+    @Override
+    public void addDatatransferProgressListener(OnDatatransferProgressListener listener) {
+        synchronized (mDataTransferListeners) {
+            mDataTransferListeners.add(listener);
+        }
+    }
+    
+    @Override
+    public void addDatatransferProgressListeners(Collection<OnDatatransferProgressListener> listeners) {
+        synchronized (mDataTransferListeners) {
+            mDataTransferListeners.addAll(listeners);
+        }