如何在UWP MVVM模型中使我的构造函数异步？(MVVM Lighttoolkit)

Question

我有一个想要读取StorageFolder VideosLibrary的UWP项目,并在带缩略图的Views中显示mp4文件列表.

使用MVVM ligth工具包,我已经使用xaml设置了这4只苍蝇.Xaml正在使用UWP社区工具箱包装面板.

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>

我想在我的VideoListModel中为构造函数执行类似下面的操作.

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

}

如何以异步方式完成此初始化？为了获得缩略图,我创建了GetVideoItem()的方法,但是我找不到在构造函数中异步调用GetVideoItem的方法.有谁知道如何解决这个任务？

Answer 1

简短的回答是:你不能做一个构造函数async.

但是有一些选择可以解决这个问题.以下是两个提案:

解决方案1:ViewModel生命周期

很多MVVM框架使用lifecylce方法来解决这个问题.您可以添加一个ActivateAsync方法,在实例化之后由框架调用ViewModel.

在您的示例中,这可以在ViewModelLocator中完成.

interface IActivate
{
    Task ActivateAsync();
}

// Call it like this:
(model as IActivate)?.ActivateAsync(); // this will work even if the model does not implement IActivate

解决方案2:使用工厂

另一个选择是使用a factory method来创建ViewModel.在factory method可以获取所有async数据,并创建该对象的所有数据汇总之后.

public static async Task<CustomViewModel> Create()
{
    var data = await FetchAsyncData();
    return new CustomViewModel(data);
}

例:

这里有一个关于如何使用该activate模式的简短片段.

public class ViewModelLocator 
{
    // existing implementation goes here

    public async Task<TViewModel> Create<TViewodel>
    {
        var model = ServiceLocator.Current.GetInstance<TViewodel>(); 
        var activate = model as IActivate;
        if(activate != null)
            await activate.ActivateAsync();

        return model;
    }
}

现在,工厂方法仅返回完全激活的模型.这种模式的优点是创建者不需要知道它正在创建的模型.它检查模型是否需要激活并调用它.然后可以将所有激活逻辑放入ViewModel.

Answer 2

我建议使用异步任务通知程序，如我有关异步 MVVM 数据绑定的文章中所述。

例如，使用NotifyTask此帮助程序库：

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

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

然后，您的数据绑定将从 更改ItemsSource="{Binding VideoItems}"为ItemsSource="{Binding VideoItems.Result}"。此外，VideoItems还有其他几个属性，例如IsNotCompleted和 ，IsFaulted以便您的数据绑定可以根据任务的状态显示/隐藏元素。

这种方法避免了和的微妙问题Result。ContinueWith