Add PlayerGameModeChangeEvent

src/main/java/net/minestom/server/entity/Player.java

@@ -1286,8 +1286,18 @@ public class Player extends LivingEntity implements CommandSender, Localizable,
      * Changes the player {@link GameMode}
      *
      * @param gameMode the new player GameMode
+     * @return true if the gamemode was changed successfully, false otherwise (cancelled by event)
      */
-    public void setGameMode(@NotNull GameMode gameMode) {
+    public boolean setGameMode(@NotNull GameMode gameMode) {
+        PlayerGameModeChangeEvent playerGameModeChangeEvent = new PlayerGameModeChangeEvent(this, gameMode);
+        EventDispatcher.call(playerGameModeChangeEvent);
+        if (playerGameModeChangeEvent.isCancelled()) {
+            // Abort
+            return false;
+        }
+
+        gameMode = playerGameModeChangeEvent.getNewGameMode();
+
         this.gameMode = gameMode;
         // Condition to prevent sending the packets before spawning the player
         if (isActive()) {
@@ -1320,6 +1330,8 @@ public class Player extends LivingEntity implements CommandSender, Localizable,
         if (isActive()) {
             refreshAbilities();
         }
+
+        return true;
     }
 
     /**

src/main/java/net/minestom/server/event/player/PlayerGameModeChangeEvent.java

@@ -0,0 +1,56 @@
+package net.minestom.server.event.player;
+
+import net.minestom.server.entity.GameMode;
+import net.minestom.server.entity.Player;
+import net.minestom.server.event.trait.CancellableEvent;
+import net.minestom.server.event.trait.PlayerInstanceEvent;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Called when the gamemode of a player is being modified.
+ */
+public class PlayerGameModeChangeEvent implements PlayerInstanceEvent, CancellableEvent {
+
+    private final Player player;
+    private GameMode newGameMode;
+
+    private boolean cancelled;
+
+    public PlayerGameModeChangeEvent(@NotNull Player player, @NotNull GameMode newGameMode) {
+        this.player = player;
+        this.newGameMode = newGameMode;
+    }
+
+    /**
+     * Gets the target gamemode.
+     *
+     * @return the target gamemode
+     */
+    public @NotNull GameMode getNewGameMode() {
+        return newGameMode;
+    }
+
+    /**
+     * Changes the target gamemode.
+     *
+     * @param newGameMode the new target gamemode
+     */
+    public void setNewGameMode(@NotNull GameMode newGameMode) {
+        this.newGameMode = newGameMode;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @Override
+    public @NotNull Player getPlayer() {
+        return player;
+    }
+}