UILabel UILongPressGestureRecognizer 不起作用？

我怎样才能得到UILongPressGestureRecognizer在 uilabel.当我实现以下代码时，它不会调用该函数。那么请告诉我我做错了什么？

UILongPressGestureRecognizer *longPress = [[UILongPressGestureRecognizer alloc]      initWithTarget:self action:@selector(LabelLongPressed:)];
longPress.minimumPressDuration = 0.5;  // Seconds
longPress.numberOfTapsRequired = 0;

[objlblDuplicate addGestureRecognizer:longPress];
[longPress release];

默认情况下，UILabel 无法获取触摸事件。

objlblDuplicate.userInteractionEnabled = YES;