c# - 在 LINQ 中调用 Task<> 方法并返回数据

Question

我在调用 Task<> 时遇到问题LINQ 语句中的方法并尝试从 Task<> 返回数据(图像)方法。我无法获得 Task<>返回数据(图像)的方法，我收到以下错误:

Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'System.Uri'.

这是带有 LINQ 语句的 DownloadStringCompleted 方法，我在其中调用 GetTest Task<> 方法:

private async void GetGamesListRequestCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    if (e.Error == null)
    {
    var feedXml = XDocument.Parse(e.Result);

    var gameData = feedXml.Root.Descendants("Game").Select(x => new GetGamesList
    {
        ID = (int)x.Element("id"),
        GameTitle = (string)x.Element("GameTitle"),
        ReleaseDate = (string)x.Element("ReleaseDate"),
        Platform = (string)x.Element("Platform"),
        Front = GetTest((int)x.Element("id")), // THE METHOD WITH PROBLEM.
    })
    .ToList();

    foreach (var item in gameData)
    {
        GetGamesListItems.Add(item);
    }
}
}

我在尝试返回数据(图像)时遇到问题的 Task<> GetTest 方法:

public Task<string> GetTest(int id)
{
    var tcs = new TaskCompletionSource<string>();
    var client = new WebClient();
    client.DownloadStringCompleted += (s, e) =>
    {
        if (e.Error == null)
        {
            var feedXml = XDocument.Parse(e.Result);

            var gameData = feedXml.Root.Descendants("Images").Select(x => new GetArt
            {
                BoxArtFrontThumb = new Uri(GetBoxArtFront(x)),
            })
            .ToList();

            foreach (var item in gameData) GetArtItems.Add(item);
            foreach (var i in GetArtItems)
            {
                tcs.SetResult("http://thegamesdb.net/banners/" + i.BoxArtFrontThumb.ToString());
            }
        }
        else
        {
            tcs.SetException(e.Error);
        }
    };

    client.DownloadStringAsync(new Uri("http://thegamesdb.net/api/GetArt.php?id=" + id.ToString()));
    return tcs.Task;
}

我存储图像的 ObservableCollection: 私有(private) ObservableCollection _GetArtItems = new ObservableCollection();

public ObservableCollection<GetArt> GetArtItems
{
    get
    {
        return this._GetArtItems;
    }
}

我从 XML 获取图像的位置:

private static string GetBoxArtFront(XElement gameNode)
{
    return "http://thegamesdb.net/banners/" + (string)gameNode.Descendants("boxart")
        .FirstOrDefault(b => (string)b.Attribute("side") == "front");
}

这是我用来存储数据(图像)的类:

public class GetArt
{
    public Uri BoxArtFrontThumb { get; set; }
}

这是我的 LongListSelector，我想在其中显示数据(图像):

<phone:LongListSelector Name="llsGameList" Background="#242424" ItemsSource="{Binding}" Tap="llsGameList_Tap" Margin="0,90,0,0">
                    <phone:LongListSelector.ItemTemplate>
                        <DataTemplate>
                            <Grid>
                                <toolkit:ContextMenuService.ContextMenu>
                                    <toolkit:ContextMenu Name="ContextMenu">
                                        <toolkit:MenuItem 
                            Name="addToFavorite"  
                            Header="Add to favorite" 
                            Click="addToFavorite_Click"/>
                                    </toolkit:ContextMenu>
                                </toolkit:ContextMenuService.ContextMenu>
                                <StackPanel>
                                    <Border Background="{StaticResource PhoneAccentBrush}" 
        Padding="{StaticResource PhoneTouchTargetOverhang}"
        Margin="{StaticResource PhoneTouchTargetOverhang}">
                                        <TextBlock Name="tblGameTitle" Style="{StaticResource PhoneTextGroupHeaderStyle}" ManipulationStarted="tblGameTitle_ManipulationStarted" ManipulationCompleted="tblGameTitle_ManipulationCompleted">
                                        <Run Text="{Binding GameTitle}"></Run>
                                        </TextBlock>
                                    </Border>
                                    <Image Source="{Binding Front}" Height="200" Width="200"></Image> // HERE IM BINDING TO MY FRONT PROPERTY TO SHOW THE IMAGES
                                    <TextBlock TextWrapping="Wrap" Foreground="YellowGreen" Style="{StaticResource PhoneTextNormalStyle}" Padding="{StaticResource PhoneTouchTargetOverhang}" 
        FontSize="{StaticResource PhoneFontSizeNormal}">
                                        <Run Text="Platform: "></Run>
                                        <Run Text="{Binding Platform}"></Run>
                                    </TextBlock>
                                    <TextBlock Foreground="YellowGreen" Style="{StaticResource PhoneTextNormalStyle}" Padding="{StaticResource PhoneTouchTargetOverhang}" 
        FontSize="{StaticResource PhoneFontSizeNormal}">
                                        <Run Text="Release Date: "></Run>
                                        <Run Text="{Binding ReleaseDate}"></Run>
                                    </TextBlock>
                                </StackPanel>
                            </Grid>
                        </DataTemplate>
                    </phone:LongListSelector.ItemTemplate>
                </phone:LongListSelector>

这是我的 DataContext，它获取 gd.GetGamesListItems，其中包含我想在 LongListSelector 中显示的图像:

public MainPage()
    {
        InitializeComponent();
        llsGameList.DataContext = gd.GetGamesListItems;
    }

我希望有人可以帮助我 :)。谢谢。

Answer 1

每当你有一个Task<T> ，您需要花一些时间考虑您希望如何(异步)等待结果。请记住，一个 Task<T>表示 future 结果，而不是当前结果。

一个选项是根据@Sriram 的回答，它将(异步地)一次等待一个结果，并在它们到达时将它们添加到列表中。另一种选择是做这样的事情:

var feedXml = XDocument.Parse(e.Result);
var gameDataTasks = feedXml.Root.Descendants("Game").Select(
    async x => new GetGamesList
    {
      ID = (int)x.Element("id"),
      GameTitle = (string)x.Element("GameTitle"),
      ReleaseDate = (string)x.Element("ReleaseDate"),
      Platform = (string)x.Element("Platform"),
      Front = new Uri(await GetTestAsync((int)x.Element("id"))),
    }).ToList();
var gameData = await Task.WhenAll(gameDataTasks);
foreach (var item in gameData)
{
  GetGamesListItems.Add(item);
}

这将启动所有 GetTestAsync调用所有元素，然后(异步地)等待它们全部完成(并检索结果)。

其他说明:

• 我确实将您的方法重命名为 GetTestAsync符合 Task-based Asynchronous Pattern .
• 我发现如果使用较新的 HttpClient，代码会更简洁，内置支持 await .

这是一个使用 HttpClient 的例子:

public async Task<string> GetTestAsync(int id)
{
  var client = new HttpClient();
  var result = await client.GetStringAsync("http://thegamesdb.net/api/GetArt.php?id=" + id);
  var feedXml = XDocument.Parse(result);
  var gameData = feedXml.Root.Descendants("Images").Select(x => new GetArt
  {
    BoxArtFrontThumb = new Uri(GetBoxArtFront(x)),
  }).ToList();
  foreach (var item in gameData) GetArtItems.Add(item);
  return "http://thegamesdb.net/banners/" + gameData.Single().BoxArtFrontThumb.ToString()
}