Ext中的tabpanle怎么判断哪一个tab被选中？

Question

<ext:TabPanel ID="TabPanel1" runat="server" ActiveTab="5">
                                    <Tabs>
                                        <ext:Tab runat="server" Title="机构职能类" ID="Tab1">
                                            <Content>
                                                <asp:TextBox ID="txtKind1" runat="server" Width="100%" BorderStyle="None" Height="20px"></asp:TextBox>
                                            </Content>
                                        </ext:Tab>
                                        <ext:Tab runat="server" Title="法律法规类" ID="Tab2" >
                                            <Content>
                                                <asp:TextBox ID="txtKind2" runat="server" Width="100%" BorderStyle="None" Height="20px"></asp:TextBox>
                                            </Content>
                                        </ext:Tab>
</Tabs>
</TabPanel>
请问后台我怎么才能获得我当前选择的是哪个tab页呢？