为什么radioButtonGroup不立即设置numRadioButtons?

            var x : int = 50;
            var group : RadioButtonGroup = new RadioButtonGroup();
            for (var i : int = 0; i < 5; i++) {
                var rb : RadioButton = new RadioButton();
                rb.id = i.toString();
                rb.group = group;
                rb.label = i.toString();
                rb.x = x;
                x += 40;
                cnv_subContent.addElement(rb);//a BorderContainer
            }
            Alert.show(group.numRadioButtons.toString());

当我运行应用程序时,它显示“0”。为什么是这个?