如何用Xamarin 4 Shell导航实现Android标准导航？

using Xamarin.Forms;

namespace YourAppName
{
    public partial class App : Shell
    {
        public App()
        {
            InitializeComponent();
            // 设置主页
            MainPage = new AppShell();
        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<Shell xmlns="http://xamarin.com/schemas/2014/forms"
       xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
       xmlns:views="clr-namespace:YourAppName.Views"
       Title="Your App Name"
       x:Class="YourAppName.AppShell">

    <TabBar>
        <Tab Title="Page 1" Icon="page1.png">
            <ShellContent ContentTemplate="{DataTemplate views:Page1}" />
        </Tab>
        <Tab Title="Page 2" Icon="page2.png">
            <ShellContent ContentTemplate="{DataTemplate views:Page2}" />
        </Tab>
        <!-- 添加更多页面 -->
    </TabBar>

</Shell>

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="YourAppName.Views.Page1">
    <!-- 页面内容 -->
</ContentPage>