feat: PlayerSwapWithEquipmentSlotEvent (#13687)

Co-authored-by: Hannes Greule <SirYwell@users.noreply.github.com>

Author: kashike

paper-api/src/main/java/io/papermc/paper/event/player/PlayerSwapWithEquipmentSlotEvent.java

@@ -0,0 +1,75 @@
+package io.papermc.paper.event.player;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.player.PlayerEvent;
+import org.bukkit.inventory.EquipmentSlot;
+import org.bukkit.inventory.ItemStack;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Triggered when a {@link Player} swaps an item with an equipment slot.
+ */
+@NullMarked
+public class PlayerSwapWithEquipmentSlotEvent extends PlayerEvent implements Cancellable {
+    private static final HandlerList HANDLERS = new HandlerList();
+    private final ItemStack itemInHand;
+    private final EquipmentSlot slot;
+    private final ItemStack itemToSwap;
+    private boolean cancelled;
+
+    @ApiStatus.Internal
+    public PlayerSwapWithEquipmentSlotEvent(
+        final Player player,
+        final ItemStack itemInHand,
+        final EquipmentSlot slot,
+        final ItemStack itemToSwap
+    ) {
+        super(player);
+        this.itemInHand = itemInHand;
+        this.slot = slot;
+        this.itemToSwap = itemToSwap;
+    }
+
+    /**
+     * {@return the item in one of the hand slots}
+     */
+    public ItemStack getItemInHand() {
+        return this.itemInHand.clone();
+    }
+
+    /**
+     * {@return the slot to swap into}
+     */
+    public EquipmentSlot getSlot() {
+        return this.slot;
+    }
+
+    /**
+     * {@return the item to swap}
+     */
+    public ItemStack getItemToSwap() {
+        return this.itemToSwap.clone();
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return this.cancelled;
+    }
+
+    @Override
+    public void setCancelled(final boolean cancelled) {
+        this.cancelled = cancelled;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return HANDLERS;
+    }
+
+    public static HandlerList getHandlerList() {
+        return HANDLERS;
+    }
+}

paper-server/patches/sources/net/minecraft/world/item/equipment/Equippable.java.patch

@@ -0,0 +1,22 @@
+--- a/net/minecraft/world/item/equipment/Equippable.java
++++ b/net/minecraft/world/item/equipment/Equippable.java
+@@ -129,6 +_,19 @@
+             ItemStack itemBySlot = player.getItemBySlot(this.slot);
+             if ((!EnchantmentHelper.has(itemBySlot, EnchantmentEffectComponents.PREVENT_ARMOR_CHANGE) || player.isCreative())
+                 && !ItemStack.isSameItemSameComponents(stack, itemBySlot)) {
++                // Paper start
++                final io.papermc.paper.event.player.PlayerSwapWithEquipmentSlotEvent event = new io.papermc.paper.event.player.PlayerSwapWithEquipmentSlotEvent(
++                    (org.bukkit.entity.Player) player.getBukkitEntity(),
++                    org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(stack),
++                    org.bukkit.craftbukkit.CraftEquipmentSlot.getSlot(this.slot),
++                    org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(itemBySlot)
++                );
++                event.callEvent();
++                if (event.isCancelled()) {
++                    ((org.bukkit.entity.Player) player.getBukkitEntity()).updateInventory();
++                    return InteractionResult.FAIL;
++                }
++                // Paper end
+                 if (!player.level().isClientSide()) {
+                     player.awardStat(Stats.ITEM_USED.get(stack.getItem()));
+                 }