Add ClientChatReceivedEvent for system messages (#9284)

src/main/java/net/minecraftforge/client/ForgeHooksClient.java

@@ -72,10 +72,12 @@ import net.minecraft.core.Direction;
 import net.minecraft.locale.Language;
 import net.minecraft.network.Connection;
 import net.minecraft.network.chat.ChatType;
+import net.minecraft.network.chat.ChatTypeDecoration;
 import net.minecraft.network.chat.CommonComponents;
 import net.minecraft.network.chat.Component;
 import net.minecraft.network.chat.FormattedText;
 import net.minecraft.network.chat.PlayerChatMessage;
+import net.minecraft.network.chat.Style;
 import net.minecraft.network.protocol.status.ServerStatus;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.packs.resources.Resource;
@@ -974,6 +976,17 @@ public class ForgeHooksClient
         return MinecraftForge.EVENT_BUS.post(event) ? null : event.getMessage();
     }
 
+    private static final ChatTypeDecoration SYSTEM_CHAT_TYPE_DECORATION = new ChatTypeDecoration("forge.chatType.system", List.of(ChatTypeDecoration.Parameter.CONTENT), Style.EMPTY);
+    private static final ChatType SYSTEM_CHAT_TYPE = new ChatType(SYSTEM_CHAT_TYPE_DECORATION, SYSTEM_CHAT_TYPE_DECORATION);
+    private static final ChatType.Bound SYSTEM_CHAT_TYPE_BOUND = SYSTEM_CHAT_TYPE.bind(Component.literal("System"));
+
+    @Nullable
+    public static Component onClientSystemChat(Component message, boolean overlay)
+    {
+        ClientChatReceivedEvent.System event = new ClientChatReceivedEvent.System(SYSTEM_CHAT_TYPE_BOUND, message, overlay);
+        return MinecraftForge.EVENT_BUS.post(event) ? null : event.getMessage();
+    }
+
     @NotNull
     public static String onClientSendMessage(String message)
     {

src/main/java/net/minecraftforge/client/event/ClientChatReceivedEvent.java

@@ -118,4 +118,33 @@ public class ClientChatReceivedEvent extends Event
             return this.playerChatMessage;
         }
     }
+
+    /**
+     * Fired when a system chat message is received on the client.
+     *
+     * <p>This event is {@linkplain Cancelable cancellable}, and does not {@linkplain HasResult have a result}.
+     * If the event is cancelled, the message is not displayed in the chat message window or in the overlay.</p>
+     *
+     * <p>This event is fired on the {@linkplain MinecraftForge#EVENT_BUS main Forge event bus},
+     * only on the {@linkplain LogicalSide#CLIENT logical client}.</p>
+     */
+    public static class System extends ClientChatReceivedEvent
+    {
+        private final boolean overlay;
+
+        @ApiStatus.Internal
+        public System(ChatType.Bound boundChatType, Component message, boolean overlay)
+        {
+            super(boundChatType, message, Util.NIL_UUID);
+            this.overlay = overlay;
+        }
+
+        /**
+         * {@return whether the message goes to the overlay}
+         */
+        public boolean isOverlay()
+        {
+            return this.overlay;
+        }
+    }
 }