Added captive defaults

eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java

@@ -2,10 +2,12 @@ package com.willfp.eco.core.gui.slot;
 
 import com.willfp.eco.core.Eco;
 import com.willfp.eco.core.gui.slot.functional.SlotProvider;
+import com.willfp.eco.core.items.TestableItem;
 import org.bukkit.Material;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.function.Function;
 
@@ -28,6 +30,14 @@ public interface Slot {
      */
     boolean isCaptive();
 
+    /**
+     * Get default captive item.
+     *
+     * @param player The player.
+     * @return The item, or null if no captive default was found.
+     */
+    @Nullable TestableItem getCaptiveDefault(@NotNull Player player);
+
     /**
      * Create a builder for an ItemStack.
      *

eco-api/src/main/java/com/willfp/eco/core/gui/slot/SlotBuilder.java

@@ -3,10 +3,14 @@ package com.willfp.eco.core.gui.slot;
 import com.willfp.eco.core.gui.slot.functional.SlotHandler;
 import com.willfp.eco.core.gui.slot.functional.SlotModifier;
 import com.willfp.eco.core.gui.slot.functional.SlotUpdater;
+import com.willfp.eco.core.items.TestableItem;
+import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.function.BiConsumer;
+import java.util.function.Function;
 
 /**
  * Builder to create slots.
@@ -130,7 +134,17 @@ public interface SlotBuilder {
      *
      * @return The builder.
      */
-    SlotBuilder setCaptive();
+    default SlotBuilder setCaptive() {
+        return setCaptive(null);
+    }
+
+    /**
+     * Set slot to be a captive default with a default provider.
+     *
+     * @param provider The provider.
+     * @return The builder.
+     */
+    SlotBuilder setCaptive(@Nullable Function<Player, TestableItem> provider);
 
     /**
      * Build the slot.