Added menu signals

eco-api/src/main/java/com/willfp/eco/core/gui/menu/Menu.java

@@ -135,6 +135,17 @@ public interface Menu {
      */
     void refresh(@NotNull Player player);
 
+    /**
+     * Send a signal to the menu.
+     *
+     * @param player The player.
+     * @param signal The signal.
+     */
+    default void sendSignal(@NotNull final Player player,
+                            @NotNull final Signal signal) {
+        // Override when needed.
+    }
+
     /**
      * Write data.
      *

eco-api/src/main/java/com/willfp/eco/core/gui/menu/MenuBuilder.java

@@ -122,7 +122,7 @@ public interface MenuBuilder extends PageBuilder {
     }
 
     /**
-     * Set the menu close handler.
+     * Add a menu close handler.
      *
      * @param action The handler.
      * @return The builder.
@@ -132,7 +132,7 @@ public interface MenuBuilder extends PageBuilder {
     }
 
     /**
-     * Set the menu close handler.
+     * Add a menu close handler.
      *
      * @param action The handler.
      * @return The builder.
@@ -140,7 +140,7 @@ public interface MenuBuilder extends PageBuilder {
     MenuBuilder onClose(@NotNull CloseHandler action);
 
     /**
-     * Set the menu open handler.
+     * Add a menu open handler.
      *
      * @param action The handler.
      * @return The builder.
@@ -148,13 +148,23 @@ public interface MenuBuilder extends PageBuilder {
     MenuBuilder onOpen(@NotNull OpenHandler action);
 
     /**
-     * Set the action to run on render.
+     * Add an action to run on render.
      *
      * @param action The action.
      * @return The builder.
      */
     MenuBuilder onRender(@NotNull BiConsumer<Player, Menu> action);
 
+    /**
+     * Add an action to run on signal receive.
+     *
+     * @param action The action.
+     * @return THe builder.
+     */
+    default MenuBuilder onSignalReceive(@NotNull final SignalHandler action) {
+        return this;
+    }
+
     /**
      * Build the menu.
      *

eco-api/src/main/java/com/willfp/eco/core/gui/menu/Signal.java

@@ -0,0 +1,8 @@
+package com.willfp.eco.core.gui.menu;
+
+/**
+ * Represents a signal sent to a menu.
+ */
+public interface Signal {
+
+}

eco-api/src/main/java/com/willfp/eco/core/gui/menu/SignalHandler.java

@@ -0,0 +1,45 @@
+package com.willfp.eco.core.gui.menu;
+
+import org.bukkit.entity.Player;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Handles signals sent to menus.
+ */
+public abstract class SignalHandler<T extends Signal> {
+    /**
+     * The class of signal.
+     */
+    private final Class<T> signalClass;
+
+    /**
+     * Create a new signal handler.
+     *
+     * @param signalClass The class of signal to handle.
+     */
+    protected SignalHandler(@NotNull final Class<T> signalClass) {
+        this.signalClass = signalClass;
+    }
+
+
+    /**
+     * Performs this operation on the given arguments.
+     *
+     * @param player The player.
+     * @param menu   The menu.
+     * @param signal The signal.
+     */
+    public abstract void handle(@NotNull Player player,
+                                @NotNull Menu menu,
+                                @NotNull T signal);
+
+    /**
+     * Get if this handler can handle a certain signal.
+     *
+     * @param signal The signal
+     * @return If the signal can be handled.
+     */
+    public boolean canHandleSignal(@NotNull final Signal signal) {
+        return signalClass.isAssignableFrom(signal.getClass());
+    }
+}

eco-api/src/main/java/com/willfp/eco/core/gui/page/PageChangeSignal.java

@@ -0,0 +1,16 @@
+package com.willfp.eco.core.gui.page;
+
+import com.willfp.eco.core.gui.menu.Signal;
+
+/**
+ * Represents a page change.
+ *
+ * @param newPage The new page.
+ * @param oldPage The old page.
+ */
+public record PageChangeSignal(
+        int newPage,
+        int oldPage
+) implements Signal {
+
+}

eco-api/src/main/java/com/willfp/eco/core/gui/page/PageChanger.java

@@ -43,7 +43,16 @@ public final class PageChanger implements GUIComponent {
                                     Page.getMaxPage(player, menu)
                             )
                     );
+
+                    if (newPage == page) {
+                        return;
+                    }
+
                     menu.addState(player, Page.PAGE_KEY, newPage);
+                    menu.sendSignal(player, new PageChangeSignal(
+                            newPage,
+                            page
+                    ));
                 })
                 .build();
     }

eco-api/src/main/kotlin/com/willfp/eco/core/gui/GUIHelpers.kt

@@ -4,6 +4,8 @@ package com.willfp.eco.core.gui
 
 import com.willfp.eco.core.gui.menu.Menu
 import com.willfp.eco.core.gui.menu.MenuBuilder
+import com.willfp.eco.core.gui.menu.Signal
+import com.willfp.eco.core.gui.menu.SignalHandler
 import com.willfp.eco.core.gui.page.Page
 import com.willfp.eco.core.gui.page.PageBuilder
 import com.willfp.eco.core.gui.slot.Slot
@@ -140,6 +142,14 @@ fun MenuBuilder.addPage(page: Int, creation: PageBuilder.() -> Unit): MenuBuilde
     return this.addPage(Page(page, builder.build()))
 }
 
+/** @see MenuBuilder.onSignalReceive */
+inline fun <reified T : Signal> MenuBuilder.onSignalReceive(crossinline handler: (Player, Menu, T) -> Unit): MenuBuilder {
+    return this.onSignalReceive(object : SignalHandler<T>(T::class.java) {
+        override fun handle(player: Player, menu: Menu, signal: T) =
+            handler(player, menu, signal)
+    })
+}
+
 /** Kotlin builder for menus. */
 fun menu(
     rows: Int,