feat: 目标不在线时进行提示和更新UI

WebRTCSignalServer/src/main/java/com/ttstd/signaling/handler/SignalWebSocketHandler.java

@@ -57,13 +57,17 @@ public class SignalWebSocketHandler extends TextWebSocketHandler {
                     break;
                 case "OFFER":
                 case "ANSWER":
+                    // 关键信令：目标不在线时回送 TARGET_OFFLINE 提醒发送方
+                    forwardMessage(signalMessage, true);
+                    break;
                 case "ICE_CANDIDATE":
                 case "CONTROL_COMMAND":
-                    forwardMessage(signalMessage);
+                    // 普通信令：目标不在线时静默丢弃，不返回提示
+                    forwardMessage(signalMessage, false);
                     break;
                 default:
-                    // 其他消息类型直接转发
-                    forwardMessage(signalMessage);
+                    // 其他消息类型直接转发，目标不在线时静默丢弃
+                    forwardMessage(signalMessage, false);
                     break;
             }
         } catch (Exception e) {
@@ -122,7 +126,7 @@ public class SignalWebSocketHandler extends TextWebSocketHandler {
         sendToSession(session, response);
     }
 
-    private void forwardMessage(SignalMessage message) {
+    private void forwardMessage(SignalMessage message, boolean notifyOffline) {
         String toDeviceId = message.getToDeviceId();
         if (toDeviceId == null) {
             logger.warn("Cannot forward message: toDeviceId is null");
@@ -132,6 +136,10 @@ public class SignalWebSocketHandler extends TextWebSocketHandler {
         WebSocketSession targetSession = sessionManager.getSession(toDeviceId);
         if (targetSession == null || !targetSession.isOpen()) {
             logger.warn("Target device {} is not online", toDeviceId);
+            // 仅 OFFER / ANSWER 在目标不在线时回送 TARGET_OFFLINE，其余类型静默丢弃
+            if (notifyOffline) {
+                notifySenderTargetOffline(message, toDeviceId);
+            }
             return;
         }
 
@@ -144,6 +152,31 @@ public class SignalWebSocketHandler extends TextWebSocketHandler {
         }
     }
 
+    /**
+     * 目标被控端不在线时，向发送方（主控端）回送 TARGET_OFFLINE 提醒。
+     */
+    private void notifySenderTargetOffline(SignalMessage message, String offlineDeviceId) {
+        String fromDeviceId = message.getFromDeviceId();
+        if (fromDeviceId == null) {
+            logger.warn("Cannot notify offline state: fromDeviceId is null");
+            return;
+        }
+
+        WebSocketSession senderSession = sessionManager.getSession(fromDeviceId);
+        if (senderSession == null || !senderSession.isOpen()) {
+            logger.warn("Sender {} session not found, cannot notify target offline", fromDeviceId);
+            return;
+        }
+
+        Map<String, Object> response = new HashMap<>();
+        response.put("type", "TARGET_OFFLINE");
+        response.put("toDeviceId", offlineDeviceId);
+        response.put("payload", "目标被控端（" + offlineDeviceId + "）不在线，请确认设备已开启并连接到信令服务器");
+
+        sendToSession(senderSession, response);
+        logger.info("Notified sender {} that target {} is offline", fromDeviceId, offlineDeviceId);
+    }
+
     private void sendToSession(WebSocketSession session, Object data) {
         try {
             String json = objectMapper.writeValueAsString(data);