UWP 下载文件显示下载进度

<Page
    x:Class="WgscdProject.TestDownloadPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WgscdProject"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid Name="d">
        <TextBlock Name="txt2" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="0,-164,0,0" Text="file path"></TextBlock>
        <TextBlock Name="txt" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="0,-84,0,0" Text="100%"></TextBlock>
        <Grid  Name="gridPercent" Background="WhiteSmoke"  Height="22">
            <Rectangle Name="lbPercent" Fill="Blue" Width="1" HorizontalAlignment="Left"  Height="22"></Rectangle>
        </Grid>
        <Button VerticalAlignment="Top" Height="55" Width="222"  HorizontalAlignment="Center" Content="test download" Click="Button_Click"></Button>
    </Grid>

</Page>

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.Storage;
using Windows.Storage.Streams;
using Windows.UI.Popups;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238

namespace WgscdProject
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class TestDownloadPage : Page
    {
        public TestDownloadPage()
        {
            this.InitializeComponent();

            worker = new BackgroundWorker();
            worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted); ;
            worker.DoWork += new DoWorkEventHandler(worker_DoWork);

        }

        BackgroundWorker worker;
        bool isDownloading = false;
        void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {

        }

        void worker_DoWork(object sender, DoWorkEventArgs e)
        {
            dowloadFile();
        }

        public async void Invoke(Action action, Windows.UI.Core.CoreDispatcherPriority Priority = Windows.UI.Core.CoreDispatcherPriority.Normal)

        {

            await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(Priority, () => { action(); });

        }

        async void dowloadFile()
        {

            if (isDownloading) return;

            isDownloading = true ;
            string serverUrl = "http://files.cnblogs.com/files/pcat/hackbar.zip";//"https://files.cnblogs.com/files/wgscd/jyzsUpdate.zip"; 

            try
            {
                System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(serverUrl);
                System.Net.WebResponse response = await request.GetResponseAsync();

                System.IO.Stream ns = response.GetResponseStream();
                long totalSize = response.ContentLength;
                double  hasDownSize = 0;
                byte[] nbytes = new byte[512];//521,2048 etc
                int nReadSize = 0;
                nReadSize = ns.Read(nbytes, 0, nbytes.Length);
                StorageFolder folder;
                folder = ApplicationData.Current.LocalFolder; // await StorageFolder.GetFolderFromPathAsync("F:\\");
                string localFile = "1.rar";
                StorageFile file = await folder.CreateFileAsync(localFile, CreationCollisionOption.ReplaceExisting);

                using (StorageStreamTransaction transaction = await file.OpenTransactedWriteAsync())
                {
                    using (DataWriter dataWriter = new DataWriter(transaction.Stream))
                    {
                        while (nReadSize > 0)
                        {

                            dataWriter.WriteBytes(nbytes);

                            nReadSize = ns.Read(nbytes, 0, 512);
                            hasDownSize += nReadSize;
                            this.Invoke(new Action(() =>
                            {
                                txt.Text = "" + (hasDownSize /1024.0).ToString("0.00")+ " KB/" +( totalSize / 1024.0).ToString("0.00") + " KB   (" + (((double)hasDownSize * 100 / totalSize).ToString("0")) + "%)";//显示下载百分比
                                 lbPercent.Width = gridPercent.RenderSize.Width * ((double)hasDownSize / totalSize);
                                txt.UpdateLayout();
                             }));

                        }

                        transaction.Stream.Size = await dataWriter.StoreAsync();
                        await dataWriter.FlushAsync ();
                        await transaction.CommitAsync();

                        this.Invoke(new Action(() =>
                        {
                            txt.Text = "100%";//显示下载百分比
                             txt.UpdateLayout();
                          }));
                        // MessageBox.Show("下载完成");

                    }
                }

            }
            catch (Exception ex)
            {
                // fs.Close();
                new MessageDialog("出现错误:" + ex.ToString()).ShowAsync();
            }

            isDownloading = false;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            worker.RunWorkerAsync();
        }
    }
}