedSlotChanges = [];
+	/** @var int */
+	private $clientSelectedHotbarSlot = -1;
 
 	public function __construct(Player $player, NetworkSession $session){
 		$this->player = $player;
@@ -84,6 +86,10 @@ class InventoryManager{
 		$this->add(ContainerIds::INVENTORY, $this->player->getInventory());
 		$this->add(ContainerIds::ARMOR, $this->player->getArmorInventory());
 		$this->add(ContainerIds::UI, $this->player->getCursorInventory());
+
+		$this->player->getInventory()->getHeldItemIndexChangeListeners()->add(function() : void{
+			$this->syncSelectedHotbarSlot();
+		});
 	}
 
 	private function add(int $id, Inventory $inventory) : void{
@@ -206,13 +212,21 @@ class InventoryManager{
 		}
 	}
 
+	public function onClientSelectHotbarSlot(int $slot) : void{
+		$this->clientSelectedHotbarSlot = $slot;
+	}
+
 	public function syncSelectedHotbarSlot() : void{
-		$this->session->sendDataPacket(MobEquipmentPacket::create(
-			$this->player->getId(),
-			TypeConverter::getInstance()->coreItemStackToNet($this->player->getInventory()->getItemInHand()),
-			$this->player->getInventory()->getHeldItemIndex(),
-			ContainerIds::INVENTORY
-		));
+		$selected = $this->player->getInventory()->getHeldItemIndex();
+		if($selected !== $this->clientSelectedHotbarSlot){
+			$this->session->sendDataPacket(MobEquipmentPacket::create(
+				$this->player->getId(),
+				TypeConverter::getInstance()->coreItemStackToNet($this->player->getInventory()->getItemInHand()),
+				$selected,
+				ContainerIds::INVENTORY
+			));
+			$this->clientSelectedHotbarSlot = $selected;
+		}
 	}
 
 	public function syncCreative() : void{

src/network/mcpe/handler/InGamePacketHandler.php

@@ -448,6 +448,7 @@ class InGamePacketHandler extends PacketHandler{
 	}
 
 	public function handleMobEquipment(MobEquipmentPacket $packet) : bool{
+		$this->session->getInvManager()->onClientSelectHotbarSlot($packet->hotbarSlot);
 		if(!$this->player->selectHotbarSlot($packet->hotbarSlot)){
 			$this->session->getInvManager()->syncSelectedHotbarSlot();
 		}

src/player/Player.php

@@ -1373,7 +1373,7 @@ class Player extends Human implements CommandSender, ChunkListener, IPlayer{
 			return false;
 		}
 
-		$this->inventory->setHeldItemIndex($hotbarSlot, false);
+		$this->inventory->setHeldItemIndex($hotbarSlot);
 		$this->setUsingItem(false);
 
 		return true;