Fixed some bugs and added QoL features

src/main/java/io/github/apickledwalrus/skriptgui/elements/sections/SecMakeGUI.java

@@ -167,4 +167,7 @@ public String toString(@Nullable Event e, boolean debug) {
 		}
 	}
 
+	public Expression<Object> getSlots() {
+		return slots;
+	}
 }

src/main/java/io/github/apickledwalrus/skriptgui/elements/sections/SecSlotChange.java

@@ -0,0 +1,92 @@
+package io.github.apickledwalrus.skriptgui.elements.sections;
+
+import ch.njol.skript.Skript;
+import ch.njol.skript.config.SectionNode;
+import ch.njol.skript.doc.Description;
+import ch.njol.skript.doc.Examples;
+import ch.njol.skript.doc.Name;
+import ch.njol.skript.doc.Since;
+import ch.njol.skript.lang.Expression;
+import ch.njol.skript.lang.Section;
+import ch.njol.skript.lang.SkriptParser.ParseResult;
+import ch.njol.skript.lang.Trigger;
+import ch.njol.skript.lang.TriggerItem;
+import ch.njol.skript.variables.Variables;
+import ch.njol.util.Kleenean;
+import io.github.apickledwalrus.skriptgui.SkriptGUI;
+import io.github.apickledwalrus.skriptgui.gui.GUI;
+import org.bukkit.event.Event;
+import org.bukkit.event.inventory.InventoryClickEvent;
+import org.eclipse.jdt.annotation.Nullable;
+
+import java.util.List;
+
+@Name("GUI Slot Change")
+@Description("Sections that will run when a gui slot is changed. This section is optional.")
+@Examples({
+		"create a gui with virtual chest inventory with 3 rows named \"My GUI\"",
+		"\trun when slot 12 changes:",
+		"\t\tsend \"You changed slot 12!\" to player",
+		"\trun on slot 14 changed:",
+		"\t\tcancel event"
+})
+@Since("1.3")
+public class SecSlotChange extends Section {
+
+	static {
+		Skript.registerSection(SecSlotChange.class,
+				"run when [gui] slot[s] %integers% change[s]",
+				"run when [gui] slot[s] %integers% [(are|is)] [being] changed",
+				"run on change of [gui] slot[s] %integers%"
+		);
+	}
+
+	@SuppressWarnings("NotNullFieldNotInitialized")
+	private Trigger trigger;
+	private Expression<Integer> guiSlots;
+
+	@SuppressWarnings("unchecked")
+	@Override
+	public boolean init(Expression<?>[] exprs, int matchedPattern, Kleenean isDelayed, ParseResult parseResult, SectionNode sectionNode, List<TriggerItem> triggerItems) {
+		if (!getParser().isCurrentSection(SecCreateGUI.class)) {
+			Skript.error("You can't listen for changes of a slot outside of a GUI creation or editing section.");
+			return false;
+		}
+
+		trigger = loadCode(sectionNode, "inventory click", InventoryClickEvent.class);
+
+		guiSlots = (Expression<Integer>) exprs[0];
+		return true;
+	}
+
+	@Override
+	@Nullable
+	public TriggerItem walk(Event e) {
+		GUI gui = SkriptGUI.getGUIManager().getGUI(e);
+		Integer[] slots = guiSlots.getAll(e);
+
+		for (Integer slot : slots) {
+			if (gui != null && slot != null && slot >= 0 && slot + 1 <= gui.getInventory().getSize()) {
+				Object variables = Variables.copyLocalVariables(e);
+				GUI.SlotData slotData = gui.getSlotData(gui.convert(slot));
+				if (variables != null && slotData != null) {
+					slotData.setRunOnChange(event -> {
+						Variables.setLocalVariables(event, variables);
+						trigger.execute(event);
+					});
+				} else if (slotData != null) {
+					slotData.setRunOnChange(trigger::execute);
+				}
+			}
+		}
+
+		// We don't want to execute this section
+		return walk(e, false);
+	}
+
+	@Override
+	public String toString(@Nullable Event e, boolean debug) {
+		return "run on change of slot " + guiSlots.toString(e, debug);
+	}
+
+}

src/main/java/io/github/apickledwalrus/skriptgui/gui/GUI.java

@@ -38,6 +38,15 @@ public void onClick(InventoryClickEvent e) {
 				// Only cancel if this slot can't be removed AND all items aren't removable
 				e.setCancelled(!isRemovable(slotData));
 
+				// Call onChange if the slot is being changed
+				if (!e.isCancelled() && (e.getCursor() != null || e.getCurrentItem() != null)) {
+					if (e.getCursor() == null || e.getCurrentItem() == null ||
+							!e.getCursor().isSimilar(e.getCurrentItem()) ||
+							e.getCurrentItem().getAmount() < e.getCurrentItem().getMaxStackSize()) {
+						onChange(e);
+					}
+				}
+
 				Consumer<InventoryClickEvent> runOnClick = slotData.getRunOnClick();
 				if (runOnClick != null) {
 					SkriptGUI.getGUIManager().setGUI(e, GUI.this);
@@ -48,6 +57,28 @@ public void onClick(InventoryClickEvent e) {
 			}
 		}
 
