[UWP][Prism] NavigationServiceの対象FrameをMainPageのSpritViewに設定するやつ

App.xaml.cs

using Prism.Unity.Windows;
using RssReaderApp.Views;
using System.Threading.Tasks;
using Windows.ApplicationModel.Activation;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace RssReaderApp
{
    /// <summary>
    /// 既定の Application クラスを補完するアプリケーション固有の動作を提供します。
    /// </summary>
    sealed partial class App : PrismUnityApplication
    {
        private Frame _rootFrame;

        public App()
        {
            InitializeComponent();
        }
        protected override UIElement CreateShell(Frame rootFrame)
        {
            _rootFrame = rootFrame;
            return base.CreateShell(rootFrame);
        }

        protected override Task OnLaunchApplicationAsync(LaunchActivatedEventArgs args)
        {
            var  mainPage = Window.Current.Content as MainPage;
            if (mainPage == null)
            {
                mainPage = new MainPage();
                mainPage.frameContainer.Content = _rootFrame;
                Window.Current.Content = mainPage;
            }
            if (_rootFrame.Content == null)
            {
                NavigationService.Navigate("Init", null);
            }
            
            return Task.CompletedTask;  
        }

        
    }
}

MainPage.xaml

<Page
    x:Class="RssReaderApp.Views.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:RssReaderApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:mvvm="using:Prism.Windows.Mvvm"
    mvvm:ViewModelLocator.AutoWireViewModel="True"
    mc:Ignorable="d">

    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal">
            <ToggleButton Content="&#xE700;"
                          FontFamily="{StaticResource SymbolThemeFontFamily}"
                          IsChecked="{Binding IsPainOpen,ElementName=SplitView}"/>
            <TextBlock Text="RSS リーダー" Style="{StaticResource TitleTextBlockStyle}"/>
        </StackPanel>
        <SplitView Grid.Row="1" x:Name="SplitView">
            <SplitView.Pane>
                <!-- Menu -->
                <ListView ItemsSource="{x:Bind ViewModel.Feeds}" ></ListView>
            </SplitView.Pane>
            <ContentControl x:Name="frameContainer" x:FieldModifier="public" >
                <!-- ここにFrameが入る -->
            </ContentControl>
        </SplitView>
    </Grid>
</Page>