Do not push only meta data to the client (#11490)

Change-Id: I589ede89583be90e99fbed6fe5c0c6c1ac4d7c0a

server/src/com/vaadin/ui/ConnectorTracker.java

@@ -32,9 +32,9 @@ import org.json.JSONException;
 import org.json.JSONObject;
 
 import com.vaadin.server.AbstractClientConnector;
-import com.vaadin.server.LegacyCommunicationManager;
 import com.vaadin.server.ClientConnector;
 import com.vaadin.server.GlobalResourceHandler;
+import com.vaadin.server.LegacyCommunicationManager;
 import com.vaadin.server.StreamVariable;
 
 /**
@@ -464,6 +464,15 @@ public class ConnectorTracker implements Serializable {
         return dirtyConnectors;
     }
 
+    /**
+     * Checks if there a dirty connectors.
+     * 
+     * @return true if there are dirty connectors, false otherwise
+     */
+    public boolean hasDirtyConnectors() {
+        return !getDirtyConnectors().isEmpty();
+    }
+
     /**
      * Returns a collection of those {@link #getDirtyConnectors() dirty
      * connectors} that are actually visible to the client.

server/src/com/vaadin/ui/UI.java

@@ -1141,6 +1141,11 @@ public abstract class UI extends AbstractSingleComponentContainer implements
     public void push() {
         VaadinSession session = getSession();
         if (session != null) {
+            if (getConnectorTracker().hasDirtyConnectors()) {
+                // Do not push if there is nothing to push
+                return;
+            }
+
             if (session.getPushMode() == PushMode.DISABLED) {
                 throw new IllegalStateException("Push not enabled");
             }