How to make my constructor async in UWP MVVM model? (MVVM Lighttoolkit)

Question

I have a UWP project want to reads StorageFolder VideosLibrary and show a list of mp4 files at Views with thumbnail.

With MVVM ligth toolkit I have setup this 4 flies with xaml. The Xaml is using UWP community toolkit wrap panel.

1)ViewModelLocator.cs

namespace UWP.ViewModels
{
/// <summary>
/// This class contains static reference to all the view models in the 
/// application and provides an entry point for the bindings.
/// </summary>

class ViewModelLocator
{
    /// <summary>
    /// Initializes a new instance of the ViewModelLocator class.
    /// </summary>
    public ViewModelLocator()
    {
        ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
        if (ViewModelBase.IsInDesignModeStatic)
        {
            // Create  design time view services and models
        }
        else
        {
            // Create run Time view services and models
        }
        //Register services used here

        SimpleIoc.Default.Register<VideoListModel>();
    }


    public VideoListModel VideoListModel
    {
        get { return ServiceLocator.Current.GetInstance<VideoListModel>(); 
    }
}
}

2) VideoListItem.cs

namespace UWP.Models
{
class VideoListItem : ViewModelBase
{
    public string VideoName { get; set; }
    public string Author { get; set; }
    public Uri Vid_url { get; set; }
    public BitmapImage Image { get; set; }

    public VideoListItem(string videoname,string author,Uri url, BitmapImage img)
    {
        this.VideoName = videoname;
        this.Author = author;
        this.Vid_url = url;
        this.Image = img;
    }
}
}

3) VideoListModel.cs

namespace UWP.ViewModels
{
class VideoListModel : ViewModelBase
{
    public ObservableCollection<VideoListItem> VideoItems { get; set; }

    private VideoListItem videoItems;

    public VideoListModel()
    {

    }

    public async static Task<List<VideoListItem>> GetVideoItem()
    {
        List<VideoListItem> videoItems = new List<VideoListItem>();
        StorageFolder videos_folder = await KnownFolders.VideosLibrary.CreateFolderAsync("Videos");
        var queryOptions = new QueryOptions(CommonFileQuery.DefaultQuery, new[] { ".mp4" });
        var videos = await videos_folder.CreateFileQueryWithOptions(queryOptions).GetFilesAsync();


        foreach (var video in videos)
        {
            //Debug.WriteLine(video.Name);
            //videoItems.Add(new VideoListItem());
            var bitmap = new BitmapImage();
            var thumbnail = await video.GetThumbnailAsync(ThumbnailMode.SingleItem);
            await bitmap.SetSourceAsync(thumbnail);
            videoItems.Add(new VideoListItem(video.DisplayName, "", new Uri(video.Path),bitmap));

        }

        //foreach(var video in videoItems)
        //{
        //    Debug.WriteLine("Name:{0} , Author:{1}, Uri:{2}, Bitmap:{3}", video.VideoName, video.Author, video.Vid_url, video.Image.UriSource);
        //}


        return videoItems;
    }


}
}

4) Video.xaml

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="using:UWP.Views"
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:Controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
  x:Class="UWP.Views.Video"
  mc:Ignorable="d"
  NavigationCacheMode="Enabled"
  DataContext="{Binding Source={StaticResource ViewModelLocator},Path=VideoListModel}">
<!--NavigationCacheMode Enable for the page state save-->
<Page.Resources>
    <DataTemplate x:Key="VideoTemplate">
        <Grid Width="{Binding Width}"
              Height="{Binding Height}"
              Margin="2">
            <Image HorizontalAlignment="Center"
                   Stretch="UniformToFill"
                   Source="{Binding Image}" />
            <TextBlock Text="{Binding VideoName}"/>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="Author" />
                <TextBlock Text="{Binding Author}" />
            </StackPanel>
        </Grid>
    </DataTemplate>
</Page.Resources>

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <ListView Name="VideosListWrapPanal"
              ItemTemplate="{StaticResource VideoTemplate}">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <Controls:WrapPanel />
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ListView>

</Grid>
</Page>

I wanted to do something like below in my VideoListModel for constructor.

public async MainViewModel()
{
   VideoItems = new ObservableCollection<MainMenuItem>(await GetVideoItem());

}

how can I accomplish this initialization in an asynchronous way? To get the thumbnail I have created the method of GetVideoItem(), But I can't find a way to call the GetVideoItem asynchronously in a constructor. Does anyone know how to solve this task?

Answer 1

I recommend using an asynchronous task notifier, as described in my article on async MVVM data binding.

E.g., using NotifyTask from this helper library:

public NotifyTask<List<VideoListItem>> VideoItems { get; }

public VideoListModel(IKnownFolderReader knownFolder)
{
  _knownFolder = knownFolder;
  VideoItems = NotifyTask.Create(() => _knownFolder.GetData());
}

Your data binding would then change from ItemsSource="{Binding VideoItems}" to ItemsSource="{Binding VideoItems.Result}". In addition, VideoItems has several other properties such as IsNotCompleted and IsFaulted so that your data binding can show/hide elements based on the state of the task.

This approach avoids the subtle problems with Result and problems with ContinueWith.