Refactor: Trigger the listener added for the first time (#225)

* refactor: Trigger the listener added for the first time

* refactor: Trigger the watch listener before adding to watchers

polaris-discovery/polaris-discovery-client/src/main/java/com/tencent/polaris/discovery/client/flow/WatchFlow.java

@@ -36,15 +36,18 @@
 import com.tencent.polaris.client.util.NamedThreadFactory;
 import com.tencent.polaris.client.util.Utils;
 import com.tencent.polaris.logging.LoggerFactory;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executor;
 import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.BiConsumer;
+import java.util.stream.Collectors;
 import org.slf4j.Logger;
 
 /**
@@ -77,7 +80,18 @@ public WatchServiceResponse commonWatchService(CommonWatchServiceRequest request
         InstancesResponse response = syncFlow.commonSyncGetAllInstances(request.getAllRequest());
         watchers.computeIfAbsent(request.getSvcEventKey().getServiceKey(),
                 key -> Collections.synchronizedSet(new HashSet<>()));
-        boolean result = watchers.get(serviceKey).addAll(request.getWatchServiceRequest().getListeners());
+        List<ServiceListener> addListeners = request.getWatchServiceRequest().getListeners();
+        Set<ServiceListener> existListeners = watchers.get(serviceKey);
+        List<ServiceListener> firstAddedListeners = addListeners.stream()
+                .filter(serviceListener -> !existListeners.contains(serviceListener)).collect(Collectors.toList());
+        if (CollectionUtils.isNotEmpty(firstAddedListeners)) {
+            ServiceChangeEvent event = ServiceChangeEvent.builder().serviceKey(serviceKey)
+                    .addInstances(Arrays.asList(response.getInstances()))
+                    .allInstances(Arrays.asList(response.getInstances())).build();
+            firstAddedListeners.forEach(
+                    serviceListener -> executor.execute(event.getServiceKey(), () -> serviceListener.onEvent(event)));
+        }
+        boolean result = existListeners.addAll(addListeners);
         return new WatchServiceResponse(response, result);
     }