feat: add some more features to (multi page) guis

src/main/java/minevalley/core/api/Core.java

@@ -9,6 +9,7 @@
 import minevalley.core.api.discord.Webhook;
 import minevalley.core.api.economy.BankAccount;
 import minevalley.core.api.enums.DebugType;
+import minevalley.core.api.gui.FillItem;
 import minevalley.core.api.gui.InventoryGui;
 import minevalley.core.api.gui.MultiPageGui;
 import minevalley.core.api.npc.NPC;
@@ -547,14 +548,15 @@ public static InventoryGui createGui(@Nonnull Component title, @Nonnegative int
      * <br>
      * {@code %o%} will be replaced with the amount of pages
      *
-     * @param title title of the inventory
-     * @param size  size of the inventory
+     * @param title     title of the inventory
+     * @param size      size of the inventory
+     * @param fillItems items to fill the inventory with
      * @return new gui-builder
      * @throws IllegalArgumentException if the size is invalid (negative, higher than 54 or not a multiple of 9 while being higher than 6)
      */
     @Nonnull
-    public static MultiPageGui createMultiPageGui(@Nonnull Component title, @Nonnegative int size) throws IllegalArgumentException {
-        return server.createMultiPageGui(size);
+    public static MultiPageGui createMultiPageGui(@Nonnull Component title, @Nonnegative int size, @Nonnull List<FillItem> fillItems) throws IllegalArgumentException {
+        return server.createMultiPageGui(title, size, fillItems);
     }
 
     /**

src/main/java/minevalley/core/api/CoreServer.java

@@ -9,6 +9,7 @@
 import minevalley.core.api.discord.Webhook;
 import minevalley.core.api.economy.BankAccount;
 import minevalley.core.api.enums.DebugType;
+import minevalley.core.api.gui.FillItem;
 import minevalley.core.api.gui.InventoryGui;
 import minevalley.core.api.gui.MultiPageGui;
 import minevalley.core.api.npc.NPC;
@@ -154,7 +155,7 @@ public interface CoreServer {
     InventoryGui createGUI(@Nonnull Component title, @Nonnegative int size) throws IllegalArgumentException;
 
     @Nonnull
-    MultiPageGui createMultiPageGui(@Nonnegative int size) throws IllegalArgumentException;
+    MultiPageGui createMultiPageGui(@Nonnull Component title, @Nonnegative int size, @Nonnull List<FillItem> fillItems) throws IllegalArgumentException;
 
     @Nonnull
     World getMainWorld();

src/main/java/minevalley/core/api/gui/FillItem.java

@@ -0,0 +1,29 @@
+package minevalley.core.api.gui;
+
+import minevalley.core.api.Core;
+import minevalley.core.api.users.OnlineUser;
+import org.bukkit.entity.Player;
+import org.bukkit.event.inventory.InventoryClickEvent;
+import org.bukkit.inventory.ItemStack;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+
+public record FillItem(@Nonnull ItemStack itemStack,
+                       @Nullable Consumer<OnlineUser> callback,
+                       @Nullable BiConsumer<OnlineUser, InventoryClickEvent> advancedCallback) {
+
+    public FillItem {
+        if (itemStack == null) throw new IllegalArgumentException("ItemStack cannot be null");
+    }
+
+    @SuppressWarnings("unused")
+    public void accept(final @Nonnull InventoryClickEvent event) {
+        if (!(event.getWhoClicked() instanceof Player player)) return;
+        final OnlineUser user = Core.getOnlineUser(player);
+        if (callback != null) callback.accept(user);
+        if (advancedCallback != null) advancedCallback.accept(user, event);
+    }
+}

src/main/java/minevalley/core/api/gui/InventoryGui.java

@@ -143,6 +143,16 @@ default InventoryGui setItem(@Nonnegative int slot, @Nonnull InterfaceItem item,
     @Nonnull
     InventoryGui addCloser();
 
+    /**
+     * Empties the specified slot.
+     *
+     * @param slot the slot to empty
+     * @return this GUI
+     * @throws IllegalArgumentException if the slot is out of bounds
+     */
+    @Nonnull
+    InventoryGui empty(@Nonnegative int slot) throws IllegalArgumentException;
+
     /**
      * Gets the item in the specified slot.
      *

src/main/java/minevalley/core/api/gui/MultiPageGui.java

@@ -1,7 +1,6 @@
 package minevalley.core.api.gui;
 
 import minevalley.core.api.users.OnlineUser;
-import net.kyori.adventure.text.Component;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemStack;
@@ -116,18 +115,6 @@ MultiPageGui setPageDependantItem(@Nonnegative int slot, @Nonnull Function<Integ
         return getInventory(0);
     }
 
-    /**
-     * Updating the title of GUIs with multiple pages is not supported.
-     *
-     * @param title ignored
-     * @return nothing
-     */
-    @Override
-    @Contract("_ -> fail")
-    default @Nonnull InventoryGui updateTitle(@Nonnull Component title) {
-        throw new UnsupportedOperationException("This GUI does not support updating the title.");
-    }
-
     /**
      * Opens this GUI for the specified player on the first page.
      *