+		@Override
+		public void onChange(InventoryClickEvent e) {
+			if (isPaused() || isPaused((Player) e.getWhoClicked())) {
+				e.setCancelled(true); // Just in case
+				return;
+			}
+
+			SlotData slotData = getSlotData(convert(e.getSlot()));
+			if (slotData != null) {
+				// Only cancel if this slot can't be removed AND all items aren't removable
+				e.setCancelled(!isRemovable(slotData));
+
+				Consumer<InventoryClickEvent> runOnChange = slotData.getRunOnChange();
+				if (runOnChange != null) {
+					SkriptGUI.getGUIManager().setGUI(e, GUI.this);
+					runOnChange.accept(e);
+				}
+			} else { // If there is no slot data, cancel if this GUI doesn't have stealable items
+				e.setCancelled(!isRemovable());
+			}
+		}
+
 		@Override
 		public void onDrag(InventoryDragEvent e) {
 			if (isPaused() || isPaused((Player) e.getWhoClicked())) {
@@ -61,6 +92,7 @@ public void onDrag(InventoryDragEvent e) {
 					break;
 				}
 			}
+			onChange(e);
 		}
 
 		@Override
@@ -233,6 +265,16 @@ public Character convert(Object slot) {
 		return nextSlot();
 	}
 
+	public InventoryClickEvent setClickedSlot(InventoryClickEvent event, int slot) {
+		return new InventoryClickEvent(
+				event.getView(),
+				event.getSlotType(),
+				slot,
+				event.getClick(),
+				event.getAction()
+		);
+	}
+
 	/**
 	 * @return The next available slot in this GUI.
 	 */
@@ -477,7 +519,7 @@ public String getID() {
 	 */
 	public void setID(@Nullable String id) {
 		this.id = id;
-		if (id == null && inventory.getViewers().size() == 0) {
+		if (id == null && inventory.getViewers().isEmpty()) {
 			SkriptGUI.getGUIManager().unregister(this);
 			clear();
 		}
@@ -500,6 +542,8 @@ public static final class SlotData {
 
 		@Nullable
 		private Consumer<InventoryClickEvent> runOnClick;
+		@Nullable
+		private Consumer<InventoryClickEvent> runOnChange;
 		private boolean removable;
 
 		public SlotData(@Nullable Consumer<InventoryClickEvent> runOnClick, boolean removable) {
@@ -515,6 +559,11 @@ public Consumer<InventoryClickEvent> getRunOnClick() {
 			return runOnClick;
 		}
 
+		@Nullable
+		public Consumer<InventoryClickEvent> getRunOnChange() {
+			return runOnChange;
+		}
+
 		/**
 		 * Updates the consumer to run when a slot with this data is clicked. A null value may be used to remove the consumer.
 		 * @param runOnClick The consumer to run when a slot with this data is clicked.
@@ -523,12 +572,16 @@ public void setRunOnClick(@Nullable Consumer<InventoryClickEvent> runOnClick) {
 			this.runOnClick = runOnClick;
 		}
 
+		public void setRunOnChange(@Nullable Consumer<InventoryClickEvent> runOnChange) {
+			this.runOnChange = runOnChange;
+		}
+
 		/**
 		 * @return Whether this item can be removed from its slot, regardless of {@link GUI#isRemovable()}.
 		 * 	Please note that if {@link #getRunOnClick()} returns a non-null value, this method will <b>always</b> return false.
 		 */
 		public boolean isRemovable() {
-			return runOnClick == null && removable;
+			return removable;
 		}
 
 		/**

src/main/java/io/github/apickledwalrus/skriptgui/gui/GUIEventHandler.java

@@ -1,10 +1,7 @@
 package io.github.apickledwalrus.skriptgui.gui;
 
 import org.bukkit.entity.Player;
-import org.bukkit.event.inventory.InventoryClickEvent;
-import org.bukkit.event.inventory.InventoryCloseEvent;
-import org.bukkit.event.inventory.InventoryDragEvent;
-import org.bukkit.event.inventory.InventoryOpenEvent;
+import org.bukkit.event.inventory.*;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -68,8 +65,21 @@ public boolean isPaused(Player player) {
 	}
 
 	public abstract void onClick(InventoryClickEvent e);
+	public abstract void onChange(InventoryClickEvent e);
 	public abstract void onDrag(InventoryDragEvent e);
 	public abstract void onOpen(InventoryOpenEvent e);
 	public abstract void onClose(InventoryCloseEvent e);
 
