diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/util/ConstraintViolationNotificator.java b/ganttzk/src/main/java/org/zkoss/ganttz/util/ConstraintViolationNotificator.java
index 23c0f2d3d..06a27e6c4 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/util/ConstraintViolationNotificator.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/util/ConstraintViolationNotificator.java
@@ -25,6 +25,7 @@ import java.util.List;
 import org.zkoss.ganttz.data.constraint.Constraint;
 import org.zkoss.ganttz.data.constraint.Constraint.IConstraintViolationListener;
 import org.zkoss.ganttz.util.WeakReferencedListeners.IListenerNotification;
+import org.zkoss.ganttz.util.WeakReferencedListeners.Mode;
 
 /**
  * @author Óscar González Fernández <ogonzalez@igalia.com>
@@ -86,5 +87,9 @@ public class ConstraintViolationNotificator<T> {
         constraintViolationListeners.addListener(listener);
     }
 
+    public void addConstraintViolationListener(
+            IConstraintViolationListener<T> listener, Mode mode) {
+        constraintViolationListeners.addListener(listener, mode);
+    }
 
 }
diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/util/WeakReferencedListeners.java b/ganttzk/src/main/java/org/zkoss/ganttz/util/WeakReferencedListeners.java
index 3738da71a..435a83df1 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/util/WeakReferencedListeners.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/util/WeakReferencedListeners.java
@@ -24,8 +24,11 @@ package org.zkoss.ganttz.util;
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.LinkedList;
+import java.util.List;
 import java.util.ListIterator;
 
+import org.apache.commons.lang.Validate;
+
 public class WeakReferencedListeners<T> {
 
     public interface IListenerNotification<T> {
@@ -44,27 +47,56 @@ public class WeakReferencedListeners<T> {
 
     }
 
-    public synchronized void addListener(T listener) {
-        if (listener == null) {
-            throw new IllegalArgumentException("listener cannot be null");
+    public enum Mode {
+        RECEIVE_PENDING, FROM_NOW_ON;
+    }
+
+    public void addListener(T listener) {
+        this.addListener(listener, Mode.FROM_NOW_ON);
+    }
+
+    public synchronized void addListener(T listener, Mode mode) {
+        Validate.notNull(listener);
+
+        if (getActiveListeners().isEmpty() && mode == Mode.RECEIVE_PENDING) {
+            notifyPendingOfNotificationTo(listener);
         }
         listeners.add(new WeakReference<T>(listener));
     }
 
+    private List<IListenerNotification<? super T>> pendingOfNotification = new ArrayList<WeakReferencedListeners.IListenerNotification<? super T>>();
+
+    private void notifyPendingOfNotificationTo(T listener) {
+        for (IListenerNotification<? super T> each : pendingOfNotification) {
+            each.doNotify(listener);
+        }
+        pendingOfNotification.clear();
+    }
+
     public synchronized void fireEvent(
             IListenerNotification<? super T> notification) {
+        List<T> active = getActiveListeners();
+
+        for (T listener : active) {
+            notification.doNotify(listener);
+        }
+
+        if (active.isEmpty()) {
+            pendingOfNotification.add(notification);
+        }
+    }
+
+    private List<T> getActiveListeners() {
         ListIterator<WeakReference<T>> listIterator = listeners.listIterator();
-        ArrayList<T> active = new ArrayList<T>();
+        List<T> result = new ArrayList<T>();
         while (listIterator.hasNext()) {
             T listener = listIterator.next().get();
             if (listener == null) {
                 listIterator.remove();
             } else {
-                active.add(listener);
+                result.add(listener);
             }
         }
-        for (T listener : active) {
-            notification.doNotify(listener);
-        }
+        return result;
     }
 }