UWP在下载webview时显示进度条

Question

我想在下载内容时显示UWP Web View的进度。这就是我所做的：

main_page.xaml：

<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="768"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="20"
Foreground="#FF000000">

<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="_grid" Background="Transparent">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>

    <!--TitlePanel contains the name of the application and page title-->

    <!--ContentPanel - place additional content here-->
    <Grid x:Name="_contentPanel" Grid.Row="1" Margin="0,0,0,0">
        <WebView 
            x:Name="webView" 
            Margin="0,0,0,0" 
            Grid.Row="1" 
            />

        <ProgressBar x:Name="_progressBar" Margin="10,10,0,754" IsIndeterminate="False" Visibility="Collapsed"/>

    </Grid>

</Grid>

在NavigationStarting事件中，我显示了进度条：

 private async void WebView_Navigation_Starting(WebView sender, WebViewNavigationStartingEventArgs args)
    {
        //show progress bar
        _progressBar.Visibility = Visibility.Visible;
.......
}

我在加载内容时隐藏它：

 void WebView_LoadCompleted(object sender, NavigationEventArgs e)
    {
        //show progress bar
        _progressBar.Visibility = Visibility.Collapsed;
    }

这里是我想要更新进度条中进度值的地方。但是如何获取加载的Web视图内容的实际百分比值？

 void WebView_Content_Loading(WebView sender, WebViewContentLoadingEventArgs args) {
        webView.SetValue(???);
    }