+	public void onChange(InventoryDragEvent e) {
+		for (int slot : e.getInventorySlots()) {
+			InventoryClickEvent clickEvent = new InventoryClickEvent(
+					e.getView(),
+					e.getView().getSlotType(slot),
+					slot,
+					ClickType.UNKNOWN,
+					InventoryAction.UNKNOWN
+			);
+			onChange(clickEvent);
+		}
+	}
 }

src/main/java/io/github/apickledwalrus/skriptgui/gui/events/GUIEvents.java

@@ -2,6 +2,7 @@
 
 import io.github.apickledwalrus.skriptgui.SkriptGUI;
 import io.github.apickledwalrus.skriptgui.gui.GUI;
+import io.github.apickledwalrus.skriptgui.gui.GUIEventHandler;
 import org.bukkit.GameMode;
 import org.bukkit.Material;
 import org.bukkit.event.EventHandler;
@@ -43,6 +44,7 @@ public void onInventoryClick(InventoryClickEvent event) {
 		if (gui == null) {
 			return;
 		}
+		GUIEventHandler eventHandler = gui.getEventHandler();
 
 		// Don't process unknown clicks for safety reasons - cancel them to prevent unwanted GUI changes
 		if (event.getClick() == ClickType.UNKNOWN) {
@@ -61,36 +63,37 @@ public void onInventoryClick(InventoryClickEvent event) {
 					if (clicked != null) {
 						Inventory guiInventory = gui.getInventory();
 
-						if (!guiInventory.contains(clicked.getType())) {
-							int firstEmpty = guiInventory.firstEmpty();
-							if (firstEmpty != -1 && gui.isRemovable(gui.convert(firstEmpty))) { // Safe to be moved into the GUI
-								return;
-							}
-						}
-
 						int size = guiInventory.getSize();
 
 						for (int slot = 0; slot < size; slot++) {
 							ItemStack item = guiInventory.getItem(slot);
 							if (item != null && item.getType() != Material.AIR && item.isSimilar(clicked)) {
+								InventoryClickEvent clickEvent = gui.setClickedSlot(event, slot);
+
 								if (!gui.isRemovable(gui.convert(slot))) {
-									if (item.getAmount() == 64) { // It wouldn't be able to combine
+									if (item.getAmount() >= item.getMaxStackSize()) { // It wouldn't be able to combine
 										continue;
 									}
 									event.setCancelled(true);
+
+									eventHandler.onChange(clickEvent);
 									return;
 								}
 
-								if (item.getAmount() + clicked.getAmount() <= 64) { // This will only modify a modifiable slot
+								if (item.getAmount() + clicked.getAmount() <= item.getMaxStackSize()) { // This will only modify a modifiable slot
+									eventHandler.onChange(clickEvent);
 									return;
 								}
 
 							}
 						}
 
 						int firstEmpty = guiInventory.firstEmpty();
-						if (firstEmpty != -1 && gui.isRemovable(gui.convert(firstEmpty))) { // Safe to be moved into the GUI
-							return;
+						if (firstEmpty != -1) { // Safe to be moved into the GUI
+							InventoryClickEvent clickEvent = gui.setClickedSlot(event, firstEmpty);
+							eventHandler.onChange(clickEvent);
+							if (gui.isRemovable(gui.convert(firstEmpty)))
+								return;
 						}
 
 					}
@@ -103,18 +106,42 @@ public void onInventoryClick(InventoryClickEvent event) {
 					// If that item is mergeable but it isn't stealable, we will cancel the event now
 					Inventory guiInventory = gui.getInventory();
 					int size = guiInventory.getSize();
-					ItemStack cursor = event.getWhoClicked().getItemOnCursor();
-					for (int slot = 0; slot < size; slot++) {
-						ItemStack item = guiInventory.getItem(slot);
-						if (item != null && item.isSimilar(cursor) && !gui.isRemovable(gui.convert(slot))) {
-							event.setCancelled(true);
-							break;
+					ItemStack cursor = event.getCursor();
+					if (cursor != null) {
+						for (int slot = 0; slot < size; slot++) {
+							ItemStack item = guiInventory.getItem(slot);
+							if (item != null && item.isSimilar(cursor)) {
+								if (!gui.isRemovable(gui.convert(slot))) {
+									event.setCancelled(true);
+									break;
+								}
+								if (cursor.getAmount() < cursor.getMaxStackSize()) {
+									InventoryClickEvent clickEvent = gui.setClickedSlot(event, slot);
+									eventHandler.onChange(clickEvent);
+								}
+							}
 						}
 					}
 					return;
 				default:
 					return;
 			}
+		} else {
+			// Call onChange if a slot is changed due to interactions within the gui itself
+			if (event.getClick() == ClickType.DOUBLE_CLICK) {
+				Inventory guiInventory = gui.getInventory();
+				int size = guiInventory.getSize();
+				ItemStack cursor = event.getCursor();
+				if (cursor != null && cursor.getAmount() < cursor.getMaxStackSize()) {
+					for (int slot = 0; slot < size; slot++) {
+						ItemStack item = guiInventory.getItem(slot);
+						if (item != null && item.isSimilar(cursor)) {
+							InventoryClickEvent clickEvent = gui.setClickedSlot(event, slot);
+							eventHandler.onChange(clickEvent);
+						}
+					}
+				}
+			}
 		}
 
 		gui.getEventHandler().onClick(event);